How to add type annotation in let binding

Question

I\'m a beginner(ish) in Haskell and I find error message really hard to understand (I guess it comes with time). Anyway, to help me understanding my mistakes, I tried to add int

Answer 1

This should work:

f :: s -> s
f x =
   let y = undefined :: s
   in  y

Just use the normal :: operator as shown in the above example for type annotation.

Update:

It doesn't seem to work with polymorphic types. But it works with an concrete type.

The following typechecks:

f :: Int -> Int
f x = let m = x :: Int
      in m

This produces error:

f1 :: a -> a
f1 x = let m = x :: a
       in m