Question

I’ve been looking at Haskell and I’d quite like to write a compiler in it (as a learning exercise), since a lot of its innate features can be readily applied to a compiler (particularly a recursive descent compiler).

What I can’t quite get my head around is how to represent a language’s grammar in a Haskell-ian way. My first thought was to use recursive data type definitions, but I can’t see how I use them to match against keywords in the language (‘if’) for example.

Thoughts and suggestions greatly appreciated,

Pete

Answer 1

A recursive data type is fine for this. For example, given the language:

expr ::= var       |  'true'       |  'false'       |  'if' expr 'then' expr 'else' expr       |  '(' expr ')' 

an example expression in this language would be:

if true then x else (if false then y else true) 

Your Haskell data type would look something like this:

data Expr = Var String           | Lit Bool           | If Expr Expr Expr 

Your parser then takes care to translate, e.g., x into Var 'x', and true into Lit True, etc. I.e.:

parse 'if x then false else true'    ==  If (Var 'x') (Lit False) (Lit True) 

For writing parsers you can roll your own using the techniques mentioned in Norman’s answer, or using Parsec or use parser generators like Happy.