How to get the ASCII value of a character in Haskell?

As Travis Brown indicated in a comment, you have to import the ord function from the module Data.Char:

import Data.Char (ord)

main = print (ord 'a')

Only the Prelude module is loaded by default, all other modules have to be imported explicitly.

You can also use fromEnum. (defined in Enum class, from Prelude.)

Prelude> :i Char
data Char = GHC.Types.C# GHC.Prim.Char#     -- Defined in `GHC.Types'
instance Enum Char -- Defined in `GHC.Enum'
instance Eq Char -- Defined in `GHC.Classes'
...

So you can use fromEnum and toEnum, which uses the ASCII code as the Int value.

Prelude> fromEnum 'A'
65
Prelude> fromEnum 'a'
97
Prelude> toEnum 9 :: Char
'\t'
Prelude> toEnum 100 :: Char
'd'