The funpat package
This library provides pattern matching with restricted function patterns (RFPs). An expression is an RFP iff exists an equivalent valid Haskell pattern. For example ("abc" ++ xs) is an RFP, because ('a' : 'b' : 'c' : xs) is an equivalent valid Haskell pattern. On the other hand, (xs ++ "abc") is not an RFP. Details are discussed in the paper Restricted Function Patterns presented at TFP 2011.
Example 1. Here is a function to chop off the prefix "prefix" of strings:
unprefix :: String -> String unprefix s = match s $ do with $ \z -> "prefix" ++ z ~> z with $ \z -> z ~> z
Example 2. Let's have a small embedded language:
data Expr = Symbol String | Expr :$ Expr deriving (Eq,Show,Typeable) instance Num Expr where fromInteger n = Symbol $ show n a + b = Symbol "+" :$ a :$ b a * b = Symbol "*" :$ a :$ b ...
In order to allow pattern matching on expressions of type Expr, the following Matchable instance is needed:
instance Matchable Expr where Symbol s .=. Symbol z = Just [s :=: z] (e :$ f) .=. (g :$ h) = Just [e :=: g, f :=: h] _ .=. _ = Nothing
Now we can pattern match on expressions even if the constructors of the Expr type were hidden:
transform :: Expr -> Expr transform e = match e $ do with $ \a -> 0 + a ~> a with $ \a b c -> a * (b + c) ~> a * b + a * c with $ \a -> a ~> a
|Dependencies||base (>=22.214.171.124 && <4.4), mtl (>=126.96.36.199 && <3) [details]|
|Uploaded||Fri Jul 1 21:55:23 UTC 2011 by GergelyDevai|
|Downloads||311 total (3 in the last 30 days)|
|Status||Docs uploaded by user
Build status unknown [no reports yet]
For package maintainers and hackage trustees