[Haskell-cafe] MultiCase alternative

[MarLinn]

> Hello, Li-yao! I mean this: https://wiki.haskell.org/MultiCase
>
>> Hi Paul,
>>
>> This looks like the or-patterns proposal:
>>
>> https://github.com/ghc-proposals/ghc-proposals/pull/43
>>
>>

You can see on github that it's just "dormant". But the examples in this 
thread made me have an interesting, even more general idea: we have 
functions on term-level and type-level – why not on pattern-level?

Here's a very rough draft of what I imagine:

	{-# LANGUAGE PatternFamilies, PatternKinds #-}
	
	
	data Dt a = A a | B a | C a | D a deriving ( Show )

	pattern (∥) ∷ (Pattern a → T) → (Pattern a → T) → (T → U) → (a → U)
	pattern (a ∥ _) f = f a
	pattern (_ ∥ b) f = f b

	infixl 2 pattern ∥
  
	foo ∷ (Show a) ⇒ Dt a → String
	foo (A a) = …
	foo (B b) = …
	foo (C c ∥ D d) = …


No, I don't think that's worth drawing up a proposal yet. I just want to 
document the idea. Maybe it can inspire someone who's building their own 
language or something.


Cheers,
MarLinn