[Haskell-cafe] Type inference for lambda function

[Matthias Güdemann]

Hi cafe,

today while explaining lambda functions I came across this case which
I don't really understand.

This is what I get using ghci

λ> :t (\a b -> a + b)
(\a b -> a + b) :: Num a => a -> a -> a

but when loading a .hs file with just this function (without signature)

add = \a b -> a + b

I get

λ> :t add
add :: Integer -> Integer -> Integer

but I can explicitly define add to have this type:

add :: Num a => a -> a -> a
add = \a b -> a + b

Also ghc infers this polymorphic type for example for

add0 a = \b -> a + b

Why is the type of add without a signature inferred to Integer and not Num a?

Best regards
Matthias