MoonBit Proof-Carrying Code

Use this skill when the task is to write, extend, or debug verified MoonBit code.

Typical triggers:

• add contracts to executable MoonBit code
• define an abstract model or representation invariant
• verify a recursive data structure
• connect a concrete representation to a set/map model
• replace trusted proof bridges with lemmas
• debug proof failures, timeouts, or frontend lowering limits

Goal

Write proof-carrying code, not proof-shaped comments.

That means:

• the runtime code remains executable and readable
• the proof model is explicit
• contracts talk about named predicates/functions with consistent roles
• local proof_assert steps explain why the implementation satisfies the model

Naming

Prefer model(...) as the default name for the proof-side semantic view of a value.

Examples:

• model(set) : Fset[Int]
• model(map) : Fmap[Int, Int]
• model(tree) : Seq[Int]

Use a more specific name only when it is materially clearer:

• elements(node) when the model is literally the set of elements stored in a recursive subtree
• domain(bitmap) when the model is specifically the occupied index set
• height(tree) when it is a structural measure, not the main semantic model

Default rule:

• use model for the main semantic abstraction
• use specialized names like elements, domain, height, or rank for auxiliary views

Default Structure

Split the package into two layers.

• .mbtp
  • model functions such as model(x)
  • representation invariants such as tree_inv(x) or sparse_array_inv(x)
  • named proof predicates such as insert_pre(...) and insert_post(...)
  • reusable lemmas
• .mbt
  • executable code
  • contracts over the proof-side predicates
  • local proof_assert steps after construction, branching, and loops

Use this default unless there is a strong reason not to.

Recommended Workflow

1. Choose the abstract model first.
2. Define the smallest useful invariant.
3. State contracts with named *_pre / *_post predicates.
4. Implement the runtime code.
5. Add loop invariants for every proof-relevant loop.
6. Add local proof_assert steps where the solver needs help.
7. Introduce helper lemmas only after seeing actual failing VCs.
8. Shrink trusted bridges from constructors outward.

Do not start by writing a large pile of lemmas.

Step 1: Pick the Right Abstract Model

Prefer the simplest model that matches the observable behavior.

• Membership-only structure: use a finite set.
• Key-value structure: use a finite map.
• Recursive tree/trie: use model(node) or, if clearer, elements(node).
• Packed layout: separate domain/layout facts from semantic meaning.

Example:

fn model(set : HashSet) -> Fset[Int] {
  match set.root {
    None => @fset.fset_empty()
    Some(node) => model(node)
  }
}

If a recursive helper really denotes the element set of a subtree, this is also reasonable:

fn elements(node : Node) -> Fset[Int] {
  match node {
    Empty => @fset.fset_empty()
    Branch(l, x, r) => elements(l).union(elements(r)).add(x)
  }
}

Avoid putting the whole semantics directly into every contract.

Step 2: Keep the Invariant Small

The invariant should mostly describe:

• shape
• bounds
• layout
• well-formedness

Semantic equalities usually belong in postconditions or lemmas, not inside *_inv.

Good:

predicate sparse_ok(sa : SparseArray) {
  sa.data.length() == count_value(sa.bitmap, 0) &&
  (∀ i : Int,
    valid_idx(i) && mem_value(sa.bitmap, i) →
      0 <= rank_value(sa.bitmap, i, 0) &&
      rank_value(sa.bitmap, i, 0) < sa.data.length())
}

Not good:

• putting every update theorem into the invariant
• encoding the entire semantic equality into *_inv

Step 3: Use Named Postconditions

Prefer named predicates over repeating large formulas. As a default naming convention, use *_inv, *_pre, and *_post.

Good:

predicate singleton_post(res : SparseArray, idx : Int, value : Int) {
  sparse_ok(res) &&
  model(res).eq(@fmap.fmap_empty().add(idx, value))
}

pub fn singleton(idx : Int, value : Int) -> SparseArray where {
  proof_require: valid_idx(idx),
  proof_ensure: result => singleton_post(result, idx, value),
} {
  ...
}

This keeps contracts short and gives the solver a reusable target.

Step 4: Put the Math in .mbtp

Proof-side material belongs in .mbtp.

Examples:

fn model(t : Tree) -> Fset[Int] {
  match t {
    Empty => @fset.fset_empty()
    Node(l, x, r, _) => model(l).union(model(r)).add(x)
  }
}

predicate avl(t : Tree) {
  match t {
    Empty => true
    Node(l, x, r, h) =>
      avl(l) &&
      avl(r) &&
      all_lt(model(l), x) &&
      all_gt(x, model(r)) &&
      h == 1 + max2(height(l), height(r))
  }
}

Keep .mbtp focused on:

• logic definitions
• predicates
• lemmas

Avoid filling .mbtp with runtime implementation details.

Two recurring helper patterns are especially useful:

1. Extensional equality hypotheses for abstract structures.

Example:

predicate fmap_eq_hyp(m1 : Fmap[Int, Int], m2 : Fmap[Int, Int]) {
  (∀ k : Int, m1.mem(k) == m2.mem(k)) &&
  (∀ k : Int, m1.mem(k) → m1.find(k) == m2.find(k))
}

lemma fmap_eq_intro(m1 : Fmap[Int, Int], m2 : Fmap[Int, Int]) where {
  proof_require: fmap_eq_hyp(m1, m2),
  proof_ensure: m1.eq(m2),
} {
}

This is often the cleanest way to finish map-refinement proofs.

2. Small transport lemmas for updates.

Examples:

• add/remove mem at self and other keys
• add/remove find at self and other keys
• set cardinality after adding/removing an absent/present element

Prefer several small transport lemmas over one giant “everything changed correctly” theorem.

Step 5: Guide the Solver in .mbt

After constructing data, assert the concrete facts the solver may miss.

Example:

let data = FixedArray::make(1, value)
proof_assert data.length() == 1
proof_assert data[0] == value
let result = { bitmap, data }
proof_assert sparse_ok(result)
proof_assert singleton_post(result, idx, value)
result

Use proof_assert:

• after record construction
• after array writes
• after case splits
• after loop bodies establish a stronger relation

Prefer this over introducing a callable trusted wrapper function.

Step 6: Write Loop Invariants Early

Any loop that is relevant to the proof should get invariants as soon as the loop shape stabilizes.

In practice, proof-carrying MoonBit code often relies on loops for:

• copying array prefixes or suffixes
• accumulating counts or ranks
• building a result structure incrementally
• iterating over a subtree or packed representation

Do not wait for the prover to fail before writing the obvious invariants.

Typical invariants:

• index bounds
• relationship between the accumulator and the abstract model so far
• prefix/suffix copy facts
• preservation of unchanged fields

Example:

for j = 0, acc = 0; j < idx; {
  let next_acc = if bitmap_mem(bitmap, j) { acc + 1 } else { acc }
  proof_assert next_acc == rank_value(bitmap, j + 1, 0)
  continue j + 1, next_acc
} nobreak {
  acc
} where {
  proof_invariant: 0 <= j,
  proof_invariant: j <= idx,
  proof_invariant: acc == rank_value(bitmap, j, 0),
}

For array updates, use staged invariants that match the proof shape.

Example:

for i = 0; i < pos; {
  new_data[i] = old_data[i]
  continue i + 1
} where {
  proof_invariant: 0 <= i,
  proof_invariant: i <= pos,
  proof_invariant: add_prefix_ok(old_data, new_data, pos, i),
}

Then strengthen to a second invariant after the inserted/removed element is handled.

Default rule:

• if a loop contributes to a postcondition, its invariant should mention the proof-side progress explicitly
• if a loop only mutates concrete state, the invariant should still state the concrete relation needed by the next abstraction lemma
• if the loop's final yielded value matters semantically, add proof_yield so the prover knows what the yielded result satisfies

Example:

for i = 0, acc = 0; i < xs.length(); {
  continue i + 1, acc + xs[i]
} nobreak {
  acc
} where {
  proof_invariant: 0 <= i,
  proof_invariant: i <= xs.length(),
  proof_invariant: acc == prefix_sum(xs, i),
  proof_yield: res => res == prefix_sum(xs, xs.length()),
}

Use proof_yield when the proof needs a fact about the value produced by the whole loop expression, not just the state maintained during iteration.

Step 7: Verify the Natural API Surface

If the public API is method-oriented, verify the methods directly.

Example:

pub fn HashSet::contains(self : HashSet, key : Int) -> Bool where {
  proof_require: set_inv(self),
  proof_ensure: result => result == model(self).mem(key),
} {
  ...
}

Use top-level verified helper functions only when they improve structure or reuse, not as a workaround for method contracts.

Step 8: Use Structural Proof Shape for Recursive Code

For recursive data structures:

• define a semantic view like model(node) or elements(node)
• define a shape invariant like node_ok(node, level)
• recurse structurally
• add proof_decrease

Example:

fn contains_at(node : Node, key : Int, level : Int) -> Bool where {
  proof_decrease: node,
  proof_require: node_ok(node, level),
  proof_ensure: result => result == model(node).mem(key),
} {
  match node {
    Flat(k) => key == k
    Branch(children) => ...
  }
}

If the solver resists tail-recursive loops in contracted functions, try structurally recursive code first.

Step 9: For Packed or Indexed Representations, Prove Concrete Updates Before Semantic Meaning

When a representation is packed, indexed, or incrementally rebuilt, do not jump straight from low-level mutation to the final semantic theorem.

First prove concrete update facts that match the implementation structure, then connect them to the abstract model.

A common progression is:

1. basic domain or indexing facts
2. local bounds or position facts
3. concrete update facts for unchanged and changed regions
4. a full concrete-update predicate
5. the final semantic *_post theorem

The exact intermediate predicates depend on the implementation. Choose names that reflect the actual stages in the code.

Typical stages are:

• unchanged region
• updated region
• shifted or rebuilt region
• full concrete-update predicate
• final semantic postcondition

Example pattern:

predicate update_prefix_ok(before_data, after_data, upto) { ... }
predicate update_middle_ok(before_data, after_data, pos, value, upto) { ... }
predicate update_data_ok(before, key, value, after) { ... }

lemma update_model_lemma(...) where {
  proof_require: update_data_ok(...),
  proof_ensure: update_post(...),
} {
  ...
}

For sparse or dense-array code, a more specific ladder like *_prefix_ok, *_fill_ok, and *_data_ok is often effective, but treat that as one useful instance of the general technique rather than a universal template.

Step 10: Keep Shared Shim Packages Small

If you have reusable proof imports or theories, put them in shim packages.

Typical examples:

• finite-set wrappers
• finite-map wrappers
• bitmap domain/rank/count helpers

The benefit is:

• client packages stay focused
• imports are not duplicated
• shared reasoning is easier to test for regressions

But keep shared shims minimal. Large shared lemma sets can perturb unrelated proofs.

Also account for lowering quirks:

• methods may work in contracts while static constructors do not
• a free wrapper like fmap_mk(...) may still be needed even if Fmap::mk(...) parses
• keep those wrappers in the shim package, not duplicated in every client

If a helper is only needed by one package, prefer a local lemma there rather than exporting it from a shared shim.

Step 11: Treat Trust as Temporary

Trusted helpers are acceptable as narrow bridges, but they should not be the design endpoint.

If trust is unavoidable:

• keep preconditions concrete
• target one named predicate
• keep the mathematical statement in .mbtp

Good temporary bridge:

fn singleton_bridge(res : SparseArray, idx : Int, value : Int) -> Unit where {
  proof_axiomatized: true,
  proof_require: valid_idx(idx),
  proof_require: res.data.length() == 1,
  proof_require: res.data[0] == value,
  proof_ensure: singleton_ok(res, idx, value),
} {
  ()
}

Then remove trusted bridges in this order:

1. constructors
2. observers
3. update functions
4. primitive machine-word bridges

Debugging Rule: Inspect the Actual Failure First

After a proof failure:

• run moon prove <pkg>
• inspect _build/verif/<pkg>/<pkg>.proof.json

Classify the problem before editing:

• missing arithmetic/index fact
• missing semantic bridge
• bad quantifier instantiation
• solver perturbation from a new lemma
• frontend/lowering limitation

Different causes need different fixes.

Examples:

• missing index fact → add a local proof_assert
• missing model bridge → add a helper lemma or predicate
• solver perturbation → move a lemma out of a shared shim
• lowering limitation → simplify the proof surface or probe a smaller reproducer

Common reproducer strategy:

• isolate the construct in a tiny probe package
• check whether moon check fails, moon prove crashes, or the VC merely times out
• only then decide whether the issue is modeling, solver guidance, or compiler lowering

Regression Discipline

After every proof edit:

moon check <pkg>
moon prove <pkg>
moon test <pkg>   # if runtime code changed

After editing shared proof layers, rerun dependent packages too.

Do not assume a local fix is safe globally.

Anti-Patterns

Avoid:

• repeating raw #proof_import in every client package
• large inline contract formulas instead of named predicates
• changing abstraction design and solver guidance in one step
• adding many helper lemmas without checking the proof report first
• storing semantic theorems only in trusted .mbt functions
• verifying methods first when top-level functions would be simpler
• introducing generic abstractions too early when a monomorphic first slice will prove faster

Minimal Checklist

Before handing off a verified MoonBit change, confirm:

• a semantic model(...) exists, or there is a clear reason to use a more specific name like elements(...)
• a *_inv predicate exists
• contracts mention named *_pre / *_post predicates when appropriate
• extensional equality is handled explicitly when the abstract model is a set/map
• proof-specific logic is mostly in .mbtp
• runtime code has local proof_assert where needed
• proof-relevant loops have explicit proof_invariant
• the trusted surface is explicit and as small as possible
• moon check, moon prove, and any needed moon test commands were run