[Haskell-cafe] Parsers for Text Adventures
S. Doaitse Swierstra
doaitse at swierstra.net
Tue Jan 19 09:44:43 EST 2010
How about using one of the existing libraries, in this case uu-
module Parse where
data Verb = Go | Get | Jump | Climb | Give deriving (Show)
pCommand :: Pars String
pCommand = foldr (<|>) pFail (map str2com [(Go, "G0"), (Get, "Get"),
(Jump, "Jump"), (Give, "Climb"), (Climb, "Give")])
str2com (comm, str) = show comm <$ pToken str
and then (the show is for demonstration purposes only; not the swap in
the last two elements in the list)
*Parse> :load "../Test.hs"
[1 of 1] Compiling Parse ( ../Test.hs, interpreted )
Ok, modules loaded: Parse.
*Parse> test pCommand "Go"
*Parse> test pCommand "G0"
Deleted '0' at position 1 expecting 'o',
Inserted 'o' at position 2 expecting 'o'])
*Parse> test pCommand "o"
Inserted 'G' at position 0 expecting one of ['G', 'G', 'J', 'C', 'G']])
*Parse> test pCommand "Clim"
Inserted 'b' at position 4 expecting 'b'])
On 17 jan 2010, at 14:30, Mark Spezzano wrote:
> 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
> Everything that I have works (so far...) except for the following
> I want to define a grammar using a series of Verbs like this:
> data Verb = Go | Get | Jump | Climb | Give etc, etc deriving (Show,
> 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-
> 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.
> Mark Spezzano
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
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