Question

Suppose I have a lambda expression in my program like:

\x -> f $ x + 1

and I want to specify for type safety that x must be an Integer. Something like:

-- WARNING: bad code
\x::Int -> f $ x + 1

Answer 1

You can just write \x -> f $ (x::Int) + 1 instead. Or, perhaps more readable, \x -> f (x + 1 :: Int). Note that type signatures generally encompass everything to their left, as far left as makes syntactic sense, which is the opposite of lambdas extending to the right.

The GHC extension ScopedTypeVariables incidentally allows writing signatures directly in patterns, which would allow \(x::Int) -> f $ x + 1. But that extension also adds a bunch of other stuff you might not want to worry about; I wouldn’t turn it on just for a syntactic nicety.