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

You can use GHCI to learn a little more about this. Use the command :t to get the type of an expression.

Prelude> :t 1
1 :: Num a => a

So 1 is a constant which can be any numeric type (Double, Integer, etc.)

Prelude> let a = 1
Prelude> :t a
a :: Integer

So in this case, Haskell inferred the concrete type for a is Integer. Similarly, if you write let b = 2.0 then Haskell infers the type Double. Using let made Haskell infer a more specific type than (perhaps) was necessary, and that leads to your problem. (Someone with more experience than me can perhaps comment as to why this is the case.) Since (+) has type Num a => a -> a -> a, the two arguments need to have the same type.

You can fix this with the fromIntegral function:

Prelude> :t fromIntegral
fromIntegral :: (Num b, Integral a) => a -> b

This function converts integer types to other numeric types. For example:

Prelude> let a = 1
Prelude> let b = 2.0
Prelude> (fromIntegral a) + b
3.0