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
