a generic matcher

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+{- A generic matcher.
+ -
+ - Can be used to check if a user-supplied condition,
+ - like "foo and ( bar or not baz )" matches. The condition must already
+ - be tokenized, and can contain arbitrary operations.
+ -
+ - If operations are not separated by and/or, they are defaulted to being
+ - anded together, so "foo bar baz" all must match.
+ -
+ - Is forgiving about misplaced closing parens, so "foo and (bar or baz"
+ - will be handled, as will "foo and ( bar or baz ) )"
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Matcher (
+	Operation(..),
+	Token(..),
+	toMatcher,
+	match,
+	runMatch	
+) where
+
+import Control.Monad
+
+{- An Operation is a command and some parameters. -}
+data Operation = Operation String [String]
+	deriving (Show, Eq)
+
+{- A Token can either be a single word, or an Operation. -}
+data Token = Token String | TokenOp Operation
+	deriving (Show, Eq)
+
+data Matcher = Any
+	| And Matcher Matcher
+	| Or Matcher Matcher
+	| Not Matcher
+	| Op Operation
+	deriving (Show, Eq)
+
+{- Converts a list of Tokens into a Matcher. -}
+toMatcher :: [Token] -> Matcher
+toMatcher ts = toMatcher' Any ts
+toMatcher' :: Matcher -> [Token] -> Matcher
+toMatcher' m [] = m
+toMatcher' m ts = toMatcher' m' rest
+	where
+		(m', rest) = consume m ts
+
+{- Consumes one or more tokens, constructs a new Matcher,
+ - and returns unconsumed tokens. -}
+consume :: Matcher -> [Token] -> (Matcher, [Token])
+consume m [] = (m, [])
+consume m ((TokenOp o):ts) = (m `And` Op o, ts)
+consume m ((Token t):ts)
+	| t == "and" = cont $ m `And` next
+	| t == "or" = cont $ m `Or` next
+	| t == "not" = cont $ m `And` (Not next)
+	| t == "(" = let (n, r) = consume next rest in (m `And` n, r)
+	| t == ")" = (m, ts)
+	| otherwise = (m, ts) -- ignore unknown token
+	where
+		(next, rest) = consume Any ts
+		cont v = (v, rest)
+
+{- Checks if a Matcher matches, using a supplied function to check
+ - the value of Operations. -}
+match :: (Operation -> Bool) -> Matcher -> Bool
+match a = go
+	where
+		go Any = True
+		go (And m1 m2) = go m1 && go m2
+		go (Or m1 m2) = go m1 || go m2
+		go (Not m1) = not (go m1)
+		go (Op v) = a v
+
+{- Runs a Matcher in an arbitrary monadic contex, using a supplied
+ - action to evaluate Operations. -}
+runMatch :: Monad m => (Operation -> m Bool) -> Matcher -> m Bool
+runMatch a = go
+	where
+		go Any = return True
+		go (And m1 m2) = liftM2 (&&) (go m1) (go m2)
+		go (Or m1 m2) =  liftM2 (||) (go m1) (go m2)
+		go (Not m1) = liftM not (go m1)
+		go (Op v) = a v