Hmm, Sylvain, this is more interesting than my variant. Thank you a lot!
Hi,
You can combine ViewPatterns and PatternSynonyms to obtain this desired effect:
{-# LANGUAGE ViewPatterns #-} {-# LANGUAGE PatternSynonyms #-}
data X a b c = A a | B b | C c | D c deriving (Show)
cOrD :: X a b c -> Maybe c cOrD (A _) = Nothing cOrD (B _) = Nothing cOrD (C c) = Just c cOrD (D c) = Just c
pattern CorD :: c -> X a b c pattern CorD c <- (cOrD -> Just c)
main :: IO () main = do let -- x = A "An a" :: X String String String -- x = B "A b" :: X String String String x = C "A c" :: X String String String --x = D "A d" :: X String String String
case x of A a -> putStrLn ("A:" ++ show a) B b -> putStrLn ("B:" ++ show b) CorD c -> putStrLn ("CorD:" ++ show c)
Note that you lose completeness checking:
Test.hs:30:4: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: (C _) (D _)
Cheers, Sylvain
On 15/06/2017 16:11, Baa wrote:
Hello, Everyone!
As I understand "MultiCase" proposal was not approved, so my question is: is there alternatives to multi-case? I have code like:
case x of A a -> .. B b -> .. C c -> --same-- D c -> --same--
and I want to merge code of "C c" with "D c" branch. Anywhere, in general: is any alternatives to make "OR"s in patterns? I don't see how to do it with pattern guards, pattern synonyms, views extensions and don't see another related extensions.
Something like "(C|D) c -> ..."
=== Best regards, Paul _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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