322 lines
14 KiB
Rust
322 lines
14 KiB
Rust
//! Type substitution + unification helpers for monomorphisation.
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//!
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//! Free functions extracted from `lib.rs` during the 18g tidy split.
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//! The four-step pipeline is: `derive_substitution` walks declared
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//! params against actual arg types, calling `unify_for_subst` to bind
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//! `Type::Var`s; `apply_subst_to_type` / `apply_subst_to_term`
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//! specialise a polymorphic def under that binding;
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//! `qualify_local_types_codegen` rewrites bare ADT names into
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//! `module.Type` form when a sig crosses an import boundary;
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//! `descriptor_for_subst` produces the stable mangling suffix used in
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//! the specialised symbol's name.
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use ailang_core::ast::*;
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use std::collections::{BTreeMap, BTreeSet};
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use super::{CodegenError, Result};
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use crate::synth::type_descriptor;
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/// Iter 12b: derive a name → concrete-type substitution from the
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/// declared params of a `Forall` body and the actual arg types at a
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/// call site. Walks both sides in parallel; whenever a `Type::Var`
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/// (rigid name) appears on the params side, binds it to the
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/// corresponding concrete type. Conflicts (same var bound to two
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/// different types) surface as an internal error — the typechecker
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/// would already have rejected such a call.
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pub(crate) fn derive_substitution(
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vars: &[String],
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params: &[Type],
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arg_tys: &[Type],
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) -> Result<BTreeMap<String, Type>> {
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if params.len() != arg_tys.len() {
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return Err(CodegenError::Internal(format!(
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"derive_substitution: arity mismatch ({} params vs {} args)",
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params.len(),
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arg_tys.len(),
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)));
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}
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let var_set: BTreeSet<&str> = vars.iter().map(|s| s.as_str()).collect();
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let mut subst: BTreeMap<String, Type> = BTreeMap::new();
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for (p, a) in params.iter().zip(arg_tys.iter()) {
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unify_for_subst(p, a, &var_set, &mut subst)?;
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}
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// Any forall var not pinned by the args is left unbound. For the
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// MVP this is an error — we can't specialise without a concrete
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// type. The typechecker's body should have constrained it already
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// through return-type unification, but at the call site we only
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// see args; if needed, callers can extend this with expected-ret
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// info.
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// Iter 15a: a forall var that the args couldn't pin (e.g.
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// `is_none(Nothing) : forall a. (Maybe a) -> Bool` — `a` is
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// genuinely unobservable from the args alone) defaults to `Unit`.
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// The specialised body must not actually read an `a`-typed value,
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// or it would have failed type-checking; a dummy concrete type is
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// sound and lets monomorphisation proceed deterministically. The
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// descriptor uses the same default, so all such call sites
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// converge on a single specialisation.
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for v in vars {
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if !subst.contains_key(v) {
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subst.insert(v.clone(), Type::unit());
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}
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}
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Ok(subst)
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}
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/// Walks `param` and `arg` in parallel, treating any `Type::Var { name }`
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/// on the param side whose name is in `vars` as an unknown to be bound
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/// in `subst`. Identical concrete shapes pass through; structural
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/// mismatches yield an internal error.
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pub(crate) fn unify_for_subst(
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param: &Type,
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arg: &Type,
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vars: &BTreeSet<&str>,
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subst: &mut BTreeMap<String, Type>,
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) -> Result<()> {
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// Iter 14a fix, extended in 15g-aux: a `$u`-prefixed var is a
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// synth-only wildcard produced by `synth_arg_type` for nullary
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// ctors of a parameterised ADT (e.g. `Nil : List<$u>`). It
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// carries no real constraint — accept without binding so a
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// sibling arg can pin the type var instead. Without this,
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// `Cons(Int, Nil)` synth would unify `a = Int` (from head) and
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// then `a = $u` (from tail's recursive `List<a>` slot) and
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// falsely error.
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//
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// 15g-aux: the early-return must accept `$u` on **either** side.
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// `$u` enters in arg position from synth, but the prev-binding
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// recursion below (`unify_for_subst(&prev, arg, ...)`) can swap
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// a `$u` onto the param side when a previously-bound type is
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// unified against a fresher arg whose roles differ. Reduced
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// repro: `length [Left 1, Right 10]` — `a` first binds to
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// `Either<Int, $u>` from `Left 1`, then a recursive unification
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// against `Either<$u, Int>` from `Right 10` lands `$u` in the
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// param-pos[1] slot. Symmetric early-return is correct because
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// `$u` is a synth-only wildcard regardless of which side carries
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// it after the prev-binding swap.
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if let Type::Var { name } = arg {
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if name.starts_with("$u") {
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return Ok(());
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}
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}
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if let Type::Var { name } = param {
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if name.starts_with("$u") {
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return Ok(());
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}
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}
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match (param, arg) {
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(Type::Var { name }, _) if vars.contains(name.as_str()) => {
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if let Some(prev) = subst.get(name).cloned() {
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// Iter 15b: the previously-bound type may be more
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// concrete than `arg` (e.g. `prev = List<Int>` from a
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// sibling binding, `arg = List<$u>` from a synth-
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// wildcard nullary ctor). Use recursive unification
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// instead of strict equality so the inner `$u`
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// wildcard matches `Int`. The previous strict-
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// equality check rejected such overlaps as bogus
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// duplicate bindings.
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return unify_for_subst(&prev, arg, vars, subst);
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}
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subst.insert(name.clone(), arg.clone());
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Ok(())
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}
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(
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Type::Con { name: pn, args: pa },
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Type::Con { name: an, args: aa },
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) if pn == an && pa.len() == aa.len() => {
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for (p, a) in pa.iter().zip(aa.iter()) {
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unify_for_subst(p, a, vars, subst)?;
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}
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Ok(())
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}
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(
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Type::Fn { params: pp, ret: pr, .. },
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Type::Fn { params: ap, ret: ar, .. },
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) => {
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if pp.len() != ap.len() {
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return Err(CodegenError::Internal(
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"monomorphisation: fn arity mismatch in arg".into(),
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));
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}
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for (p, a) in pp.iter().zip(ap.iter()) {
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unify_for_subst(p, a, vars, subst)?;
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}
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unify_for_subst(pr, ar, vars, subst)
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}
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(Type::Var { name: pn }, Type::Var { name: an }) if pn == an => Ok(()),
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_ => Err(CodegenError::Internal(format!(
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"monomorphisation: cannot match param `{}` to arg `{}`",
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ailang_core::pretty::type_to_string(param),
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ailang_core::pretty::type_to_string(arg),
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))),
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}
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}
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/// Iter 15a: rewrites bare `Type::Con` references that resolve against
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/// `owner_local_types` into qualified `module.Type` form. Mirrors
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/// `ailang_check::qualify_local_types`. Used when the codegen pulls a
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/// polymorphic fn signature across the import boundary; without this
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/// the substitution derived from the call site's qualified args
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/// (`std_maybe.Maybe<Int>`) would fail to unify against the bare
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/// signature (`Maybe<a>`).
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pub(crate) fn qualify_local_types_codegen(
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t: &Type,
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owner_module: &str,
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owner_local_types: &BTreeSet<String>,
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) -> Type {
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match t {
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Type::Con { name, args } => {
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let qualified = if name.contains('.') {
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name.clone()
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} else if ailang_core::primitives::is_primitive_name(name) {
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name.clone()
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} else if owner_local_types.contains(name) {
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format!("{owner_module}.{name}")
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} else {
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name.clone()
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};
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Type::Con {
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name: qualified,
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args: args
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.iter()
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.map(|a| qualify_local_types_codegen(a, owner_module, owner_local_types))
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.collect(),
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}
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}
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Type::Fn { params, ret, effects, .. } => Type::Fn {
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params: params
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.iter()
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.map(|p| qualify_local_types_codegen(p, owner_module, owner_local_types))
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.collect(),
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ret: Box::new(qualify_local_types_codegen(ret, owner_module, owner_local_types)),
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effects: effects.clone(),
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param_modes: vec![],
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ret_mode: ParamMode::Implicit,
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},
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Type::Forall { vars, constraints, body } => Type::Forall {
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vars: vars.clone(),
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constraints: constraints.clone(),
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body: Box::new(qualify_local_types_codegen(body, owner_module, owner_local_types)),
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},
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Type::Var { .. } => t.clone(),
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}
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}
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/// Iter 12b: substitute rigid type vars in `t` according to `subst`.
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/// Used to specialise the type of a polymorphic def for a given
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/// instantiation.
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pub(crate) fn apply_subst_to_type(t: &Type, subst: &BTreeMap<String, Type>) -> Type {
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match t {
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Type::Var { name } => subst.get(name).cloned().unwrap_or_else(|| t.clone()),
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Type::Con { name, args } => Type::Con {
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name: name.clone(),
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args: args.iter().map(|a| apply_subst_to_type(a, subst)).collect(),
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},
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Type::Fn { params, ret, effects, .. } => Type::Fn {
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params: params.iter().map(|p| apply_subst_to_type(p, subst)).collect(),
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ret: Box::new(apply_subst_to_type(ret, subst)),
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effects: effects.clone(),
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param_modes: vec![],
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ret_mode: ParamMode::Implicit,
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},
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Type::Forall { vars, constraints, body } => {
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// Inner forall shadows: don't substitute re-bound names.
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let inner: BTreeMap<String, Type> = subst
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.iter()
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.filter(|(k, _)| !vars.contains(k))
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.map(|(k, v)| (k.clone(), v.clone()))
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.collect();
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Type::Forall {
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vars: vars.clone(),
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constraints: constraints.clone(),
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body: Box::new(apply_subst_to_type(body, &inner)),
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}
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}
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}
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}
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/// Iter 12b: substitute rigid type vars throughout a Term. Only
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/// `Term::Lam` carries types in the AST (params/ret), so most arms
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/// just recurse. `Term::Var` contains a name string only and is
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/// left untouched.
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pub(crate) fn apply_subst_to_term(t: &Term, subst: &BTreeMap<String, Type>) -> Term {
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match t {
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Term::Lit { .. } | Term::Var { .. } => t.clone(),
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Term::App { callee, args, tail } => Term::App {
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callee: Box::new(apply_subst_to_term(callee, subst)),
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args: args.iter().map(|a| apply_subst_to_term(a, subst)).collect(),
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tail: *tail,
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},
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Term::Let { name, value, body } => Term::Let {
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name: name.clone(),
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value: Box::new(apply_subst_to_term(value, subst)),
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body: Box::new(apply_subst_to_term(body, subst)),
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},
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Term::If { cond, then, else_ } => Term::If {
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cond: Box::new(apply_subst_to_term(cond, subst)),
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then: Box::new(apply_subst_to_term(then, subst)),
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else_: Box::new(apply_subst_to_term(else_, subst)),
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},
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Term::Do { op, args, tail } => Term::Do {
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op: op.clone(),
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args: args.iter().map(|a| apply_subst_to_term(a, subst)).collect(),
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tail: *tail,
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},
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Term::Ctor { type_name, ctor, args } => Term::Ctor {
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type_name: type_name.clone(),
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ctor: ctor.clone(),
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args: args.iter().map(|a| apply_subst_to_term(a, subst)).collect(),
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},
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Term::Match { scrutinee, arms } => Term::Match {
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scrutinee: Box::new(apply_subst_to_term(scrutinee, subst)),
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arms: arms
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.iter()
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.map(|a| Arm {
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pat: a.pat.clone(),
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body: apply_subst_to_term(&a.body, subst),
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})
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.collect(),
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},
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Term::Lam { params, param_tys, ret_ty, effects, body } => Term::Lam {
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params: params.clone(),
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param_tys: param_tys
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.iter()
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.map(|t| apply_subst_to_type(t, subst))
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.collect(),
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ret_ty: Box::new(apply_subst_to_type(ret_ty, subst)),
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effects: effects.clone(),
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body: Box::new(apply_subst_to_term(body, subst)),
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},
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Term::Seq { lhs, rhs } => Term::Seq {
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lhs: Box::new(apply_subst_to_term(lhs, subst)),
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rhs: Box::new(apply_subst_to_term(rhs, subst)),
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},
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Term::LetRec { .. } => {
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// Iter 16b.1: eliminated by desugar before any
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// monomorphisation pass runs.
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unreachable!("Term::LetRec eliminated by desugar")
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}
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Term::Clone { value } => Term::Clone {
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// Iter 18c.1: structural recursion through the wrapper.
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value: Box::new(apply_subst_to_term(value, subst)),
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},
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Term::ReuseAs { source, body } => Term::ReuseAs {
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// Iter 18d.1: structural recursion through both children.
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source: Box::new(apply_subst_to_term(source, subst)),
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body: Box::new(apply_subst_to_term(body, subst)),
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},
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}
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}
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/// Iter 12b: deterministic descriptor string for a substitution. Used
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/// as the suffix in the mangled name `@ail_<m>_<def>__<descriptor>`.
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/// Vars are emitted in the order given by the FnDef's forall vars
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/// (so two call sites with the same instantiation map to the same
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/// descriptor regardless of internal BTreeMap ordering).
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pub(crate) fn descriptor_for_subst(vars: &[String], subst: &BTreeMap<String, Type>) -> String {
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let mut parts: Vec<String> = Vec::with_capacity(vars.len());
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for v in vars {
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let ty = subst.get(v).cloned().unwrap_or_else(|| Type::unit());
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parts.push(type_descriptor(&ty));
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}
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parts.join("_")
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}
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