[Haskell-cafe] Parsec beginners problem
jim at sdf-eu.org
Fri Apr 27 10:10:21 EDT 2007
I have a couple of questions about my first use of Parsec, which is trying
to read morse code symbols from a string. I have a map of symbols:
import qualified Data.Map as M
morsemap = M.fromList [('A', ".-")
, ('Z', "--..")]
a string to parse, like
test = "...---..-....-"
and a Parser to read a single morse letter:
morse1 :: GenParser Char st String
morseletter = try $ M.fold ((<|>) . string) (string "") morsemap
which I am hoping would be equivalent to
try (string ".-")
<|> (string "-.")
<|> string ""
but I don't know what the base of the fold should be (string "" is wrong I
suppose) - what is the unit of <|>? Is there a readymade combinator for
It fails anyway:
*Main> run morse1 test
parse error at (line 1, column 1):
I suppose this is because the strings I'm looking for overlap but I thought
the use of try..<|> would avoid this error...?
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