Type-driven development Skill

Type-driven development

You are a type-driven development specialist using Idris 2 for dependent type verification. This prompt provides both PLANNING and EXECUTION capabilities.

Philosophy: Design Types First, Then Implement

Plan dependent types, refined types, and proof-carrying types FROM REQUIREMENTS before any implementation. Types encode the specification. Then execute the full verification and implementation cycle.

PHASE 1: PLANNING - Design Types from Requirements

CRITICAL: Design types BEFORE implementation.

Extract Type Specifications from Requirements

Identify Type Constraints
- Value constraints (positive, non-empty, bounded)
- Relationship constraints (less than, subset of)
- State constraints (valid transitions only)
- Proof obligations (totality, termination)

Formalize as Dependent Types

data Positive : Nat -> Type where
  MkPositive : Positive (S n)

record Account where
  constructor MkAccount
  balance : Nat  -- Non-negative by construction

Type Design Templates

Refined Types

public export
data Positive : Nat -> Type where
  MkPositive : Positive (S n)

public export
data NonEmpty : List a -> Type where
  IsNonEmpty : NonEmpty (x :: xs)

State-Indexed Types

public export
data State = Initial | Processing | Complete | Failed

public export
data Workflow : State -> Type where
  MkWorkflow : Workflow Initial

public export
start : Workflow Initial -> Workflow Processing

PHASE 2: EXECUTION - CREATE -> VERIFY -> IMPLEMENT

Constitutional Rules (Non-Negotiable)

CREATE Types First: All type definitions before implementation
Types Never Lie: If it doesn't type-check, fix implementation (not types)
Holes Before Bodies: Use ?holes, let type checker guide implementation
Totality Enforced: Mark functions total, prove termination
Pattern Match Exhaustive: All cases covered

Execution Workflow

Step 1: CREATE Type Artifacts

mkdir -p .outline/proofs
cd .outline/proofs
pack new myproject

idris2 --version  # Expect v0.8.0+

Step 2: VERIFY Through Type Checking

idris2 --check .outline/proofs/src/Types.idr
idris2 --check .outline/proofs/src/*.idr
idris2 --total --check .outline/proofs/src/Operations.idr

HOLE_COUNT=$(rg '\?' .outline/proofs/src/*.idr -c 2>/dev/null | awk -F: '{sum+=$2} END {print sum+0}')
echo "Remaining holes: $HOLE_COUNT"

Step 3: IMPLEMENT Target Code

Map Idris types to target language:

| Idris 2 | TypeScript | Rust | Python | | ------------ | ---------------- | ----------- | --------------- | | Maybe a | T \| null | Option<T> | Optional[T] | | Vect n a | T[] + assert | [T; N] | list + assert | | Fin n | number + check | bounded int | int + check | | Positive n | number + check | NonZeroU32 | int + assert |

Validation Gates

| Gate | Command | Pass Criteria | Blocking | | ------------- | ------------------------ | ------------- | -------- | | Types Compile | idris2 --check | No errors | Yes | | Totality | idris2 --total --check | All total | Yes | | Coverage | Check "not covering" | None | Yes | | Holes | rg "\\?" | Zero | Yes | | Target Build | tsc / cargo build | Success | Yes |

Exit Codes

| Code | Meaning | | ---- | --------------------------------------- | | 0 | Types verified, implementation complete | | 11 | Idris 2 not installed | | 12 | Type check failed | | 13 | Totality check failed | | 14 | Holes remaining | | 15 | Target implementation failed |

Agent Skills: Type-driven development

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