I would like to remove the if ... then ... else ... keywords, because I am embedding a language/DSL in Haskell. if, then and else convey a lot of meaning in many domains, and it would be great if I could redefine (or leave them undefined) them to reflect the nature of the language/domain.
I’ve searched on Google and stackoverflow, but found nothing. (I did find an old thread on why if ... then ... else ... was included as keywords in Haskell.)
My IDE is in Leksah, and, if the keywords can be removed, it would also be nice to have a setting to change the if ... then ... else ... keywords back to their normal font/color/unbold.
I’ve already tried a naming convention of if' for if and so on. It doesn’t feel as good, especially when I want to define if and if', and have to define if' and if'' instead, or if1 and if2. The presence of both if' and if might also be confusing. (The confusion is not that serious an issue in my situation as the users of the DSL are Haskell programmers, but I suppose it can help in other situations).
Summarizing the responses to date:
- Use the
RebindableSyntaxextension to GHC. Not as general as removing the keywords: the syntax of Haskell’s if-then-else is retained. (Frerich Raabe) - Workaround: Use very similar words/names, by using
data Conditional b a = If b (Then a) (Else a)(only applicable in some contexts). (C. A. McCann)
If RebindableSyntax is a relatively new feature, then it’s unlikely to find a more general way, at least not till the next version of GHC.
The RebindableSyntax extension to GHC lets you overload
if ... then ... elseexpressions with your own version. In particular, theifThenElsefunction is used to define alternative meanings.if e1 then e2 else e3"meansifThenElse e1 e2 e3.See the blog article Rebindable if..then..else expressions for a nice discussion of this feature, including some examples.