[Haskell-cafe] Partial application in case expression rules

[Patrick Premont]

Occasionally I find it would be nice to be able to omit some or all
constructor parameters from a pattern matching rule. 

Perhaps the compiler could put them in for you and apply them to the right
hand side of the rule ?

 

data Tree a = Leaf a| Node (Tree a) (Tree a)

 

treeFold :: (a -> a -> a) -> Tree a -> a

treeFold f = \case

  Leaf ->  id

  Node -> f `on` treeFold f

 

This could be desugared at some point to

 

treeFold f = \case

  Leaf a ->  id a 

  Node a b -> (f `on` treeFold f) a b

 

I suppose such an extension has been considered before, but I could not find
such a discussion.

(In
http://haskell.1045720.n5.nabble.com/Partial-pattern-matching-td3087200.html
the thread focused instead on case expressions over functions)

 

Any pointers to previous discussions ? Comments ?

 

Thank you!

 

Patrick Prémont

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