Haskell Error: Couldn't match expected type `Integer' against inferred type `Int'

Question

I have a haskell function that that calculates the size of the list of finite Ints. I need the output type to be an Integer because the value will actually be larger than the maximum bound of Int (the result will be -1 to be exact if the output type is an Int)

size :: a -> Integer
size a =  (maxBound::Int) - (minBound::Int)

I understand the difference between Ints (bounded) and Integers (unbounded) but I'd like to make an Integer from an Int. I was wondering if there was a function like fromInteger, that will allow me to convert an Int to an Integer type.

Answer 1

You'll need to convert the values to Integers, which can be done by the fromIntegral function (numeric casting for Haskell):

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

It converts any type in the Integral class to any type in the (larger) Num class. E.g.

fromIntegral (maxBound::Int) - fromIntegral (minBound::Int)

However, I would not really trust the approach you're taking -- it seems very fragile. The behaviour in the presence of types that admit wraparound is pretty suspect.

What do you really mean by: "the size of the list of finite Ints". What is the size in this sense, if it isn't the length of the list?