ABI Layer 2: prove bounded array access (Fin-indexed, total) — flagship Idris2 proof#42
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Raise the Idrisiser Idris2 ABI to Layer 2 with a genuine, machine-checked semantic proof of the repo's headline domain property (proven-correct wrappers via dependent types). Module Idrisiser.ABI.Semantics models a length-indexed SafeArray (Vect n a) with a total accessor safeIndex : SafeArray n a -> Fin n -> a, and proves: - InBounds i n is inhabited exactly when i < n (carries an LT i n proof); the out-of-range case has no constructor. - Uninhabited (InBounds i 0): no index exists into an empty array, so out-of-range access is unrepresentable. - decInBounds : sound + complete Dec (InBounds i n), backed by isLT. - finToNatNatToFinLT / safeIndexUsesExactSlot: the proven index round-trips exactly to the requested raw position (no off-by-one, no runtime failure). - certifyBounded + certifyBoundedSound: certifier into the ABI Totality witness, returning Total only when genuinely in bounds. - Positive control (idx2InBounds reads "gamma") and negative controls (Not (InBounds 5 3), Not (InBounds i 0)), all machine-checked. No believe_me / postulate / assert; the headline property is non-vacuous (a deliberately-false InBounds 5 3 witness is rejected by idris2 0.7.0). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01A6PSzJWpRxtzGDjUCEh7Mx
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Summary
Raises idrisiser's Idris2 ABI to Layer 2 with its first flagship semantic proof. Idrisiser's headline is generating proven-correct wrappers with dependent types; this proves bounded array access: indexing a length-
narray with aFin nis always in-bounds and total (no runtime failure), and an out-of-range index is unrepresentable.Mirrors the estate flagship-proof pattern:
Fin/InBoundsmodel,decInBounds : Dec, certifier proven sound, positive + negative controls.Changes
src/interface/abi/Idrisiser/ABI/Semantics.idr—SafeArray/safeIndex(total Fin index),InBounds,decInBounds : Dec,certifyBoundedSound, and negative controlsidx5OutOfBounds : Not (InBounds 5 3),noIndexIntoEmpty : Not (InBounds i 0)..ipkg.RSR Quality Checklist
Required
As Applicable
Testing
Verified with Idris2 0.7.0:
idris2 --build idrisiser-abi.ipkg→ exit 0, zero warnings. Adversarial check: a deliberately-false proof was rejected.build/removed.🤖 Generated with Claude Code
https://claude.ai/code/session_01A6PSzJWpRxtzGDjUCEh7Mx
Generated by Claude Code