How to get value type in haskell

Question

I'm trying to debug and would like to be able to get a function which returns a type of a value. Is such a thing possible?

I dont wan't :t, I wan't something which can be used in code

such as:

[showType i | i <- [0..5] ]

returns

[Int,Int,Int,Int,Int]

Answer 1

The Typeable class encodes each type uniquely at compile time and can be used for this purpose:

import Data.Typeable

showType :: Typeable a => a -> String
showType = show . typeOf

with a result of:

*Main> showType (+)
"Integer -> Integer -> Integer"
*Main> showType [1..5]
"[Integer]"
*Main> map showType [1..5]
["Integer","Integer","Integer","Integer","Integer"]

All this said, Bheklilr is right. What you actually want might be something different so more details would help us help you.

Answer 2

Maybe you rather want something like TypedHoles. It allows you to write _ instead of some expression, and the compiler will tell you what type this “hole” has, or should have.