Dan Doel wrote:
On Sunday 20 July 2008, John Meacham wrote:
I do not believe that is the case, since the return type of runParser "Either ParseError a" means that before you can extract the result of the parse from the 'Right' branch, it must evaluate whether the result is 'Left' or 'Right' meaning it needs to parse the whole input in order to determine whether the parse was succesful.
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It doesn't stop it from parsing the entire file strictly. However, what it does do is prevent the parser from backtracking out of arbitrary amounts of lookahead. So, unless you use try (which allows for lookahead), when any token is consumed by the parser, it can be garbage collected (assuming the parser is the only thing pointing to the token stream). So, it consumes its input strictly, but with limited overhead (ideally using try only for some small bounded lookahead where it's needed).
So with Parsec, you can keep the *input* from filling up memory, but if you do, the *result* will still take up space (e.g. Right (value)). For a simple transformation where the output is a similar string to the input, it will be just as large, so not much space is actually saved (maybe a factor of 2 -- just keeping the output, not also the input), it seems. -Isaac