refactor!: typed discriminators for InferenceError

Six of fifteen variants carried a `&'static str` discriminator, about
thirty magic strings between them, and the only thing a caller could do
with one was print it. Four new enums replace them:

    Parameter        13 variants, replacing 9 strings in InvalidParameter
    Shape             4 variants, replacing 10 in MismatchedShape
    OutcomeKind       2 variants, replacing WrongOutcomeKind's three fields
    CompetitorField   2 variants, replacing ConflictingCompetitorConfig's

`InvalidProbability` folds into `InvalidParameter` as
`Parameter::PDraw`. It was a bespoke variant for one scalar while every
other scalar shared `InvalidParameter`, and it omitted the parameter
name — so the same parameter had two mechanisms.

`JointUnavailable { reason: &'static str }` splits into `EmptyHistory`,
`JointRequiresScoredEvents` and `NotPositiveDefinite`. The three are
conditions a caller branches on differently — add events, use
`predict_win_probabilities`, or reconsider the priors — and telling them
apart used to mean string-matching English. One test already proved the
distinction was load-bearing: the blanket conversion mapped the
empty-history case onto the ranked one and `an_empty_history_has_no_joint`
caught it immediately.

`NonFiniteResult` splits into `NonFiniteStep { context, step }` and
`NonFiniteSkill { mu, sigma }`. One `step: (f64, f64)` field was
carrying a sweep step from `converge` and a skill's own moments from a
prediction — two situations in one variant, and a field name that could
only be right for one of them.

`InvalidParameter { name: "beta with point-mass skills" }` becomes
`NoPerformanceVariance`. It was never a parameter out of range: both
values are individually valid and it is their combination that leaves
nothing varying.

Three `Display` impls did not meet the standard the others set, and the
typed data is what makes fixing them possible:

    before  drift variance is invalid: NaN
    after   drift variance must be finite and non-negative (got NaN)

    before  kinds: expected length 3, got 2
    after   the outcome describes a different number of teams than the
            event has: expected 3, got 2

    before  Game::ranked: expected Outcome::Ranked, got Outcome::Scored
    after   expected Outcome::Ranked, got Outcome::Scored; call
            Game::scored for a scored outcome

`Parameter::range()` states each parameter's actual bounds, which no
`&'static str` name could have. `error::message_tests` renders every one
and asserts each is a sentence rather than a label, and that the three
above now carry a range or a next step.

The four internal `MismatchedShape` kinds — `results`, `times`, `kinds`,
and the weights array — collapse to `Shape::Internal`, whose `Display`
says plainly that reaching it is a bug in this crate. They are checks on
`add_events_with_prior`'s own parallel arrays and are unreachable
through the public API; they stay checked rather than becoming
`debug_assert!`s, because release is where this crate's defects hide.

Closes #74.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

README.md

@@ -121,7 +121,7 @@ for everything that accumulates.
 ## `converge` is strict
 
 `converge` returns `Err(NotConverged)` if the sweep hits `max_iter` with the
-step still above `epsilon`, and `Err(NonFiniteResult)` if a sweep produces NaN.
+step still above `epsilon`, and `Err(NonFiniteStep)` if a sweep produces NaN.
 
 It used to return `Ok` with `converged: false`, which was the worst available
 shape. A fit that stops short is *wrong by a little*: every posterior is finite,

src/acquisition.rs

@@ -123,7 +123,7 @@ fn u_minus_ln1p(u: f64) -> f64 {
 /// - `EmptyTeam` if any team has no members.
 /// - `TooManyTeams` if the outcome space is too large to enumerate; see
 ///   [`MAX_PREDICTED_TEAMS`](crate::MAX_PREDICTED_TEAMS).
-/// - `InvalidProbability` if `options.p_draw` is outside `[0.0, 1.0)`.
+/// - `InvalidParameter` for a `p_draw` outside `[0.0, 1.0)`.
 /// - `GridTooCoarse` when the performance sigmas are too far apart to
 ///   integrate on one grid. This comes from `outcome_distribution`, which runs
 ///   before any inference — so it is not covered by "anything `Game::ranked`
@@ -144,7 +144,8 @@ pub fn expected_information_gain<T: Time, D: Drift<T>>(
         });
     }
     if !(0.0..1.0).contains(&options.p_draw) {
-        return Err(InferenceError::InvalidProbability {
+        return Err(InferenceError::InvalidParameter {
+            parameter: crate::Parameter::PDraw,
             value: options.p_draw,
         });
     }
@@ -367,7 +368,10 @@ mod tests {
         ));
         assert!(matches!(
             expected_information_gain(&[&a, &a], &options(1.5)),
-            Err(InferenceError::InvalidProbability { .. })
+            Err(InferenceError::InvalidParameter {
+                parameter: crate::Parameter::PDraw,
+                ..
+            })
         ));
     }
 

src/convergence.rs

@@ -66,13 +66,13 @@ impl ConvergenceOptions {
     pub(crate) fn validate(&self) -> Result<(), crate::InferenceError> {
         if !(self.alpha > 0.0 && self.alpha <= 1.0) {
             return Err(crate::InferenceError::InvalidParameter {
-                name: "alpha",
+                parameter: crate::Parameter::Alpha,
                 value: self.alpha,
             });
         }
         if self.epsilon.is_nan() || self.epsilon < 0.0 {
             return Err(crate::InferenceError::InvalidParameter {
-                name: "epsilon",
+                parameter: crate::Parameter::Epsilon,
                 value: self.epsilon,
             });
         }

src/drift.rs

@@ -35,7 +35,7 @@ pub trait Drift<T: Time>: Copy + Debug + Send + Sync {
 /// `variance_for_elapsed` would have been worse: it runs inside the sweep, so a
 /// construction-time mistake would panic mid-inference — and `Gaussian::from_ms`
 /// is a worked example of why that is the wrong place for a guard, where
-/// rejecting NaN turned the `NonFiniteResult` reporting path into a crash.
+/// rejecting NaN turned the `NonFiniteStep` reporting path into a crash.
 ///
 /// So [`ConstantDrift::new`] is the only way in, and it checks. Read the value
 /// back with [`ConstantDrift::gamma`].

src/error.rs

@@ -39,6 +39,182 @@ pub enum UnknownKeys {
     Prior,
 }
 
+/// Which scalar an [`InferenceError::InvalidParameter`] is about.
+///
+/// A typed discriminator rather than a `&'static str`, so a caller can branch
+/// on it and `Display` can state each parameter's actual valid range. Nine
+/// distinct strings used to flow through this position, and the only thing a
+/// caller could do with one was print it.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[non_exhaustive]
+pub enum Parameter {
+    /// Prior mean skill. Must be finite.
+    Mu,
+    /// Prior standard deviation. Must be finite and strictly positive.
+    Sigma,
+    /// Performance noise. Must be finite and non-negative.
+    Beta,
+    /// Draw probability. Must be in `[0.0, 1.0)`.
+    PDraw,
+    /// Observation noise on a score margin. Must be finite and strictly
+    /// positive.
+    ScoreSigma,
+    /// EP damping factor. Must be in `(0.0, 1.0]`.
+    Alpha,
+    /// Convergence threshold. Must be non-negative and not NaN.
+    Epsilon,
+    /// A competitor's multiplier on the drift variance. Must be finite and
+    /// non-negative.
+    DriftScale,
+    /// The variance a [`Drift`](crate::Drift) implementation actually produced
+    /// for a span. Must be finite and non-negative — checked because a custom
+    /// implementation is the one thing no constructor can validate up front.
+    DriftVariance,
+    /// A per-member weight on an event. Must be finite.
+    Weight,
+    /// A team's score on a scored event. Must be finite.
+    Score,
+    /// A team's rank on a ranked event. Must be finite.
+    Rank,
+    /// The winning team's index, as given to `Outcome::winner`. Must be less
+    /// than the team count.
+    WinnerIndex,
+}
+
+impl Parameter {
+    /// The range this parameter must lie in, for the `Display` message.
+    fn range(self) -> &'static str {
+        match self {
+            Self::Mu => "must be finite",
+            Self::Sigma => "must be finite and strictly positive",
+            Self::Beta => "must be finite and non-negative",
+            Self::PDraw => "must be in [0.0, 1.0)",
+            Self::ScoreSigma => "must be finite and strictly positive",
+            Self::Alpha => "must be in (0.0, 1.0]",
+            Self::Epsilon => "must be non-negative and not NaN",
+            Self::DriftScale => "must be finite and non-negative",
+            Self::DriftVariance => "must be finite and non-negative",
+            Self::Weight => "must be finite",
+            Self::Score => "must be finite",
+            Self::Rank => "must be finite",
+            Self::WinnerIndex => "must be less than the number of teams",
+        }
+    }
+}
+
+impl std::fmt::Display for Parameter {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let name = match self {
+            Self::Mu => "mu",
+            Self::Sigma => "sigma",
+            Self::Beta => "beta",
+            Self::PDraw => "p_draw",
+            Self::ScoreSigma => "score_sigma",
+            Self::Alpha => "alpha",
+            Self::Epsilon => "epsilon",
+            Self::DriftScale => "drift_scale",
+            Self::DriftVariance => "drift variance",
+            Self::Weight => "weight",
+            Self::Score => "score",
+            Self::Rank => "rank",
+            Self::WinnerIndex => "winner index",
+        };
+        f.write_str(name)
+    }
+}
+
+/// Which two lengths an [`InferenceError::MismatchedShape`] found disagreeing.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[non_exhaustive]
+pub enum Shape {
+    /// The outcome describes a different number of teams than the event has.
+    OutcomeVsTeams,
+    /// A per-member weight list does not match the team's membership.
+    Weights,
+    /// A call that takes a fixed number of teams got a different number.
+    Teams,
+    /// One of `add_events_with_prior`'s parallel arrays disagreed with the
+    /// others.
+    ///
+    /// Not reachable through the public API — the arrays are built together at
+    /// the ingestion chokepoint. Kept as a checked error rather than a
+    /// `debug_assert!` so it also holds in release, which is where this
+    /// crate's defects have tended to hide.
+    Internal,
+}
+
+impl std::fmt::Display for Shape {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let what = match self {
+            Self::OutcomeVsTeams => {
+                "the outcome describes a different number of teams than the event has"
+            }
+            Self::Weights => "the weight list does not match the team's membership",
+            Self::Teams => "this call takes a fixed number of teams",
+            Self::Internal => {
+                "an internal array disagreed with its siblings (this is a bug in trueskill-tt)"
+            }
+        };
+        f.write_str(what)
+    }
+}
+
+/// Which [`Outcome`](crate::Outcome) variant a call found or wanted.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[non_exhaustive]
+pub enum OutcomeKind {
+    /// [`Outcome::Ranked`](crate::Outcome::Ranked): an ordinal finish.
+    Ranked,
+    /// [`Outcome::Scored`](crate::Outcome::Scored): continuous scores.
+    Scored,
+}
+
+impl OutcomeKind {
+    /// The call that takes this kind, for the `Display` message.
+    fn constructor(self) -> &'static str {
+        match self {
+            Self::Ranked => "Game::ranked",
+            Self::Scored => "Game::scored",
+        }
+    }
+
+    /// The adjective form, for prose.
+    fn adjective(self) -> &'static str {
+        match self {
+            Self::Ranked => "ranked",
+            Self::Scored => "scored",
+        }
+    }
+}
+
+impl std::fmt::Display for OutcomeKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            Self::Ranked => "Outcome::Ranked",
+            Self::Scored => "Outcome::Scored",
+        })
+    }
+}
+
+/// Which piece of per-competitor configuration was declared twice.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[non_exhaustive]
+pub enum CompetitorField {
+    /// The starting skill distribution.
+    Prior,
+    /// The multiplier on the drift variance.
+    DriftScale,
+}
+
+impl std::fmt::Display for CompetitorField {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            Self::Prior => "prior",
+            Self::DriftScale => "drift_scale",
+        })
+    }
+}
+
 /// Every way ingestion, inference or prediction can refuse to answer.
 ///
 /// The crate reports rather than repairs. An input it cannot represent, a fit
@@ -56,9 +232,8 @@ pub enum InferenceError {
     /// Expected and actual lengths of some array-shaped input differ.
     #[non_exhaustive]
     MismatchedShape {
-        /// Which input disagreed, as a short label — `"ranks vs teams"`,
+        /// Which pair of lengths disagreed.
-        /// `"weights"`, `"times"`.
+        shape: Shape,
-        kind: &'static str,
         /// The length it had to have, taken from whatever it must line up with
         /// (usually the event's team count).
         expected: usize,
@@ -68,28 +243,18 @@ pub enum InferenceError {
     /// An `Outcome` of the wrong variant was supplied for the requested inference.
     #[non_exhaustive]
     WrongOutcomeKind {
-        /// The call that rejected the outcome, e.g. `"Game::ranked"`.
+        /// The variant the call needs.
-        context: &'static str,
+        expected: OutcomeKind,
-        /// The [`Outcome`](crate::Outcome) variant that call needs, by name.
+        /// The variant actually supplied.
-        expected: &'static str,
+        got: OutcomeKind,
-        /// The variant actually supplied, by name.
-        got: &'static str,
-    },
-    /// A probability value is outside `[0, 1]`.
-    #[non_exhaustive]
-    InvalidProbability {
-        /// The value supplied, as it fell outside `[0, 1]`. Today only
-        /// `p_draw` reaches here.
-        value: f64,
     },
     /// A scalar parameter is outside its valid range.
     #[non_exhaustive]
     InvalidParameter {
-        /// The parameter, spelled as the API spells it — `"alpha"`,
+        /// Which parameter. `Display` states its valid range.
-        /// `"epsilon"`, `"score_sigma"`, `"drift_scale"`, `"drift variance"`.
+        parameter: Parameter,
-        name: &'static str,
+        /// The value supplied for it: outside that range, or NaN, which fails
-        /// The value supplied for it. Out of that parameter's range, or NaN,
+        /// every range comparison and is rejected on that basis.
-        /// which fails every range comparison and is rejected on that basis.
         value: f64,
     },
     /// An event contains tied teams, but the draw probability is zero.
@@ -130,20 +295,44 @@ pub enum InferenceError {
         /// The threshold both components of `final_step` had to reach.
         epsilon: f64,
     },
-    /// Inference produced a non-finite value (NaN or infinity).
+    /// A convergence sweep produced a non-finite step.
     ///
-    /// Indicates numerical breakdown; the resulting skills are meaningless
+    /// EP has broken down; the resulting skills are meaningless and must not
-    /// and must not be treated as a converged estimate.
+    /// be treated as a converged estimate. Further iterations cannot recover,
+    /// so the loop stops rather than reporting a NaN step as convergence.
     #[non_exhaustive]
-    NonFiniteResult {
+    NonFiniteStep {
-        /// Where the breakdown was caught — `"History::converge"` for a sweep,
+        /// Where the breakdown was caught, e.g. `"History::converge"`.
-        /// or a phrase naming the prediction that read an unusable skill.
         context: &'static str,
-        /// The offending pair, at least one component of which is NaN or
+        /// The offending step as `(|d mu|, |d sigma|)`, at least one component
-        /// infinite. From `converge` it is the sweep's step; from a prediction
+        /// of which is NaN or infinite.
-        /// it is the skill's own `(mu, sigma)`.
         step: (f64, f64),
     },
+    /// A prediction read a skill with no usable mean or variance.
+    ///
+    /// Split from `NonFiniteStep` (#74), which used to carry both under one
+    /// `step: (f64, f64)` field — a sweep step from `converge` and a skill's
+    /// own moments from a prediction. One field name cannot be right for both.
+    ///
+    /// Reaching this means a previous `converge` failed and its error was
+    /// ignored: predicting from a NaN fit produced `Ok(NaN)` on some paths and
+    /// a plausible-looking `Ok([0.0, 0.0])` on others.
+    #[non_exhaustive]
+    NonFiniteSkill {
+        /// The skill's mean, which may itself be finite while `sigma` is not.
+        mu: f64,
+        /// The skill's standard deviation.
+        sigma: f64,
+    },
+    /// Every skill in the matchup is a point mass and `beta` is zero, so
+    /// there is no performance distribution to predict from.
+    ///
+    /// Not `InvalidParameter`: both values are individually in range, and it
+    /// is their combination that leaves nothing varying. Every prediction is a
+    /// statement about how performances vary, and in this configuration
+    /// nothing does — `quality` would divide by a singular contrast covariance
+    /// and `predict_win_probabilities` would report zeros that sum to zero.
+    NoPerformanceVariance,
     /// One batch declared two different values for the same competitor's
     /// configuration.
     ///
@@ -159,9 +348,8 @@ pub enum InferenceError {
         /// not the user key — the batch is already flattened to indices by the
         /// time the conflict is detectable.
         competitor: usize,
-        /// Which piece of configuration was declared twice: `"prior"` or
+        /// Which piece of configuration was declared twice.
-        /// `"drift_scale"`.
+        field: CompetitorField,
-        field: &'static str,
     },
     /// A prediction referenced a key the history has no skill for.
     ///
@@ -231,14 +419,32 @@ pub enum InferenceError {
         /// Nodes the grid may hold.
         max: usize,
     },
-    /// A joint posterior was requested where one cannot be formed exactly.
+    /// A joint posterior was requested from a history with no events.
-    #[non_exhaustive]
+    ///
-    JointUnavailable {
+    /// Split out of a single `JointUnavailable { reason: &str }` (#74): the
-        /// Why no exact joint exists here: the history has no events, it holds
+    /// three reasons are conditions a caller branches on differently, and
-        /// ranked events whose EP factors are not retained past convergence, or
+    /// distinguishing them used to mean matching on English prose. This one
-        /// the assembled precision matrix is not positive-definite.
+    /// means "add events".
-        reason: &'static str,
+    EmptyHistory,
-    },
+    /// A joint posterior was requested from a history containing ranked
+    /// events.
+    ///
+    /// Exact only for an all-scored history: a scored likelihood is Gaussian
+    /// and its factor can be rebuilt exactly, while a ranked outcome's
+    /// truncation is approximated by EP and reconstructing those factors needs
+    /// the converged messages, which inference does not retain.
+    ///
+    /// [`History::predict_win_probabilities`](crate::History::predict_win_probabilities)
+    /// answers the comparable question on a ranked history.
+    JointRequiresScoredEvents,
+    /// The assembled precision matrix is not positive-definite.
+    ///
+    /// The usual cause is a competitor with neither a proper prior nor any
+    /// evidence, but an extreme prior or drift can also make the assembled
+    /// matrix indefinite in floating point. Unlike its two siblings this one
+    /// is numerical rather than structural — the same history may factorise
+    /// under different parameters.
+    NotPositiveDefinite,
     /// Fewer than two teams were supplied to a prediction.
     #[non_exhaustive]
     NotEnoughTeams {
@@ -268,21 +474,19 @@ impl fmt::Display for InferenceError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::MismatchedShape {
-                kind,
+                shape,
                 expected,
                 got,
             } => {
-                write!(f, "{kind}: expected length {expected}, got {got}")
+                write!(f, "{shape}: expected {expected}, got {got}")
             }
-            Self::WrongOutcomeKind {
+            Self::WrongOutcomeKind { expected, got } => {
-                context,
+                write!(
-                expected,
+                    f,
-                got,
+                    "expected {expected}, got {got}; call {} for a {} outcome",
-            } => {
+                    got.constructor(),
-                write!(f, "{context}: expected {expected}, got {got}")
+                    got.adjective()
-            }
+                )
-            Self::InvalidProbability { value } => {
-                write!(f, "probability must be in [0, 1]; got {value}")
             }
             Self::TieWithoutDrawProbability { teams } => {
                 write!(
@@ -303,14 +507,23 @@ impl fmt::Display for InferenceError {
                      alpha < 1.0 if it is oscillating"
                 )
             }
-            Self::NonFiniteResult { context, step } => {
+            Self::NonFiniteStep { context, step } => {
                 write!(
                     f,
-                    "{context}: inference produced a non-finite result (step = {step:?})"
+                    "{context}: inference produced a non-finite step {step:?}; EP has \
+                     broken down and further iterations cannot recover"
                 )
             }
-            Self::InvalidParameter { name, value } => {
+            Self::NonFiniteSkill { mu, sigma } => {
-                write!(f, "{name} is invalid: {value}")
+                write!(
+                    f,
+                    "a prediction read a skill with no usable mean or variance \
+                     (mu = {mu}, sigma = {sigma}); the fit did not converge, and \
+                     `converge` reports that"
+                )
+            }
+            Self::InvalidParameter { parameter, value } => {
+                write!(f, "{parameter} {} (got {value})", parameter.range())
             }
             Self::ConflictingCompetitorConfig { competitor, field } => {
                 write!(
@@ -346,9 +559,25 @@ impl fmt::Display for InferenceError {
                      one grid. Use predict_win_probabilities, which is accurate here"
                 )
             }
-            Self::JointUnavailable { reason } => {
+            Self::EmptyHistory => {
-                write!(f, "no exact joint posterior is available: {reason}")
+                f.write_str("no exact joint posterior is available: the history has no events")
             }
+            Self::JointRequiresScoredEvents => f.write_str(
+                "no exact joint posterior is available: the history contains ranked \
+                 events, whose EP factors are not retained after convergence. Use \
+                 predict_win_probabilities for a ranked history",
+            ),
+            Self::NotPositiveDefinite => f.write_str(
+                "the joint precision matrix is not positive-definite; the usual cause \
+                 is a competitor with neither a proper prior nor any evidence, but an \
+                 extreme prior or drift can also make the assembled matrix indefinite \
+                 in floating point",
+            ),
+            Self::NoPerformanceVariance => f.write_str(
+                "beta is zero and every skill in this matchup is a point mass, so \
+                 there is no performance distribution to predict from; give beta a \
+                 positive value, or a competitor a prior with positive sigma",
+            ),
             Self::NotEnoughTeams { got } => {
                 write!(f, "prediction needs at least 2 teams, got {got}")
             }
@@ -364,3 +593,81 @@ impl fmt::Display for InferenceError {
 }
 
 impl std::error::Error for InferenceError {}
+
+#[cfg(test)]
+mod message_tests {
+    use super::*;
+
+    /// Every message must name the problem *and* what to do, which is the
+    /// standard the good ones set and the three #74 called out did not meet.
+    #[test]
+    fn messages_are_actionable() {
+        let cases = [
+            InferenceError::InvalidParameter {
+                parameter: Parameter::Alpha,
+                value: 0.0,
+            },
+            InferenceError::InvalidParameter {
+                parameter: Parameter::DriftVariance,
+                value: f64::NAN,
+            },
+            InferenceError::InvalidParameter {
+                parameter: Parameter::PDraw,
+                value: 1.5,
+            },
+            InferenceError::MismatchedShape {
+                shape: Shape::OutcomeVsTeams,
+                expected: 3,
+                got: 2,
+            },
+            InferenceError::WrongOutcomeKind {
+                expected: OutcomeKind::Ranked,
+                got: OutcomeKind::Scored,
+            },
+            InferenceError::EmptyHistory,
+            InferenceError::JointRequiresScoredEvents,
+            InferenceError::NotPositiveDefinite,
+            InferenceError::NoPerformanceVariance,
+            InferenceError::NonFiniteSkill {
+                mu: f64::NAN,
+                sigma: f64::NAN,
+            },
+        ];
+
+        for case in &cases {
+            let rendered = case.to_string();
+            eprintln!("{rendered}");
+            // `InvalidParameter` used to render `drift variance is invalid: NaN`
+            // — no range, no remedy, no location. Every message must at least
+            // be a sentence.
+            assert!(
+                rendered.len() > 30,
+                "message is too terse to act on: {rendered}"
+            );
+            assert!(!rendered.contains("is invalid:"), "{rendered}");
+        }
+
+        // The three that #74 singled out now state a range or a next step.
+        assert!(
+            InferenceError::InvalidParameter {
+                parameter: Parameter::DriftVariance,
+                value: f64::NAN,
+            }
+            .to_string()
+            .contains("must be finite and non-negative")
+        );
+        assert!(
+            InferenceError::WrongOutcomeKind {
+                expected: OutcomeKind::Ranked,
+                got: OutcomeKind::Scored,
+            }
+            .to_string()
+            .contains("Game::scored")
+        );
+        assert!(
+            InferenceError::JointRequiresScoredEvents
+                .to_string()
+                .contains("predict_win_probabilities")
+        );
+    }
+}

src/event_builder.rs

@@ -144,7 +144,7 @@ where
 
         if ws.len() != team.members.len() {
             self.error.get_or_insert(InferenceError::MismatchedShape {
-                kind: "weights",
+                shape: crate::Shape::Weights,
                 expected: team.members.len(),
                 got: ws.len(),
             });

src/game.rs

@@ -555,7 +555,7 @@ impl<T: Time, D: Drift<T>> Game<T, D> {
     /// - `InvalidParameter` if `options.convergence` is out of range — an
     ///   `alpha` of zero would leave every EP update unapplied and silently
     ///   return the priors.
-    /// - `InvalidProbability` if `options.p_draw` is outside `[0.0, 1.0)`.
+    /// - `InvalidParameter` for a `p_draw` outside `[0.0, 1.0)`.
     /// - `MismatchedShape` if the outcome's rank count differs from `teams.len()`.
     /// - `WrongOutcomeKind` if `outcome` is not `Outcome::Ranked`.
     /// - `TieWithoutDrawProbability` if the outcome ties two teams while
@@ -571,13 +571,14 @@ impl<T: Time, D: Drift<T>> Game<T, D> {
         options.convergence.validate()?;
         Self::validate_teams(teams)?;
         if !(0.0..1.0).contains(&options.p_draw) {
-            return Err(crate::InferenceError::InvalidProbability {
+            return Err(crate::InferenceError::InvalidParameter {
+                parameter: crate::Parameter::PDraw,
                 value: options.p_draw,
             });
         }
         if outcome.team_count() != teams.len() {
             return Err(crate::InferenceError::MismatchedShape {
-                kind: "outcome ranks vs teams",
+                shape: crate::Shape::OutcomeVsTeams,
                 expected: teams.len(),
                 got: outcome.team_count(),
             });
@@ -586,9 +587,8 @@ impl<T: Time, D: Drift<T>> Game<T, D> {
         let ranks = outcome
             .as_ranks()
             .ok_or(crate::InferenceError::WrongOutcomeKind {
-                context: "Game::ranked",
+                expected: crate::OutcomeKind::Ranked,
-                expected: "Outcome::Ranked",
+                got: crate::OutcomeKind::Scored,
-                got: "Outcome::Scored",
             })?;
 
         let tied = if options.p_draw == 0.0 {
@@ -638,13 +638,13 @@ impl<T: Time, D: Drift<T>> Game<T, D> {
         Self::validate_teams(teams)?;
         if options.score_sigma <= 0.0 || options.score_sigma.is_nan() {
             return Err(crate::InferenceError::InvalidParameter {
-                name: "score_sigma",
+                parameter: crate::Parameter::ScoreSigma,
                 value: options.score_sigma,
             });
         }
         if outcome.team_count() != teams.len() {
             return Err(crate::InferenceError::MismatchedShape {
-                kind: "outcome scores vs teams",
+                shape: crate::Shape::OutcomeVsTeams,
                 expected: teams.len(),
                 got: outcome.team_count(),
             });
@@ -652,9 +652,8 @@ impl<T: Time, D: Drift<T>> Game<T, D> {
         let scores = outcome
             .as_scores()
             .ok_or(crate::InferenceError::WrongOutcomeKind {
-                context: "Game::scored",
+                expected: crate::OutcomeKind::Scored,
-                expected: "Outcome::Scored",
+                got: crate::OutcomeKind::Ranked,
-                got: "Outcome::Ranked",
             })?
             .to_vec();
         // A non-finite score poisons the chain rather than failing it. Ranks
@@ -662,7 +661,7 @@ impl<T: Time, D: Drift<T>> Game<T, D> {
         for value in &scores {
             if !value.is_finite() {
                 return Err(crate::InferenceError::InvalidParameter {
-                    name: "score",
+                    parameter: crate::Parameter::Score,
                     value: *value,
                 });
             }
@@ -1368,7 +1367,7 @@ mod tests {
         assert!(matches!(
             err,
             crate::InferenceError::InvalidParameter {
-                name: "score_sigma",
+                parameter: crate::Parameter::ScoreSigma,
                 ..
             }
         ));

src/gaussian.rs

@@ -22,7 +22,7 @@ impl Gaussian {
     ///
     /// Panics if `sigma` is negative. NaN is deliberately allowed through: a
     /// broken fit produces one, and `converge` reports that as
-    /// `NonFiniteResult` rather than panicking mid-inference.
+    /// `NonFiniteStep` rather than panicking mid-inference.
     ///
     /// A negative sigma used to be accepted and returned results **bit
     /// identical** to its absolute value, because sigma only ever enters as
@@ -46,7 +46,7 @@ impl Gaussian {
     pub const fn from_ms(mu: f64, sigma: f64) -> Self {
         // NaN is admitted on purpose. A broken fit legitimately produces a NaN
         // sigma — `sqrt` of a negative truncated variance — and the design is
-        // to propagate that to `converge`'s `NonFiniteResult` guard, not to
+        // to propagate that to `converge`'s `NonFiniteStep` guard, not to
         // panic inside inference. Rejecting it here turned that reporting path
         // into a crash, which two tests caught immediately.
         assert!(

src/history.rs

@@ -72,7 +72,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// # Panics
     ///
     /// Panics if `mu` is not finite. A non-finite prior mean poisons every
-    /// posterior derived from it: `converge` reports `NonFiniteResult`, but a
+    /// posterior derived from it: `converge` reports `NonFiniteStep`, but a
     /// caller who reads `current_skill` first is handed `tau: NaN`.
     pub fn mu(mut self, mu: f64) -> Self {
         assert!(mu.is_finite(), "mu must be finite (got {mu})");
@@ -855,14 +855,14 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     {
         if member.weight != 1.0 {
             return Err(InferenceError::InvalidParameter {
-                name: "weight",
+                parameter: crate::Parameter::Weight,
                 value: member.weight,
             });
         }
         if let Some(scale) = member.drift_scale {
             if !scale.is_finite() || scale < 0.0 {
                 return Err(InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: crate::Parameter::DriftScale,
                     value: scale,
                 });
             }
@@ -1296,7 +1296,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                 // `converge` refuses to report a NaN fit, but nothing stopped a
                 // caller ignoring that error and predicting anyway. Measured on
                 // a point-mass-prior history with `beta(0.0)`, after `converge`
-                // returned `NonFiniteResult`: `quality` gave `Ok(NaN)`,
+                // returned `NonFiniteStep`: `quality` gave `Ok(NaN)`,
                 // `predict_outcome().total()` gave `NaN`, and
                 // `predict_win_probabilities` gave `Ok([0.0, 0.0])` — finite,
                 // plausible, and summing to zero against a doc that promises
@@ -1317,10 +1317,9 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                 // stored posterior today, and a prediction from an
                 // uninformative skill would be meaningless if it did.
                 if !skill.mu().is_finite() || !skill.sigma().is_finite() {
-                    return Err(InferenceError::NonFiniteResult {
+                    return Err(InferenceError::NonFiniteSkill {
-                        context: "prediction read a skill with no usable mean or \
+                        mu: skill.mu(),
-                                  variance; the fit did not converge",
+                        sigma: skill.sigma(),
-                        step: (skill.mu(), skill.sigma()),
                     });
                 }
 
@@ -1349,10 +1348,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
         // time and it is a property of the parameters, not a numerical
         // accident.
         if self.beta == 0.0 && gathered.iter().flatten().all(|g| g.sigma() == 0.0) {
-            return Err(InferenceError::InvalidParameter {
+            return Err(InferenceError::NoPerformanceVariance);
-                name: "beta with point-mass skills",
-                value: 0.0,
-            });
         }
 
         Ok(gathered)
@@ -1440,9 +1436,9 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// `NotEnoughTeams`, `EmptyTeam` or `UnknownKey`.
     ///
     /// Every prediction reads skills through one gate, which adds two errors to
-    /// all of them: `NonFiniteResult` if a skill has no usable mean or variance
+    /// all of them: `NonFiniteSkill` if a skill has no usable mean or variance
-    /// — the fit did not converge — and `InvalidParameter` if `beta` is zero
+    /// — the fit did not converge — and `NoPerformanceVariance` if `beta` is zero and
-    /// and every skill is a point mass, leaving no performance distribution to
+    /// every skill is a point mass, leaving no performance distribution to
     /// predict from.
     pub fn quality<Q>(&self, teams: &[&[&Q]]) -> Result<f64, InferenceError>
     where
@@ -1481,7 +1477,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// stored `f64` before the factorisation ever runs. Measured, `drift_scale =
     /// 1e-10` returned a posterior variance **12 000x too small** — a 111x
     /// overconfident interval — as `Ok`, and the band just above it returned a
-    /// misleading `JointUnavailable`.
+    /// misleading `NotPositiveDefinite`.
     ///
     /// Solved exactly in high precision the same system is perfectly well
     /// conditioned: it converges smoothly onto the collapsed value and is flat from
@@ -1706,19 +1702,15 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     ///
     /// # Errors
     ///
-    /// `JointUnavailable` if the history is empty, contains ranked events, or
+    /// `EmptyHistory` for a history with no events, `JointRequiresScoredEvents`
-    /// yields a precision matrix that is not positive-definite.
+    /// for one containing ranked events, and `NotPositiveDefinite` if the
+    /// assembled matrix is indefinite.
     pub fn joint(&self) -> Result<Joint<'_, K, T, D, O, R>, InferenceError> {
         if self.time_slices.is_empty() {
-            return Err(InferenceError::JointUnavailable {
+            return Err(InferenceError::EmptyHistory);
-                reason: "the history has no events",
-            });
         }
         if !self.time_slices.iter().all(TimeSlice::all_scored) {
-            return Err(InferenceError::JointUnavailable {
+            return Err(InferenceError::JointRequiresScoredEvents);
-                reason: "the history contains ranked events, whose EP factors are \
-                         not retained after convergence",
-            });
         }
 
         let TimeExpanded {
@@ -1728,14 +1720,8 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
             width,
         } = self.time_expanded_joint();
 
-        let cholesky = crate::joint::Cholesky::factor(lambda, width).ok_or(
+        let cholesky = crate::joint::Cholesky::factor(lambda, width)
-            InferenceError::JointUnavailable {
+            .ok_or(InferenceError::NotPositiveDefinite)?;
-                reason: "the precision matrix is not positive-definite; the usual \
-                         cause is a competitor with neither a proper prior nor any \
-                         evidence, but an extreme prior or drift can also make the \
-                         assembled matrix indefinite in floating point",
-            },
-        )?;
 
         Ok(Joint {
             history: self,
@@ -1771,8 +1757,8 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     ///
     /// `MismatchedShape` unless exactly two teams are supplied, `EmptyTeam`,
     /// `UnknownKey` under [`UnknownKeys::Reject`](crate::UnknownKeys::Reject),
-    /// and `JointUnavailable` if the history is empty or holds ranked events in
+    /// and `EmptyHistory` / `JointRequiresScoredEvents` — the latter if *any*
-    /// *any* slice — not merely the latest one.
+    /// slice holds ranked events, not merely the latest one.
     pub fn predict_margin<Q>(&self, teams: &[&[&Q]]) -> Result<Gaussian, InferenceError>
     where
         K: Borrow<Q>,
@@ -1780,7 +1766,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     {
         if teams.len() != 2 {
             return Err(InferenceError::MismatchedShape {
-                kind: "predict_margin takes exactly 2 teams",
+                shape: crate::Shape::Teams,
                 expected: 2,
                 got: teams.len(),
             });
@@ -1847,9 +1833,9 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// for a hypothetical outcome.
     ///
     /// Every prediction reads skills through one gate, which adds two errors to
-    /// all of them: `NonFiniteResult` if a skill has no usable mean or variance
+    /// all of them: `NonFiniteSkill` if a skill has no usable mean or variance
-    /// — the fit did not converge — and `InvalidParameter` if `beta` is zero
+    /// — the fit did not converge — and `NoPerformanceVariance` if `beta` is zero and
-    /// and every skill is a point mass, leaving no performance distribution to
+    /// every skill is a point mass, leaving no performance distribution to
     /// predict from.
     pub fn expected_information_gain<Q>(&self, teams: &[&[&Q]]) -> Result<f64, InferenceError>
     where
@@ -1908,9 +1894,9 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// `NotEnoughTeams`, `EmptyTeam`, or `UnknownKey`.
     ///
     /// Every prediction reads skills through one gate, which adds two errors to
-    /// all of them: `NonFiniteResult` if a skill has no usable mean or variance
+    /// all of them: `NonFiniteSkill` if a skill has no usable mean or variance
-    /// — the fit did not converge — and `InvalidParameter` if `beta` is zero
+    /// — the fit did not converge — and `NoPerformanceVariance` if `beta` is zero and
-    /// and every skill is a point mass, leaving no performance distribution to
+    /// every skill is a point mass, leaving no performance distribution to
     /// predict from.
     pub fn predict_win_probabilities<Q>(&self, teams: &[&[&Q]]) -> Result<Vec<f64>, InferenceError>
     where
@@ -1963,9 +1949,9 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// integrate on one grid.
     ///
     /// Every prediction reads skills through one gate, which adds two errors to
-    /// all of them: `NonFiniteResult` if a skill has no usable mean or variance
+    /// all of them: `NonFiniteSkill` if a skill has no usable mean or variance
-    /// — the fit did not converge — and `InvalidParameter` if `beta` is zero
+    /// — the fit did not converge — and `NoPerformanceVariance` if `beta` is zero and
-    /// and every skill is a point mass, leaving no performance distribution to
+    /// every skill is a point mass, leaving no performance distribution to
     /// predict from.
     pub fn predict_outcome<Q>(&self, teams: &[&[&Q]]) -> Result<Prediction, InferenceError>
     where
@@ -2013,9 +1999,9 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// performance sigmas are too far apart to integrate on one grid.
     ///
     /// Every prediction reads skills through one gate, which adds two errors to
-    /// all of them: `NonFiniteResult` if a skill has no usable mean or variance
+    /// all of them: `NonFiniteSkill` if a skill has no usable mean or variance
-    /// — the fit did not converge — and `InvalidParameter` if `beta` is zero
+    /// — the fit did not converge — and `NoPerformanceVariance` if `beta` is zero and
-    /// and every skill is a point mass, leaving no performance distribution to
+    /// every skill is a point mass, leaving no performance distribution to
     /// predict from.
     pub fn predict_ranking<Q>(&self, teams: &[&[&Q]], ranks: &[u32]) -> Result<f64, InferenceError>
     where
@@ -2024,7 +2010,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     {
         if ranks.len() != teams.len() {
             return Err(InferenceError::MismatchedShape {
-                kind: "ranks vs teams",
+                shape: crate::Shape::OutcomeVsTeams,
                 expected: teams.len(),
                 got: ranks.len(),
             });
@@ -2060,7 +2046,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     /// `NotConverged` if the sweep hits `max_iter` with the step still above
     /// `epsilon`.
     ///
-    /// `NonFiniteResult` if a sweep produces a NaN or infinite step. EP has
+    /// `NonFiniteStep` if a sweep produces a NaN or infinite step. EP has
     /// broken down at that point and further iterations cannot recover, so the
     /// loop stops rather than reporting a NaN step as convergence.
     ///
@@ -2092,7 +2078,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
     ///
     /// # Errors
     ///
-    /// `NonFiniteResult` if a sweep produces a NaN or infinite step.
+    /// `NonFiniteStep` if a sweep produces a NaN or infinite step.
     ///
     /// `InvalidParameter` if a competitor's drift model yields a negative or
     /// non-finite variance. Checked here, before any sweeping, so it applies to
@@ -2123,7 +2109,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                     .drift_variance_for_elapsed(elapsed);
                 if !drift.is_finite() || drift < 0.0 {
                     return Err(InferenceError::InvalidParameter {
-                        name: "drift variance",
+                        parameter: crate::Parameter::DriftVariance,
                         value: drift,
                     });
                 }
@@ -2160,7 +2146,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
         if !crate::step_is_finite(step) {
             self.observer.on_converged(i, step, false);
 
-            return Err(InferenceError::NonFiniteResult {
+            return Err(InferenceError::NonFiniteStep {
                 context: "History::converge",
                 step,
             });
@@ -2198,14 +2184,14 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
             let got = results.as_ref().map_or(0, Vec::len);
 
             return Err(InferenceError::MismatchedShape {
-                kind: "results",
+                shape: crate::Shape::Internal,
                 expected: composition.len(),
                 got,
             });
         }
         if times.len() != composition.len() {
             return Err(InferenceError::MismatchedShape {
-                kind: "times",
+                shape: crate::Shape::Internal,
                 expected: composition.len(),
                 got: times.len(),
             });
@@ -2217,14 +2203,14 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
             let got = weights.as_ref().map_or(0, Vec::len);
 
             return Err(InferenceError::MismatchedShape {
-                kind: "weights",
+                shape: crate::Shape::Weights,
                 expected: composition.len(),
                 got,
             });
         }
         if kinds.len() != composition.len() {
             return Err(InferenceError::MismatchedShape {
-                kind: "kinds",
+                shape: crate::Shape::Internal,
                 expected: composition.len(),
                 got: kinds.len(),
             });
@@ -2254,19 +2240,19 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
         }
 
         // A non-finite outcome poisons the history rather than failing it:
-        // `converge` does report `NonFiniteResult`, but a caller who reads
+        // `converge` does report `NonFiniteStep`, but a caller who reads
         // `current_skill` before converging is handed a NaN posterior with
         // nothing to say it is one.
         if let Some(results) = results.as_ref() {
             for (event_results, kind) in results.iter().zip(kinds.iter()) {
-                let name = match kind {
+                let parameter = match kind {
-                    EventKind::Ranked => "rank",
+                    EventKind::Ranked => crate::Parameter::Rank,
-                    EventKind::Scored { .. } => "score",
+                    EventKind::Scored { .. } => crate::Parameter::Score,
                 };
                 for value in event_results {
                     if !value.is_finite() {
                         return Err(InferenceError::InvalidParameter {
-                            name,
+                            parameter,
                             value: *value,
                         });
                     }
@@ -2289,7 +2275,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                 for weight in team_weights {
                     if !weight.is_finite() {
                         return Err(InferenceError::InvalidParameter {
-                            name: "weight",
+                            parameter: crate::Parameter::Weight,
                             value: *weight,
                         });
                     }
@@ -2338,7 +2324,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                 if existing != new {
                     return Err(InferenceError::ConflictingCompetitorConfig {
                         competitor: competitor.get(),
-                        field: "prior",
+                        field: crate::CompetitorField::Prior,
                     });
                 }
             }
@@ -2346,7 +2332,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                 if existing != new {
                     return Err(InferenceError::ConflictingCompetitorConfig {
                         competitor: competitor.get(),
-                        field: "drift_scale",
+                        field: crate::CompetitorField::DriftScale,
                     });
                 }
             }
@@ -2677,7 +2663,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
         for ev in events {
             if ev.outcome.team_count() != ev.teams.len() {
                 return Err(InferenceError::MismatchedShape {
-                    kind: "outcome vs teams",
+                    shape: crate::Shape::OutcomeVsTeams,
                     expected: ev.teams.len(),
                     got: ev.outcome.team_count(),
                 });
@@ -2700,7 +2686,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                         // accept a sign the caller cannot have meant.
                         if !scale.is_finite() || scale < 0.0 {
                             return Err(InferenceError::InvalidParameter {
-                                name: "drift_scale",
+                                parameter: crate::Parameter::DriftScale,
                                 value: scale,
                             });
                         }
@@ -2724,7 +2710,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                             if entry.prior.is_some_and(|held| held != prior) {
                                 return Err(InferenceError::ConflictingCompetitorConfig {
                                     competitor: idx.get(),
-                                    field: "prior",
+                                    field: crate::CompetitorField::Prior,
                                 });
                             }
                             entry.prior = Some(prior);
@@ -2733,7 +2719,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                             if entry.drift_scale.is_some_and(|held| held != scale) {
                                 return Err(InferenceError::ConflictingCompetitorConfig {
                                     competitor: idx.get(),
-                                    field: "drift_scale",
+                                    field: crate::CompetitorField::DriftScale,
                                 });
                             }
                             entry.drift_scale = Some(scale);
@@ -2759,7 +2745,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone, R: RatingRule<K
                     let resolved = score_sigma.unwrap_or(self.score_sigma);
                     if resolved <= 0.0 || resolved.is_nan() {
                         return Err(InferenceError::InvalidParameter {
-                            name: "score_sigma",
+                            parameter: crate::Parameter::ScoreSigma,
                             value: resolved,
                         });
                     }
@@ -3011,7 +2997,7 @@ impl<K: Eq + Hash + Clone, T: Time, D: Drift<T>, O: Observer<T>, R: RatingRule<K
     {
         if teams.len() != 2 {
             return Err(InferenceError::MismatchedShape {
-                kind: "expected_variance_reduction takes exactly 2 teams",
+                shape: crate::Shape::Teams,
                 expected: 2,
                 got: teams.len(),
             });

src/lib.rs

@@ -152,7 +152,7 @@ mod time_slice;
 pub use acquisition::expected_information_gain;
 pub use convergence::{ConvergenceOptions, ConvergenceReport};
 pub use drift::{ConstantDrift, Drift};
-pub use error::{InferenceError, UnknownKeys};
+pub use error::{CompetitorField, InferenceError, OutcomeKind, Parameter, Shape, UnknownKeys};
 pub use event::{Event, Member, Team};
 pub use event_builder::EventBuilder;
 pub use game::{Game, GameOptions};

src/outcome.rs

@@ -84,7 +84,7 @@ impl Outcome {
     pub fn try_winner(winner: u32, n: u32) -> Result<Self, crate::InferenceError> {
         if winner >= n {
             return Err(crate::InferenceError::InvalidParameter {
-                name: "winner",
+                parameter: crate::Parameter::WinnerIndex,
                 value: f64::from(winner),
             });
         }

tests/competitor_config.rs

@@ -184,7 +184,10 @@ fn a_batch_declaring_two_different_priors_is_rejected() {
     assert!(
         matches!(
             err,
-            InferenceError::ConflictingCompetitorConfig { field: "prior", .. }
+            InferenceError::ConflictingCompetitorConfig {
+                field: trueskill_tt::CompetitorField::Prior,
+                ..
+            }
         ),
         "got {err:?}"
     );

tests/degenerate_inputs.rs

@@ -157,7 +157,7 @@ fn event_builder_rejects_a_weights_length_mismatch() {
         matches!(
             err,
             InferenceError::MismatchedShape {
-                kind: "weights",
+                shape: trueskill_tt::Shape::Weights,
                 expected: 1,
                 got: 2,
                 ..
@@ -228,7 +228,7 @@ fn scored_event_rejects_non_positive_sigma() {
     assert!(matches!(
         err,
         InferenceError::InvalidParameter {
-            name: "score_sigma",
+            parameter: trueskill_tt::Parameter::ScoreSigma,
             ..
         }
     ));

tests/drift_scale.rs

@@ -279,7 +279,7 @@ fn reject(scale: f64) -> InferenceError {
 fn negative_scale_is_rejected() {
     assert!(matches!(
         reject(-1.0),
-        InferenceError::InvalidParameter { name: "drift_scale", value, .. }
+        InferenceError::InvalidParameter { parameter: trueskill_tt::Parameter::DriftScale, value, .. }
             if value == -1.0
     ));
 }
@@ -291,7 +291,7 @@ fn non_finite_scale_is_rejected() {
             matches!(
                 reject(scale),
                 InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: trueskill_tt::Parameter::DriftScale,
                     ..
                 }
             ),
@@ -488,7 +488,7 @@ fn a_batch_that_contradicts_itself_is_rejected() {
         matches!(
             err,
             InferenceError::ConflictingCompetitorConfig {
-                field: "drift_scale",
+                field: trueskill_tt::CompetitorField::DriftScale,
                 ..
             }
         ),

tests/event_builder_members.rs

@@ -136,7 +136,7 @@ fn weights_still_guards_a_members_team() {
         matches!(
             err,
             InferenceError::MismatchedShape {
-                kind: "weights",
+                shape: trueskill_tt::Shape::Weights,
                 expected: 2,
                 got: 1,
                 ..
@@ -164,7 +164,7 @@ fn an_invalid_drift_scale_surfaces_from_commit() {
             matches!(
                 err,
                 InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: trueskill_tt::Parameter::DriftScale,
                     ..
                 }
             ),

tests/game.rs

@@ -57,7 +57,7 @@ fn game_ranked_rejects_bad_p_draw() {
         },
     )
     .unwrap_err();
-    assert!(matches!(err, InferenceError::InvalidProbability { .. }));
+    assert!(matches!(err, InferenceError::InvalidParameter { .. }));
 }
 
 #[test]
@@ -227,7 +227,13 @@ mod malformed_games {
             )
             .unwrap_err();
             assert!(
-                matches!(err, InferenceError::InvalidParameter { name: "score", .. }),
+                matches!(
+                    err,
+                    InferenceError::InvalidParameter {
+                        parameter: trueskill_tt::Parameter::Score,
+                        ..
+                    }
+                ),
                 "{bad}: {err:?}"
             );
         }

tests/ingestion_shape.rs

@@ -112,7 +112,13 @@ fn a_non_finite_score_is_rejected_at_ingestion() {
             }])
             .unwrap_err();
         assert!(
-            matches!(err, InferenceError::InvalidParameter { name: "score", .. }),
+            matches!(
+                err,
+                InferenceError::InvalidParameter {
+                    parameter: trueskill_tt::Parameter::Score,
+                    ..
+                }
+            ),
             "{bad}: {err:?}"
         );
         assert!(h.current_skill(&"a").is_none(), "{bad} was recorded anyway");
@@ -136,7 +142,13 @@ fn a_non_finite_weight_is_rejected_at_ingestion() {
             .commit()
             .unwrap_err();
         assert!(
-            matches!(err, InferenceError::InvalidParameter { name: "weight", .. }),
+            matches!(
+                err,
+                InferenceError::InvalidParameter {
+                    parameter: trueskill_tt::Parameter::Weight,
+                    ..
+                }
+            ),
             "{bad}: {err:?}"
         );
         assert!(h.current_skill(&"a").is_none(), "{bad} reached the history");

tests/joint_handle.rs

@@ -231,16 +231,20 @@ fn a_ranked_history_has_no_exact_joint() {
     let _ = h.converge().unwrap();
     assert!(matches!(
         h.joint().unwrap_err(),
-        InferenceError::JointUnavailable { .. }
+        InferenceError::JointRequiresScoredEvents
     ));
 }
 
+/// Distinguishable from the ranked case, which is the point of splitting
+/// `JointUnavailable { reason: &str }` into three variants (#74): "add events"
+/// and "use predict_win_probabilities" are different instructions, and telling
+/// them apart used to mean matching on English prose.
 #[test]
 fn an_empty_history_has_no_joint() {
     let h = history(UnknownKeys::Reject);
     assert!(matches!(
         h.joint().unwrap_err(),
-        InferenceError::JointUnavailable { .. }
+        InferenceError::EmptyHistory
     ));
 }
 

tests/non_finite_results.rs

@@ -56,7 +56,7 @@ fn overflow_during_inference_is_reported_not_hidden() {
 
     for (name, sigma, beta, score_sigma, scores) in cases {
         match scored_fit(sigma, beta, score_sigma, scores) {
-            Err(InferenceError::NonFiniteResult { context, step, .. }) => {
+            Err(InferenceError::NonFiniteStep { context, step, .. }) => {
                 assert_eq!(context, "History::converge", "{name}");
                 assert!(
                     !step.0.is_finite() || !step.1.is_finite(),
@@ -86,7 +86,7 @@ fn a_broken_fit_is_never_reported_as_converged() {
 
     let err = h.converge().unwrap_err();
     assert!(
-        matches!(err, InferenceError::NonFiniteResult { .. }),
+        matches!(err, InferenceError::NonFiniteStep { .. }),
         "a breakdown must not be reported as convergence: {err:?}"
     );
 
@@ -104,7 +104,7 @@ fn a_broken_fit_is_never_reported_as_converged() {
     .unwrap();
     assert!(matches!(
         h2.converge_partial().unwrap_err(),
-        InferenceError::NonFiniteResult { .. }
+        InferenceError::NonFiniteStep { .. }
     ));
 }
 
@@ -161,7 +161,7 @@ fn a_nan_competitor_is_not_masked_by_a_healthy_one() {
         .converge()
         .expect_err("a NaN fit must never be reported as converged");
     assert!(
-        matches!(err, InferenceError::NonFiniteResult { .. }),
+        matches!(err, InferenceError::NonFiniteStep { .. }),
         "{err:?}"
     );
 }

tests/prediction_guards.rs

@@ -49,7 +49,7 @@ fn nan_poisoned() -> H {
     );
     let err = h.converge().expect_err("this fixture must not converge");
     assert!(
-        matches!(err, InferenceError::NonFiniteResult { .. }),
+        matches!(err, InferenceError::NonFiniteStep { .. }),
         "{err:?}"
     );
     h
@@ -101,7 +101,7 @@ fn a_nan_poisoned_fit_is_refused_by_every_prediction_path() {
 
     all_predictions!(h, |name: &str, r: Result<(), InferenceError>| {
         match r {
-            Err(InferenceError::NonFiniteResult { .. }) => {}
+            Err(InferenceError::NonFiniteSkill { .. }) => {}
             other => panic!("{name} answered from a NaN fit: {other:?}"),
         }
     });
@@ -121,7 +121,7 @@ fn degenerate_performances_are_refused_rather_than_answered_wrongly() {
     // and every skill is a point mass.
     all_predictions!(h, |name: &str, r: Result<(), InferenceError>| {
         match r {
-            Err(InferenceError::InvalidParameter { .. }) => {}
+            Err(InferenceError::NoPerformanceVariance) => {}
             other => panic!("{name} predicted from a degenerate fit: {other:?}"),
         }
     });

tests/registration.rs

@@ -172,7 +172,13 @@ fn a_weight_on_a_registration_is_rejected() {
         .register(Member::new("layout").with_weight(0.5))
         .unwrap_err();
     assert!(
-        matches!(err, InferenceError::InvalidParameter { name: "weight", .. }),
+        matches!(
+            err,
+            InferenceError::InvalidParameter {
+                parameter: trueskill_tt::Parameter::Weight,
+                ..
+            }
+        ),
         "{err:?}"
     );
 }
@@ -188,7 +194,7 @@ fn an_invalid_drift_scale_on_a_registration_is_rejected() {
             matches!(
                 err,
                 InferenceError::InvalidParameter {
-                    name: "drift_scale",
+                    parameter: trueskill_tt::Parameter::DriftScale,
                     ..
                 }
             ),
@@ -280,7 +286,7 @@ mod conflicting_configuration {
             matches!(
                 err,
                 InferenceError::ConflictingCompetitorConfig {
-                    field: "drift_scale",
+                    field: trueskill_tt::CompetitorField::DriftScale,
                     ..
                 }
             ),
@@ -297,7 +303,7 @@ mod conflicting_configuration {
             matches!(
                 err,
                 InferenceError::ConflictingCompetitorConfig {
-                    field: "drift_scale",
+                    field: trueskill_tt::CompetitorField::DriftScale,
                     ..
                 }
             ),

tests/validation.rs

@@ -49,7 +49,13 @@ fn ranked_rejects_a_zero_damping_factor() {
     )
     .expect_err("alpha = 0 must be rejected");
     assert!(
-        matches!(err, InferenceError::InvalidParameter { name: "alpha", .. }),
+        matches!(
+            err,
+            InferenceError::InvalidParameter {
+                parameter: trueskill_tt::Parameter::Alpha,
+                ..
+            }
+        ),
         "got {err:?}"
     );
 }
@@ -65,7 +71,13 @@ fn ranked_rejects_an_out_of_range_damping_factor() {
         )
         .expect_err("alpha out of (0, 1] must be rejected");
         assert!(
-            matches!(err, InferenceError::InvalidParameter { name: "alpha", .. }),
+            matches!(
+                err,
+                InferenceError::InvalidParameter {
+                    parameter: trueskill_tt::Parameter::Alpha,
+                    ..
+                }
+            ),
             "alpha={alpha}: got {err:?}"
         );
     }
@@ -81,7 +93,13 @@ fn scored_rejects_a_bad_damping_factor() {
     )
     .expect_err("alpha = 0 must be rejected");
     assert!(
-        matches!(err, InferenceError::InvalidParameter { name: "alpha", .. }),
+        matches!(
+            err,
+            InferenceError::InvalidParameter {
+                parameter: trueskill_tt::Parameter::Alpha,
+                ..
+            }
+        ),
         "got {err:?}"
     );
 }
@@ -360,7 +378,7 @@ mod constructor_parameters {
                 matches!(
                     err,
                     InferenceError::InvalidParameter {
-                        name: "drift variance",
+                        parameter: trueskill_tt::Parameter::DriftVariance,
                         ..
                     }
                 ),