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
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.
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