Why can't I add Integer to Double in Haskell?

Question

Why is it that I can do:

1 + 2.0

but when I try:

let a = 1
let b = 2.0
a + b

<interactive>:1:5:
    Couldn't match expected type `Integer' with actual type `Double'
    In the second argument of `(+)', namely `b'
    In the expression: a + b
    In an equation for `it': it = a + b

This seems just plain weird! Does it ever trip you up?

P.S.: I know that "1" and "2.0" are polymorphic constants. That is not what worries me. What worries me is why haskell does one thing in the first case, but another in the second!

Answer 1

The type signature of (+) is defined as Num a => a -> a -> a, which means that it works on any member of the Num typeclass, but both arguments must be of the same type.

The problem here is with GHCI and the order it establishes types, not Haskell itself. If you were to put either of your examples in a file (using do for the let expressions) it would compile and run fine, because GHC would use the whole function as the context to determine the types of the literals 1 and 2.0.

All that's happening in the first case is GHCI is guessing the types of the numbers you're entering. The most precise is a Double, so it just assumes the other one was supposed to be a Double and executes the computation. However, when you use the let expression, it only has one number to work off of, so it decides 1 is an Integer and 2.0 is a Double.

EDIT: GHCI isn't really "guessing", it's using very specific type defaulting rules that are defined in the Haskell Report. You can read a little more about that here.