Or perhaps this?
data T = Foo Int | Bar Int | Baz
fooBar (Foo a) = Just a
fooBar (Bar a) = Just a
fooBar _ = Nothing
foo :: T -> T -> Int
foo x y = sum $ catMaybes $ map fooBar [x,y]
/Øystein
On Thu, Dec 8, 2011 at 1:15 PM, Emil Axelsson
Instead of pattern guards you can use ViewPatterns:
http://hackage.haskell.org/**trac/ghc/wiki/ViewPatternshttp://hackage.haskell.org/trac/ghc/wiki/ViewPatterns
This reduces some of the noise.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~**~~ {-# LANGUAGE ViewPatterns #-}
data T = Foo Int | Bar Int | Baz
fooBar (Foo a) = Just a fooBar (Bar a) = Just a fooBar _ = Nothing
foo :: T -> T -> Int foo x y = case (x,y) of (fooBar -> Just a, fooBar -> Just b) -> a + b (Bar a, Baz) -> -a
(Foo a, Baz) -> a _ -> 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~**~~
/ Emil
2011-12-08 11:13, Asger Feldthaus skrev:
Haskell doesn't seem to support disjunctive patterns, and I'm having a difficult time writing good Haskell code in situations that would otherwise call for that type of pattern.
Suppose for an example I have this data type:
data T = Foo Int | Bar Int | Baz
In OCaml I can write something like:
(* foo : T -> T -> int *) fun foo x y = match (x,y) with | (Foo a | Bar a, Foo b | Bar b) -> a + b | (Baz, Foo a) | (Bar a, Baz) -> -a | (Baz, Bar a) | (Foo a, Baz) -> a | _ -> 0
In Haskell I can't find any equivalent to the disjunctive pattern. If expanded naively, my Haskell version would look like this:
foo :: T -> T -> Int foo x y = case (x,y) of (Foo a, Foo b) -> a + b (Foo a, Bar b) -> a + b (Bar a, Foo b) -> a + b (Bar a, Bar b) -> a + b (Baz, Foo a) -> -a (Bar a, Baz) -> -a (Baz, Bar a) -> a (Foo a, Baz) -> a _ -> 0
While my example is still managable in size, this quickly goes out of hand in practice. I've tried using pattern guards but with limited success. For example:
foo2 :: T -> T -> Int foo2 x y = case (x,y) of (x,y) | Just a <- open x, Just b <- open y -> a+b (Baz, Foo a) -> -a (Bar a, Baz) -> -a (Baz, Bar a) -> a (Foo a, Baz) -> a _ -> 0 where open (Foo x) = Just x open (Bar x) = Just x open Baz = Nothing
I admit it doesn't look that bad in my crafted example, but this approach doesn't seem to well work for me in practice. In any case, it's still far more verbose than the disjunctive pattern version.
Nesting the case expressions instead of tuple-matching can reduce some code duplication, but in general it becomes really verbose, and it is easy to make mistakes when you have partially overlapped patterns in the disjunctive-pattern version. Here's the example with nested cases:
foo3 :: T -> T -> Int foo3 x y = case x of Foo a -> case y of Foo b -> a+b Bar b -> a+b Baz -> a Bar a -> case y of Foo b -> a+b Bar b -> a+b Baz -> -a Baz -> case y of Foo b -> -b Bar b -> b Baz -> 0
What do people do in this situation - is there a good trick I've overlooked? And is there some reason why Haskell does not support disjunctive patterns?
Thanks, Asger
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