Basically, I want to have a function to return a polymorphic function, some thing like this:
fun foo () = fn x => x
So the foo function takes in a value of type unit and returns a polymorphic identity function and the compiler is happy with that, it gives me:
val foo = fn : unit -> a -> a
but once I actually call the foo function, the return value is not what I expected
val it = fn : ?.X1 -> ?.X2
Can t generalize because of value restriction it says, any help? thanks in advance
For technical reasons, you are not allowed to generalize (i.e., make polymorphic) the results of a function call. The result of a call must have a monomorphic type. If this weren t the case, you could subvert the type system by the following dirty trick:
ref [] and get back a list of type forall a . a list refand there you are: you are now executing the contents of an arbitrary string as code. Not Good.
By insisting that the value returned by ref [] be monomorphic, you ensure that it can be used as a list of strings or a list of functions but not both. So this is part of the price we pay for type safety.
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