Haskell macros

A while ago I was looking for something in base, when I landed in Foreign.Storable.

Which made me realized that Haskell has pure C/C++ macros, not only #if/#else to check Cabal-defined flags.

A dirty thought crossed my mind, we could define something like:

{-# LANGUAGE CPP #-}

module C where

type Rule = Int -> Maybe String

#define RULE(name, n, text) \
(name :: Rule) = \x -> if mod x n == 0 then Just "text" else Nothing

RULE (fizz, 3, "Fizz")
RULE (buzz, 5, "Buzz")
RULE (fizzBuzz, 15, "FizzBuzz")

Once evaluated, it gives:

-- ...
{-# LANGUAGE CPP #-}

module C where

type Rule = Int -> Maybe String




(fizz :: Rule) = \x -> if mod x  3 == 0 then Just " Fizz" else Nothing
(buzz :: Rule) = \x -> if mod x  5 == 0 then Just " Buzz" else Nothing
(fizzBuzz :: Rule) = \x -> if mod x  15 == 0 then Just " FizzBuzz" else Nothing

A bit dirty but we have what we want:

type Rule :: Type
type Rule = Int -> Maybe String
fizz :: Rule
buzz :: Rule
fizzBuzz :: Rule

To go further : the excellent Aelve Guide.