[Haskell-cafe] MultiCase alternative

[Baa]

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
> 
> I'm not sure what you call MultiCase though. Is the above what you
> were refering to?
> 
> Here are some alternatives with current Haskell, the first one is to
> be preferred:
> 
> - Factor out the alternatives in a function.
> 
>        ...
>          case x of
>            ...
>            C c -> f c
>            D c -> f c
>        where
>          f c = ...
> 
> - If your C and D constructors have similar enough meanings, you
> might want to fuse them into a single constructor with a separate tag:
> 
>        data T = A TA | B TB | CD CorD TC
>        data CorD = C | D
> 
>    so that you can write
> 
>        case x of
>          ...
>          CD _ c -> ...
> 
> - With pattern synonyms + view patterns, although it takes some
> effort to set up and confuses the exhaustiveness checker.
> 
>        matchCorD :: T -> Maybe C
>        matchCorD (C c) = Just c
>        matchCorD (D c) = Just c
>        matchCorD _ = Nothing
> 
>        pattern CD :: C -> T
>        pattern CD c <- (matchCorD -> Just c)
> 
>    you can now write
> 
>        case x of
>          ...
>          CD c -> ...
> 
> Li-yao
> 
> 
> On 06/15/2017 10:11 AM, 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
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