Type inference of a function in GHCi differs with loaded from a file

Question

I wrote a function add\' in test.hs:

add\' = \\x y -> x + y

Then I loaded test.hs in GHCi (versio

Answer 1

You have hit upon the dreaded monomorphism restriction. Monomorphism restriction makes the type signature of your function specialized to a single type. You can turn off that using the extension NoMonomorphismRestriction:

{-# LANGUAGE NoMonomorphismRestriction #-}

add1 = \x y -> x + y
add2 = \x -> \y -> x + y
add3 x y = x + y

And then when you load the types in ghci they will be:

λ> :t add1
add1 :: Num a => a -> a -> a
λ> :t add2
add2 :: Num a => a -> a -> a
λ> :t add3
add3 :: Num a => a -> a -> a