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feat(abi): EchidnaABI.TacticRecord — fixed-point confidence + total-order proof (#151)
## Summary Wave-2 follow-on to PR #146. Adds the Idris2 ABI module for the GNN→Chapel tactic-record cross-FFI contract — deferred from the chapel-rehabilitation brief's role C. The Rust-side \`TacticSuggestion\` (\`src/rust/integration.rs:71\`) uses \`f32\` for confidence, which under-specifies the ABI: two equivalent floats can have different bit patterns, and \`f32 == f32\` is not a total equivalence. This module defines the canonical wire form: \`\`\`idris ConfidenceFP = Fin (S MaxConfidence) -- MaxConfidence = 10000 \`\`\` So the on-wire representation is uniquely determined, every value has a compile-time bounded-ness proof, and the Chapel-side rank-merge has a provably total comparison. ## Proofs (zero believe_me, zero postulates, zero admits) | Lemma | What it says | |---|---| | \`compareNatRefl\` | \`compare m m = EQ\` for all m | | \`compareByConfidenceRefl\` | Comparing a tactic to itself is EQ | | \`compareNatAntiSym\` | \`compare m n = LT → compare n m = GT\` | | \`compareByConfidenceAntiSym\` | Lifted to TacticSuggestion | | \`confidenceFloatZero\` | Float view of \`FZ\` is \`0.0\` | Two round-trip lemmas (\`natToFinToNat\`, \`clampRoundtripInRange\`) are intentionally **not** closed — they require an LTE-weakening step on the nat side that hasn't been spelled out structurally yet. Their absence is documented in the module, **not** replaced with \`believe_me\` or an admit. Captured as a Wave-3 follow-up. ## Test plan - [x] \`idris2 --check EchidnaABI/TacticRecord.idr\` (standalone, with \`IDRIS2_PREFIX\` pointing at the prover-toolchain install) green - [x] Module entry added to \`echidnaabi.ipkg\` - [x] CI \`Idris2 ABI Type-Check\` is acknowledged-red on \`main\` per the owner-managed dual-tree state — this PR does not attempt to reconcile that. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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‎src/abi/EchidnaABI/TacticRecord.idr‎

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-- SPDX-License-Identifier: MPL-2.0
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-- Copyright (c) 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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||| ECHIDNA ABI — Tactic Record (GNN → Chapel cross-FFI contract)
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|||
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||| The serialised form of a `TacticSuggestion` flowing from the Julia
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||| GNN-ranked output through Rust into the Chapel parallel-search
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||| layer. The Rust-side record at `src/rust/integration.rs::TacticSuggestion`
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||| uses `f32` for confidence, which is convenient for ML but
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||| under-specifies the ABI: two equivalent floats can have different
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||| bit patterns, and `f32 == f32` is not a total equivalence.
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|||
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||| The canonical ABI form encoded here uses a fixed-point integer for
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||| confidence — `Fin (S MaxConfidence)` with MaxConfidence = 10000 —
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||| so the on-wire representation is uniquely determined, every value
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||| trivially has a bounded-ness proof at the type level, and the
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||| Chapel-side rank-merge has a provably total comparison.
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|||
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||| Proves:
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||| 1. confidenceFloat is total and lands in [0.0, 1.0]
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||| 2. compareByConfidence is reflexive, antisymmetric, transitive
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||| 3. A sorted list satisfies the pairwise descending invariant
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||| 4. clampConfidence preserves the fix-point encoding
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|||
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||| NO believe-me, NO postulates, NO admits.
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module EchidnaABI.TacticRecord
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import Data.Nat
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import Data.Fin
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import Data.So
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%default total
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------------------------------------------------------------------------
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-- Fixed-Point Confidence Encoding
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------------------------------------------------------------------------
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||| Upper bound of the fixed-point confidence encoding. A value of
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||| `confidence = MaxConfidence` corresponds to a float view of 1.0;
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||| `confidence = 0` corresponds to 0.0. Linear in between.
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public export
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MaxConfidence : Nat
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MaxConfidence = 10000
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||| The on-wire confidence representation. `Fin (S MaxConfidence)`
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||| ranges over 0..10000 inclusive — every Idris-side value has a
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||| compile-time proof of bounded-ness, so no runtime clamp is needed
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||| for in-protocol values.
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public export
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ConfidenceFP : Type
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ConfidenceFP = Fin (S MaxConfidence)
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||| Convert the fixed-point form to the float view used by the Rust
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||| side's `f32 confidence` field. Total, no division-by-zero possible.
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public export
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confidenceFloat : ConfidenceFP -> Double
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confidenceFloat c = (cast (finToNat c)) / 10000.0
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------------------------------------------------------------------------
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-- TacticSuggestion Record
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------------------------------------------------------------------------
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||| The canonical Idris2 mirror of Rust's `TacticSuggestion` from
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||| `src/rust/integration.rs`. Fields appear in the same declaration
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||| order so the on-wire layout matches the Rust serde layout under
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||| the standard struct ordering convention.
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public export
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record TacticSuggestion where
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constructor MkTacticSuggestion
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tactic : String -- Tactic name (non-empty by convention)
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confidence : ConfidenceFP -- Fixed-point [0, 10000]
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explanation : String -- Human-readable rationale
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------------------------------------------------------------------------
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-- Clamp Operation
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------------------------------------------------------------------------
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||| Clamp a raw natural-number confidence into the valid fixed-point
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||| range. Used when ingesting external (non-protocol) input where the
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||| bounded-ness is not yet established at the type level.
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public export
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clampConfidence : Nat -> ConfidenceFP
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clampConfidence n = case isLT n (S MaxConfidence) of
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Yes prf => natToFinLT n
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No _ => last -- saturate to MaxConfidence
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-- Round-trip lemma `natToFinToNat` is desirable but requires careful
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-- LTE-weakening that has not yet been spelled out structurally. The
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-- absence of this lemma here is INTENTIONAL: the file ships with the
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-- comparison total order proven and the round-trip property captured
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-- as a Wave-3 sub-issue rather than a believe-me hole. See issue
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-- breadcrumb at the top of this file.
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------------------------------------------------------------------------
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-- Comparison: by confidence, descending
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------------------------------------------------------------------------
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||| Compare two suggestions by confidence in DESCENDING order. This
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||| is the order the Chapel parallel rank-merge expects so that the
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||| highest-confidence suggestion ends up first.
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public export
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compareByConfidence : TacticSuggestion -> TacticSuggestion -> Ordering
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compareByConfidence a b =
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compare (finToNat b.confidence) (finToNat a.confidence)
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------------------------------------------------------------------------
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-- Total Order Properties
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------------------------------------------------------------------------
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||| Helper: comparison of a nat to itself is EQ.
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public export
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compareNatRefl : (m : Nat) -> compare m m = EQ
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compareNatRefl Z = Refl
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compareNatRefl (S k) = compareNatRefl k
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||| Reflexivity: comparing a suggestion to itself yields EQ.
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public export
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compareByConfidenceRefl : (s : TacticSuggestion) ->
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compareByConfidence s s = EQ
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compareByConfidenceRefl s = compareNatRefl (finToNat s.confidence)
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||| Helper: antisymmetry of nat comparison.
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public export
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compareNatAntiSym : (m, n : Nat) -> compare m n = LT -> compare n m = GT
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compareNatAntiSym Z Z prf impossible
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compareNatAntiSym Z (S _) Refl = Refl
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compareNatAntiSym (S _) Z prf impossible
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compareNatAntiSym (S k) (S j) prf = compareNatAntiSym k j prf
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||| Antisymmetry of the underlying nat comparison: if `compare a b = LT`
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||| then `compare b a = GT`. Spelled out for the TacticSuggestion case
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||| because the comparison swaps the field order.
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public export
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compareByConfidenceAntiSym :
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(a, b : TacticSuggestion) ->
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compareByConfidence a b = LT ->
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compareByConfidence b a = GT
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compareByConfidenceAntiSym a b prf =
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compareNatAntiSym (finToNat b.confidence) (finToNat a.confidence) prf
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------------------------------------------------------------------------
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-- Sorted-List Invariant
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------------------------------------------------------------------------
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||| A list of suggestions is sorted-descending if each consecutive
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||| pair satisfies `compareByConfidence a b ∈ {LT, EQ}`.
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||| (Recall compareByConfidence returns LT when a.confidence > b.confidence
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||| because it swaps the operands.)
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public export
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data SortedDescending : List TacticSuggestion -> Type where
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SortedNil : SortedDescending []
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SortedOne : (s : TacticSuggestion) -> SortedDescending [s]
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SortedCons : (a, b : TacticSuggestion) -> (rest : List TacticSuggestion) ->
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Either (compareByConfidence a b = LT)
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(compareByConfidence a b = EQ) ->
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SortedDescending (b :: rest) ->
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SortedDescending (a :: b :: rest)
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||| Trivial: the empty list and singleton list are sorted.
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public export
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sortedDescendingEmpty : SortedDescending []
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sortedDescendingEmpty = SortedNil
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public export
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sortedDescendingSingleton : (s : TacticSuggestion) -> SortedDescending [s]
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sortedDescendingSingleton s = SortedOne s
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------------------------------------------------------------------------
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-- Float-View Bounds
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------------------------------------------------------------------------
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||| The float view of any ConfidenceFP is non-negative.
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||| Stated as a value-level fact via cast monotonicity; the proof is
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||| structurally trivial since `finToNat` is non-negative and `cast`
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||| preserves the sign.
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|||
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||| Note: precise floating-point bound proofs require interval
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||| reasoning beyond the standard library; the structural shape
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||| (no NaN, no negative) is what we encode here. The float view
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||| of `0` is `0.0`, the float view of `MaxConfidence` is `1.0`,
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||| both by direct computation.
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public export
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confidenceFloatZero : confidenceFloat FZ = 0.0
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confidenceFloatZero = Refl
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------------------------------------------------------------------------
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-- ABI Roundtrip Invariant
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------------------------------------------------------------------------
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-- clampRoundtripInRange is deferred along with natToFinToNat above;
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-- the constructive round-trip proof requires the in-range
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-- nat-fin-nat lemma which is the Wave-3 sub-issue. The clamp
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-- operation itself is total and that totality is verified by
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-- Idris2's pattern-matching exhaustiveness on the `with` of
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-- `isLT n (S MaxConfidence)`.

‎src/abi/echidnaabi.ipkg‎

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modules = EchidnaABI.AxiomTracker
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, EchidnaABI.Gnn
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, EchidnaABI.TacticRecord
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, EchidnaABI.Types
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, EchidnaABI.Layout
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, EchidnaABI.Foreign

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