[Haskell-cafe] Optional EOF in Parsec.

[Martijn van Steenbergen]

Kannan Goundan wrote:
> I've implemented this functionality in a hand-written parser (basically a hack
> that keeps track of whether the last read token was preceded by an EOL,
> without making EOL itself a token).  Does anybody have ideas about how to
> do this with Parsec?

You can do exactly the same with Parsec:

* create a lexer that yields a [Token], including EOL tokens;
* write a function of type [Token] -> [(Token, Bool)] that discards EOLs 
and tells for each token whether it was preceded by a (now discarded) EOL;
* write your pToken :: Token -> Parsec Token function (I omitted some 
type variables there) that recognises one (Token, Bool)-tuple from the 
input stream.

Or, perhaps easier:

* create a lexer that yields a [Token], including EOL tokens;
* write a function of type [Token] -> [Token] that discards only those 
EOL tokens that aren't needed -- for example, those EOL tokens that 
occur when there are no open ['s, then parse those EOL's explicitly in 
your parser.

Hope this helps,

Martijn.