Haskell: Check if integer, or check type of variable

Question

So let's say you have a variable n.

You want to check if its an integer, or even better yet check what type it is.

I know there is a function in haskell, isDigit that checks if it is a char.

However is there a function that checks if n is in integer, or even better, gives the type of n?

Solution 1

If you are using an interactive Haskell prompt (like GHCi) you can type :t <expression> and that will give you the type of an expression.

e.g.

Prelude> :t 9

gives

9 :: (Num t) => t

or e.g.

Prelude> :t (+)

gives

(+) :: (Num a) => a -> a -> a

Solution 2

import Data.Typeable
isInteger :: (Typeable a) => a -> Bool
isInteger n = typeOf n == typeOf 1

But you should think about your code, this is not very much like Haskell should be, and it probably is not what you want.

Comments

[sepp2k]

Given that he mentioned isDigit, I think he wants to check whether a string represents an integer - not whether a given variable is an integer, even though that's what the title said. Also your type signature is wrong: you're missing the Typeable constraint.

[Chris Eidhof]

This is almost always a wrong approach. It looks like the poster is a Haskell beginner, and we should try to understand his problem better, not give solutions like this.

[Displee]

What if you're using Intellij's Haskell plugin?

[Matt Ellen]

@Displee edon's answer seems to cover that.