The original author said he did not want to use existing parser
libraries, but write it himself for learning. After I read
"introduction to functional programming" from Bird, I closed the book,
and re-wrote the parser from scratch again, and seeing how all these
pieces come together was such a wonderful experience that I would
recommend everyone to this once instead of immediately using an
existing library :-)
It seems Marks parser definition lacks error information. It's been a
while since I played with Haskell, but if I recall correctly, you
could define a parser with backtracking that carries errors as:
type Error = String
newtype Parser a = Parser (String -> Either Error [(a, String)])
or was it
newtype Parser a = Parser (String -> [(Either Error a, String)])
in any case, when combining parsers using >>=, the errors must be propagated.
As you can see, I forgot the correct solution myself, so now I would
indeed use a library, since I know I could do it once ;)
On Tue, Jan 19, 2010 at 5:31 PM, S.Doaitse Swierstra
How about using one of the existing libraries, in this case uu-parsinglib:
module Parse where
import Text.ParserCombinators.UU.Parsing import Text.ParserCombinators.UU.Examples
data Verb = Go | Get | Jump | Climb | Give deriving (Show)
pCommand :: Pars String pCommand = foldr (<|>) pFail (map str2com [(Go, "Go"), (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" ("Go",[]) *Parse> test pCommand "G0" ("Go",[ Deleted '0' at position 1 expecting 'o', Inserted 'o' at position 2 expecting 'o']) *Parse> test pCommand "o" ("Go",[ Inserted 'G' at position 0 expecting one of ['G', 'G', 'J', 'C', 'G']]) *Parse> test pCommand "Clim" ("Give",[ Inserted 'b' at position 4 expecting 'b']) *Parse>
On 17 jan 2010, at 14:30, 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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