Question

Say I want to write a function to decide whether a given integer number is prime, which type signature should I use?

  isPrime :: Int -> Bool

or

  isPrime :: (Integral a) => a -> Bool

What’s the difference? Is there a particular reason to choose one over the other?
If so, in which situations should I use the two respectively?

Answer 1

The type Int -> Bool means that your function operates on values of type Int, which are size-limited integers (the maximum size being, I believe, machine-dependent).

The type (Integral a) => a -> Bool means that your function operates on values of any type that has an instance of the Integral type class–i.e., types that behave like integers in a particular way. The main reason to chose this over a concrete type is to create a more general-purpose function.

Generic forms using Integral tend to be most useful when you need to work with integer-like types in other contexts–a good example being places where the standard library fails to do so, e.g. functions like replicate :: Int -> a -> [a]. Code that operates on some specific integer-like type for its own purposes that wants to use that type with replicate therefore needs to convert to Int first, or import genericReplicate from Data.List.

What you might want to consider in your case is instead the type Integer, which represents integers of arbitrary size. Since your main goal is the calculation, there’s less value to supporting arbitrary integral types.

If memory serves me, the only instances of Integral in the standard library are Int and Integer anyhow. (EDIT: As hammar reminds me in the comments, there are also instances for fixed-size types in Data.Int and Data.Word. There are also foreign types like CInt but I was disregarding those intentionally.)