[Haskell-cafe] Question about implementing an off-side rule in Parsec 2

Neil Brown nccb2 at kent.ac.uk
Tue Apr 28 08:04:45 EDT 2009

Bas van Gijzel wrote:
> Hello everyone,
> I'm doing a bachelor project focused on comparing parsers. One of the 
> parser libraries I'm using is Parsec (2) and I'm going to implement a 
> very small subset of haskell with it, with as most important feature 
> the off-side rule (indentation based parsing) used in function 
> definitions and possibly in where clauses.
> But I'm still a bit stuck on how to implement this cleanly. I tried to 
> search for some examples on blogs but I haven't found anything yet. As 
> far as I can see the way to go would be using getState and updateState 
> methods defined in Parsec.Prim and to use the methods in Parsec.Pos to 
> compare the difference in indendation for tokens.
> But I haven't completely wrapped my head around any state monad yet 
> and I don't understand Parsec enough yet to see how to use the methods 
> Parsec.Pos and state easily. Some examples or pointers to something to 
> read would really be helpful.

I work on a compiler for occam-pi, which has indentation-based syntax.  
It's regular (two-spaces per indent) rather than 
different-number-of-spaces, and line continuations can only follow 
certain tokens, but perhaps our code might help you.

We use alex for tokenising and parsec for parsing.  We tokenise the 
file, and then use the source positions to create indent/outdent tokens 
in the token stream, and after that the parser parses things like a PAR 
block as: do {reserved "PAR"; indent; many1 subItem; outdent}.  Our code 
can be found at:


Look in the frontends subdirectory, particularly at StructureOccam.hs, 
but also LexOccam.x and ParseOccam.hs.  It may not be the most elegant 
way to do things (occam has all sorts of oddities that make parsing a 
pain), but it does work :-)



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