No higher-rank polymorphism

GAP-TYPES-001

A function parameter is a monotype, so it cannot be used at two types in one body.

Lifecycle

known

Owner

language

Scope

reference/spec/types.md#type-inference

Origin

metel-core#1218 limitation analysis; fixture typechecking/generics/limit_01_rank1_fn_arg.mtl; reference/spec/functions.md First-Class Functions

Affects · linked records

spec.functions.first-class-functions.legality-2language spec rule

RFC-0138rfc

gapKnown gaps ↗

Gap

Inference is Hindley-Milner, which is rank-1: a quantifier appears only at the outermost level of a type. Two consequences follow, and the Language Spec states the second:

• A function parameter is a monotype. An unannotated f in fun apply_both(f, x: i64, y: boolean) cannot be applied to both an i64 and a boolean; the second use is rejected (T0001).
• A generic function may not be referenced where the receiving position is itself still generic in the callee (rank-2), nor where nothing pins down a concrete instantiation; both are T0003.

Impact

A Metel programmer cannot write a function that takes a polymorphic function argument and uses it at more than one type. Each use needs its own instantiation, or the argument must be a concrete function type.

Affects

• spec.functions.first-class-functions.legality-2
• RFC-0138

Resolution

None recorded: no RFC or issue plans higher-rank polymorphism. RFC-0138 (implemented) defines the current boundary.