AILang/design/models/typeclasses.md

Typeclasses — resolution and monomorphisation whitepaper

The schema-and-diagnostics contract for typeclasses lives in typeclasses; the call-site lookup rule in method dispatch; the built-in classes shipped in the prelude in prelude classes. This file covers the design choices and the resolution / monomorphisation algorithm.

Decision 11: typeclasses — Haskell-lite, monomorphised, coherent

The design pass for typeclasses. Codified after the Feature-acceptance criterion was committed; the criterion is the explicit basis for the choices below.

AILang ships typeclasses to compress a real LLM-author redundancy: without them, every comparable function must be written per-type (int_eq, string_eq, bool_eq, int_show, string_show, …). With typeclasses behind a monomorphising compiler, the LLM author writes one signature with a class constraint and one method per concrete type, and the compiler emits the same machine code as the per-type version. No runtime cost, no dictionary passing, no vtables.

Choice. A deliberately narrow typeclass design — narrower than Haskell, narrower than Rust traits — calibrated to what an LLM author naturally produces. Five semantic axes are committed:

1. Scope. Multi-method, single-parameter, optional defaults, single-superclass relation. No multi-param classes, no functional dependencies, no associated types.
2. Constraints in signatures. Explicit and mandatory. A function that calls a class method must declare the constraint in its forall block. No constraint inference.
3. Resolution. Orphan-free coherence. An instance C T may be declared only in the module of C or in the module of T. Resolution is global type-directed against a workspace-built registry; coherence makes the lookup unambiguous.
4. Defaults. Opt-in via an explicit default keyword in the class body. Methods without default are abstract-required; methods with default may be overridden or inherited per instance.
5. Class-parameter kind. Concrete types only (kind *). No higher-kinded class params; Functor/Monad-style abstractions over type constructors are not expressible. The LLM-natural pattern is List.map / Tree.map as separate functions per type, which monomorphisation handles directly.

The five axes follow from the Feature-acceptance criterion: each rejected mechanism (multi-param, higher-kinded, FunDeps, assoc types) is one an LLM author does not unprompted produce.

Resolution and monomorphisation

Constraint collection (per function body). During typechecking of a body, each method call generates a residual constraint of shape <Class> <Type> (the schema for these lives in Data model) where <Type> may still contain type variables. After local typechecking, residual constraints are checked against the function's declared constraints (modulo α-conversion and modulo auto-expansion through superclasses; see below). Any residual not covered by declared constraints fires MissingConstraint.

Instance registry (workspace-global). At workspace load (see crates/ailang-core/src/workspace.rs), all InstanceDef nodes across all reachable modules are collected into a registry keyed by (class-name, canonical-hash-of-instance-type). Registry build performs three checks:

• Coherence. Each instance's module must be either the class's defining module or the instance type's defining module. Otherwise → OrphanInstance.
• Uniqueness. No two entries share a key. Otherwise → DuplicateInstance.
• Method completeness. Each instance specifies every required (non-default) method of its class. Otherwise → MissingMethod.

Registry build is a one-time-per-build pass that fires before any typechecking. Its errors are workspace-load errors, not per-call-site errors.

Resolution at call sites with concrete types. When the typechecker sees a method call where every type variable in the constraint is substituted to a concrete type, it queries the registry. Hit → resolved. Miss → NoInstance.

Resolution at polymorphic call sites. When type variables are still free, the constraint propagates into the surrounding function's constraint context — which the user MUST have declared explicitly (per axis 2). No constraint is implicitly hoisted.

Monomorphisation (post-typecheck, pre-codegen). A pass between typechecking and codegen replaces every call to a Type::Forall-quantified Def::Fn with a call to a synthesised monomorphic FnDef. Two source-body entry points share the same mechanics in one fixpoint:

1. Class-method entry. For each unique (method, concrete-type) pair produced by a class-constraint residual, the pass looks up the resolved instance body via Registry::entries[(class, type-hash)], substitutes the class parameter to the concrete type, and synthesises a top-level FnDef named <method>__<type-surface-name>.
2. Free-fn entry. For each call site to a polymorphic free Def::Fn with a fully-concrete substitution, the pass takes the source body directly from the polymorphic Def::Fn, applies rigid-var substitution on both the type AND the body (the body may contain inner Term::Lams whose param_tys reference the outer Forall vars), and synthesises a top-level FnDef named <name>__<type-surface-name-1>__<type-surface-name-2>__… (concatenated in Type::Forall.vars declaration order; the N-ary case extends the single-type-var class-method shape bit-stably).

Both arms share:

• A fixpoint loop that keeps collecting targets until a round adds nothing new (a synthesised free-fn body may invoke class methods at concrete types, scheduling new class-method targets; a class-method body may invoke polymorphic free fns at concrete types, scheduling new free-fn targets).
• A dedup cache keyed by (kind, base-name, type-hash-or-joined-hashes) where the first component ("class" / "free") guarantees disjoint keying across the two kinds.
• A call-site rewrite walker that rewrites bare polymorphic call sites — class-method-named OR poly-free-fn-named — to their mono symbols before codegen runs. The walker advances a single cursor over interleaved class-method and free-fn slots emitted in synth's traversal order.

After this pass, the IR contains no polymorphism, no class machinery, no polymorphic call sites — only ordinary monomorphic functions and direct calls. Codegen sees no difference between a hand-written show_int and a synthesised show__Int.

Why mono, not virtual dispatch. Monomorphisation makes the call target visible to the optimiser, unlocking inlining and downstream loop transformations that virtual dispatch prevents in principle. On a saturating branch predictor with a monomorphic indirect target, the indirect call itself is comparable in cost to a non-inlined direct call — the win is in what the optimiser can do with the visible target, not in the call instruction. The end-to-end gain shrinks toward zero on larger callee bodies and cold call sites, but the architectural claim — "mono enables optimisations vdisp forbids" — holds across the spectrum (bench/mono_dispatch.py and the corresponding JOURNAL bench-notes entry record the measured ratios).

The separator is __ rather than # or @ because # and @ are invalid in LLVM IR global identifiers (the IR verifier rejects them inside @ail_<module>_<def> mangled names). __ is legal in both LLVM IR and the C ABI used by the runtime glue, and parses unambiguously into <method>__<type-surface-name> because neither component contains __ by project convention.

No runtime dispatch, no dictionary passing. The monomorphisation pass is the ONLY specialiser. Codegen sees only monomorphic Def::Fns and direct calls; the pre-iter-23.4 codegen-time specialiser (lower_polymorphic_call + module_polymorphic_fns + mono_queue) was removed in iter 23.4. A call that cannot be monomorphised — for instance, because a constraint remains unresolved at the entry point — is a static error, not a runtime one. This is the LLVM-friendly form and is consistent with Decision 10's performance commitment.