{- |
Module : OrPatterns
Copyright : (c) Adam Vogt 2011 - 2014
License : BSD3
Maintainer : Adam Vogt
Stability : experimental
Portability : GHC>=7 -XTemplateHaskell, -XViewPatterns
Quasiquoter for /or patterns/ separated by @\" | \"@, like other languages
(ML family).
Example:
>>> :set -XQuasiQuotes -XViewPatterns -w
>>> :{
>>> let f :: Either (a,b) (b,a) -> (a,b)
>>> f [o| Left (x,y) | Right (y,x) |] = (x,y)
>>> :}
>>> map f [Left ('a',0), Right (2, 'b')]
[('a',0),('b',2)]
A more confusing example (to show that the string " | " is interpreted
correctly by the parser):
>>> :{
>>> let g [o| " | " | "|||" | " | " |] = True
>>> g _ = False
>>> :}
>>> map g [ "|", " | " , "|||" , " | "]
[False,True,True,True]
f is desugared to something like:
> f ( (\v -> case v of
> Left (x,y) -> Just (x,y)
> Right (y,x) -> Just (x,y)
> _ -> Nothing
> ) -> Just (x,y) ) = (x,y)
So failing to match will pass on to the next equation.
Limitations include:
* compilation could probably be more efficient
* incorrect patterns, such as those binding the wrong variables or the wrong
number of variables could be reported better
* generated code can have overlapped patterns, and so -Wall doesn't help the
above example. Duplicating or using ghc's check for this should be done,
in which case the the desugared code should look like /f2/.
> f2 ( (\v -> case v of
> Left (x,y) -> (x,y)
> Right (y,x) -> (x,y))
> -> a) = a
-}
module OrPatterns ( o ) where
import Language.Haskell.TH.Quote
import OrPatterns.Internal
-- | or pattern quasiquote. See above.
o :: QuasiQuoter
o = QuasiQuoter { quotePat = either fail id . pats,
quoteExp = error "OrPatterns.quoteExp",
quoteType = error "OrPatterns.quoteType",
quoteDec = error "OrPatterns.quoteDec" }