consider the follow f# code
let vl x= let tem y =(x,y) in tem in let a= vl 5 in Console.WriteLine((a "4",a 3))
where vl has the signature 'a -> 'b -> 'a * 'b so tem is 'b -> 'a * 'b but compiling the code produce the error message:
Error FS0001 This expression was expected to have type
'string'
but here has type
'int'
what is going on here?
This is because of the eternally dreaded and marvelously confusing value restriction.
The signature of vl is indeed what you expect it to be. It's the signature of a that can't be generic.
It can't be generic because it's syntactically a value, not a function, which is the essence of the value restriction. For a bit more info on it see this answer.
If a was a top-level binding, this could be fixed by explicitly giving it a generic parameter like let a<'t> = ..., but since it's a local binding, this doesn't work, and the only way to fix this is to give it a parameter, so it's no longer a value, but a function:
let vl x = let tem y = (x,y) in tem in let a z = vl 5 z in (a "4", a 3)
