OCaml Polymorphic Parameters

OCaml already supports higher-rank polymorphism in polymorphic record fields and methods, but a function parameter is always monomorphic inside the function body. A function that needs to use an argument at two different types has to take it wrapped in a record or object that every caller then builds. Jane Street's compiler has supported polymorphic parameters for a while, and the task brings the feature to the upstream compiler.

The agent implements polymorphic function parameters: arrow types with a polymorphic parameter such as ('a. 'a -> 'a) -> int, annotated polymorphic parameters in every function form, and applications whose argument must be at least as general as the parameter. The new arrows take part in unification, instantiation, generalization, type equality, signature inclusion and coercions. A polymorphic type is never inferred, and every program accepted today must keep its current type and meaning.

The offline sandbox (4 CPUs, 8 GB, 4 hours) holds the OCaml compiler repository at the starting commit.

The committed HEAD is rebuilt and bootstrapped in a clean checkout with the trusted test suite and bootstrap binaries restored. Hidden phrases in seven families are compared with pre-registered reference observations: whether each is accepted or rejected, the error messages, printed signatures, warnings including under -principal, and program output. The compiler's own test suite must pass with no regressions, and every family must reach 90%.