Haskell Parsec skip all words that arent predefined
haskell, parsec
Solution
I won't go into specifics since this is homework and the OP said the "important thing is the parsing".
The way I'd solve this problem:
tokenize the input. Break it into words; this will free the real parsing step from having to worry about token definitions (i.e. "is %#@[ part of a word?") or whitespace. This could be as simple as `words` or you could use Parsec for the tokenization. Then you'll have `[Token]` (or `[String]` if you prefer).
a parser for compass directions. You already have this (good job), but it'll have to modified a bit if the input is `[String]` instead of `String`.
a parser for words that start with a capital letter.
a parser for everything else, that succeeds whenever it sees a token that isn't a compass direction or a word starting with a caps.
a parser that works on any token, but distinguishes between good stuff and bad stuff, perhaps using an algebraic data type.
a parser that works on lots of tokens
Hopefully that's clear without being too clear; you'll still have to worry about when to discard the junk, for example. The basic idea is to break the problem down into lots of little sub-problems, solve the sub-problems, then glue those solutions together.
Problem
Learning to use the Parsec library, part of homework. EDIT: Suggestions to use other libraries are welcome, the point is the parsing. What I want, is to extract all words with a capital letter, and four compass directions from any sentence. Example: "Belgium totally lies south of Holland." should find and return "Belgium south Holland". What I can't figure is how to ignore (eat) any input that is -not- a compass direction. I was looking to find something along the lines of ``` 'many (not compassDirection >> space)' ``` but g(h)oogle isn't helping me. Following code is obviously stuck on the 'many' function. ``` readExpr :: String -> String readExpr input = case parse (parseLine) "" input of Left err -> "No match: " ++ show err Right val -> "Found: " ++ showVal val parseLine :: Parser GraphValue parseLine = do x <- parseCountry space many ( some (noneOf " ") >> space ) y <- parseCompass space many ( some (noneOf " ") >> space ) z <- parseCountry return $ Direction [x,y,z] compassDirection :: Parser String compassDirection = string "north" <|> string "south" <|> string "east" <|> string "west" parseCountry :: Parser GraphValue parseCountry = do c <- upper x <- many (lower) return $ Country (c:x) parseCompass :: Parser GraphValue parseCompass = do x <- compassDirection return $ Compass x ```