Question

As the title says im trying to build an interpreter for Imperative languages in Haskell. Iv done 90% of it, nevertheless, im trying to build If statements, and my question is, how will i define a new datatype lets say :

data x = if boolExp then exp else exp.

I understand that i could re-write this with something like doIf boolExp exp exp. But i would like to see if i could use those reserved keywords just for fun (and maybe conciseness).
Note that both boolExp and exp are defined in my language and work correctly( i even evaluate them to get the actual expression “value”).

So bottom line is, how will i add reserved keywords in my data definition as required above?

Answer 1

The closest you can get is the RebindableSyntax extension in GHC, which treats if cond then truePart else falsePart as if you had written ifThenElse cond truePart falsePart, with whatever ifThenElse is in scope.

You can’t use this for a data constructor, but you can write something like this:

{-# LANGUAGE RebindableSyntax #-}

import Prelude

data BoolExp = Foo
  deriving Show

data Exp = If BoolExp Exp Exp | Bar
  deriving Show

ifThenElse :: BoolExp -> Exp -> Exp -> Exp
ifThenElse = If

example = if Foo then Bar else Bar

main = print example

Running this prints If Foo Bar Bar.

However, unless you’re writing some kind of internal DSL where this would make sense, I strongly recommend just sticking with the regular syntax like If cond truePart falsePart. Writing it this way has no real benefit and serves only to confuse people when reading your code.