[Haskell-beginners] Simple lookahead with Parsec

Derek Thurn derek at thurn.ca
Fri Mar 19 14:24:06 EDT 2010

On Fri, Mar 19, 2010 at 11:21 AM, Brandon S. Allbery KF8NH
<allbery at ece.cmu.edu> wrote:
> On Mar 19, 2010, at 11:14 , Derek Thurn wrote:
>> Here's a simplified version of my grammar:
>> myType = try (primitiveType) <|> arrayType
>> primitiveType = do {reserved "int"; return "primitive"}
>> arrayType = do { primitiveType; symbol "["; symbol "]"; return "array"}
> What you're doing here is matching and succeeding on primitiveType and never
> even looking for the bracket; your parser then exits and the framework fails
> expecting the end of the token stream but finding the unmatched left
> bracket.
> You need to rearrange your cases:
>> myType = try arrayType <|> primitiveType
> so that your parser actually checks for the brackets before concluding that
> it has a primitiveType already.
> Even better would be to refactor the grammar since both types start with a
> primitiveType, but I'll leave that up to you since it does complicate
> returning the shape of the resulting type.
> --
> brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
> system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
> electrical and computer engineering, carnegie mellon university    KF8NH

Ah, I see. The problem is that the primitiveType parser succeeds. Is
there then no generic way to use lookahead in Parsec to make choices
of this nature? I appreciate that it might be possible to refactor my
grammar to never require examining the next token to make a parsing
decision about the current token, but I've already got an LALR(1)
grammar that I'm pretty happy with...

What I'd ideally like is some sort of general solution wherein Parsec
first tries primitiveType, moves to the next token, realizes that it
will be a parse error, and backtracks to try the other alternative.

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