Why is “(2+2.0)” Double in a .hs file but “Fractional a => a” in GHCi?

Question

Loading a = 2+2.0 from a .hs file in GHCi and doing :t a shows a :: Double.

On the other hand, doing let b = 2+2.0 and :t b in GHCi shows b :: Fractional a => a.

How are you able to deduce this from these two documents?

• 4.3.4 Ambiguous Types, and Defaults for Overloaded Numeric Operations https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-750004.3
• 2.4.8 Type defaulting in GHCi: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/interactive-evaluation.html

I find this official documentation almost incomprehensible.

Answer 1

This documentation is what you want, I think. Basically, GHC implements a different language by default in GHCi than in a module; in particular, the dreaded monomorphism restriction is enabled by default in a module (as per the language) whereas in GHCi it's disabled. The monomorphism restriction is what causes GHC to pick a monomorphic type for your a, whereas, with it turned off, GHC is free to generalize the type of b, yielding the polymorphic type you see.

Answer 2

The key is that GHCi isn't defaulting the type of b. It takes the constraints 2 :: Num a => a and 2.0 :: Fractional a => a and unifies them to give 2 + 2.0 :: Fractional a => a. When it loads a module, the compiler forces every value to have a concrete type, so Fractional a is defaulted to Double.