Why aren't the inferred types of these Haskell functions all the same?
haskell, hugs, type-inference
Solution
It is the MonomorphismRestriction at work.
Prelude> let f5 = (+)
Prelude> :t f5
f5 :: Integer -> Integer -> Integer
Prelude> :set -XNoMonomorphismRestriction
Prelude> let f5 = (+)
Prelude> :t f5
f5 :: Num a => a -> a -> a
Because of that, the type synthesizer is forced to default early on some types.
Problem
I define five functions that seem to me like they should be equivalent (and, therefore, have the same type). But the inferred types are different. I put the following five lines in type-inference.hs: ``` f1 a b = a + b f2 a = \b -> a + b f3 = \a -> \b -> a + b f4 a = (a+) f5 = (+) ``` And then I load up Hugs: ``` Hugs> :load type-inference.hs Main> :type f1 f1 :: Num a => a -> a -> a Main> :type f2 f2 :: Num a => a -> a -> a Main> :type f3 f3 :: Integer -> Integer -> Integer Main> :type f4 f4 :: Num a => a -> a -> a Main> :type f5 f5 :: Integer -> Integer -> Integer ``` What happened here?