What is going on with the types in this ghci session?

Question

I\'m learning Haskell, and I was playing around in ghci when I came across something very puzzling.

First, create a simple add function:

Prelude>          


        

Answer 1

It's the Monomorphism restriction. When you define a value with a simple pattern binding (just the name, without any function arguments) and without a type signature, it gets a monomorphic type. Any type variables are tried to be disambiguated according to the defaulting rules, if that doesn't succeed you get a type error.

In this case, the Num constrained type variable gets defaulted to Integer.

You can turn off the monomorphism restriction with

ghci> :set -XNoMonomorphismRestriction

or with the -XnoMonomorphismRestriction flag on the command line.