[Haskell-cafe] Re: Proving my point

Philippa Cowderoy wrote:

On Fri, 16 May 2008, Achim Schneider wrote:

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Philippa Cowderoy wrote:

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On Fri, 16 May 2008, Achim Schneider wrote:

Guess who ran into that with a separate token for layout-inserted braces?

It can't be me, as I attempted to be as lazy as possible, not going for a tokenising pass, and ended up being too lazy.

Nah, you just picked the wrong way to attempt discipline. I don't use separate tokenising/lexing passes in a lot of my code (though you can't really avoid it when you want to do layout), it's a matter of knowing how it's done. Unless you've got a lexical structure that prevents it (which is to say, there're situations in which two tokens following each other aren't allowed to have whitespace between them), it's a good idea to have your token productions eat any whitespace following them, and then your toplevel becomes:

do {whitespace; r <- realTopLevel; eof; return r}

and then you need never worry about it again.

My problem is that realTopLevel = expr, and that I get into an infinite recursion, never "closing" enough parens, never hitting eof. Additionally, two passes are definitely easier to reason about, and it's wise to be lazy there. Btw: Is there any way to make Parsec return a tree of things it tried? The end-user error messages are quite often just not informative enough while debugging the parser itself. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.