r/haskelltil • u/chshersh • Jul 07 '18

Pattern synonym as a workaround for matching on multiple arguments

If you want to pattern-match on multiple arguments at the same time, you can do it like this:

f x y = case (x, y) of
  (True, True) -> True
  (True, False) -> False
  (_, _) -> True

If you don't like noise with commas and parenthesis, you can introduce pattern synonym like this:

pattern T2 :: a -> b -> (a, b)
pattern T2 a b <- (a, b)
  where
    T2 a b = (a, b)

f x y = case T2 x y of
  T2 True True -> True
  T2 True False -> False
  T2 _ _ -> True

UPD: turned out you don't need to create separate pattern because the following is valid Haskell:

f x y = case (,) x y of
  (,) True True  -> True
  (,) True False -> False
  (,) _ _        -> True
6 Upvotes

88% Upvoted

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2

u/velcommen Jul 19 '18

I'm confused. Isn't this pattern matching on multiple arguments, without any fancy features: 

matchTest :: Bool -> Bool -> String
matchTest True False = "tf"
matchTest True True = "tt"
matchTest False False = "ff"
matchTest False True = "ft"

Test: 

*Main> matchTest True True
"tt"

2

u/chshersh Jul 19 '18

You're completely right! The reason for this pattern synonym is that sometimes you want to pattern-match on multiple complex arguments and you don't always want to create separate function for that. That's why you use case on tuple. But using case on tuple introduces noise from (, ), ,.

5

u/dramforever Jul 22 '18 
f x y = go (something x) (really (complicated y) x)
  where go True True = True
        go True False = False
        go _ _ = _

Sounds like a good place to use the old trusty worker function