explain weak type variable

[chenglou]

Hi, An Ocaml beginner here, what does it mean to have _ in your type signatures ? like 
val id2 : '_a -> '_a = <fun> 

IwanKaramazow - Today at 12:25 AM
A type variable '_a (the actual letter doesn't matter, the crucial thing is the underscore) is a so called weak type variable. This is a variable that cannot be generalized, i.e., it can be substituted with only one concrete type. It is like a mutable value, but in the realm of types.

A type denoted with a weak type variable '_a is not included in a type that is denoted with a generic type variable. Moreover, it even can't escape the compilation unit, and should be either hidden or concretized.

Weak type variables are created when an expression is not a pure value (that is defined syntactically). Usually, it is either a function application, or an abstraction. It is usually possible, to get rid of weak type variables by doing a so called eta-expansion, when you substitute a partially applied function to a normal function application by enumerating all function arguments
i.e. id3 = ( fun x -> id2 x)

cc @IwanKaramazow

FWIW, I would probably use a message like this, given the following:

val f : '_a -> '_b

The type signature of f includes weakly polymorphic type variables ('_a, '_b) so f may not be as polymorphic as expected. Polymorphic generalization usually only occurs if f is defined as a syntactic value. Consider rewriting f using an explicit fun to make it into a value.