Am Donnerstag, 10. April 2008 04:05 schrieb Jackm139:
I'm trying to write a function to recognize a context free grammar, but I keep getting pattern match failure errors. This is what I have:
data Grammar c = Brule c c c | Rule c c
gez = [(Brule 'S' 'p' 'D'),(Brule 'D' 't' 'E'),(Rule 'E' 'j')]
recog :: String -> String -> [Grammar Char] -> Bool recog a b list = case list of [Brule x y z] -> if a == [x] then recog [z] b list else recog a b list [Rule x y] -> True
how can I solve this pattern matching error?
By including cases for 1. empty lists 2. lists with more than one element. You probably want something like recog a b list = case list of (Brule x y z:rest) -> whatever (Rule x y:rest) -> something_else [] -> True Besides, the "else recog a b list" is a nice infinite loop.