Hi Mark,
I recently ported Conrad Barski's 'Casting SPELs in Lisp' to Haskell (a text
adventure game).
I had some of these problems as well, and you can find my code on Hackage
(the package is called Advgame).
Some things in there might be of some help.
Cheers,
- Tim
On Sun, Jan 17, 2010 at 7:30 AM, Mark Spezzano wrote: Hi, I am writing a Text Adventure game in Haskell (like Zork) I have all of the basic parser stuff written as described in Hutton's
Programming in Haskell and his associated papers. (I'm trying to avoid using
3rd party libraries, so that I can learn this myself) Everything that I have works (so far...) except for the following problem: I want to define a grammar using a series of Verbs like this: data Verb = Go | Get | Jump | Climb | Give etc, etc deriving (Show, Read) and then have my parser "get" one of these Verb tokens if possible;
otherwise it should do something (?) else like give an error message stating
"I don't know that command" Now, Hutton gives examples of parsing strings into string whereas I want to
parse Strings into my Verbs So, if the user types "get sword" then it will tokenise "get" as type
Verb's data constructor Get and perhaps "sword" into a Noun called Sword My parser is defined like this: newtype Parser a = Parser (String -> [(a, String)]) So I CAN give it a Verb type but this is where I run into a problem.... I've written a Parser called keyword keyword :: Parser Verb
keyword = do x <- many1 letter
return (read x) (read this as
"take-at-least-one-alphabetic-letter-and-convert-to-a-Verb-type") which DOES work provided that the user types in one of my Verbs. If they
don't, well, the whole thing fails with an Exception and halts processing,
returning to GHCi prompt. Question: Am I going about this the right way? I want to put together lots
of "data" types like Verb and Noun etc so that I can build a kind of "BNF
grammar". Question: If I am going about this the right way then what do I about the
"read x" bit failing when the user stops typing in a recognised keyword. I
could catch the exception, but typing an incorrect sentence is just a typo,
not really appropriate for an exception, I shouldn't think. If it IS
appropriate to do this in Haskell, then how do I catch this exception and
continue processing. I thought that exceptions should be for exceptional circumstances, and it
would seem that I might be misusing them in this context. Thanks Mark Spezzano _______________________________________________
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