[Haskell-cafe] MultiCase alternative

[Hiromi ISHII]

Hi cafe,

Here's another solution using lens (and prisms):

```
{-# LANGUAGE PatternSynonyms, RankNTypes, TemplateHaskell, ViewPatterns #-}
module Main where
import Control.Lens

data F = C Int | D Int | E String  Int | F Int | G String
       deriving (Read, Show, Eq, Ord)
makePrisms ''F

match = flip (^?)

main :: IO ()
main =
  case C 2 of
    (match (_C <|?> _D <|?> _E._2 <|?> _F) -> Just i) -> print i
    G s -> putStrLn s

infixl 8 <|?>

(<|?>) :: (Conjoined p, Applicative f) =>
          Traversing p f s t a b -> Over p f s t a b -> Over p f s t a b
(<|?>) = failing
```


> 2017/06/16 14:55、Clinton Mead <clintonmead at gmail.com>のメール:
> 
> You can record match like so "x at D{}" but then you'll need someway to access the contents of "x" (if you're interested in the contents, that is).
> 
> On Fri, Jun 16, 2017 at 3:43 PM, Baa <aquagnu at gmail.com> wrote:
> Hello, Richard.
> 
> As a result I did with "...where f c = ..." :) A way with "which" is
> interesting but duplicates unique constructors (A -> A', B -> B').
> 
> Interesting, is F# can solve this problem with active pattern?
> 
> Pattern matching in Haskell does not seem enought flexible, for
> example, if I have `data D = D a b c d`, how can I match it without
> placeholders for `D` args? Something like `D...`? I need to write
> `D _ _ _ _` if I want to match without to bind args, right?
> 
> I want to say, that may be (I'm not sure, I'm newbie in Haskell) active
> patterns can solve this and many other problems, may be active-pattern
> + reflection. For last example, I create pattern `IsD` in place where
> `D` is defined and use it - to be more independent on D args (D can be
> record and I can use getters/lens only to access its args, so I need a
> way to be similar independent from its args in pattern-matching too).
> 
> But again, I'm not sure about right approaches - I'm newbie yet.
> 
> 
> ===
> Best regards, Paul
> 
> > There is another elementary alternative.  If you need to treat C and
> > D the same in just one place, you don't really have a problem.  If
> > you need to treat them the same in several places, do this:
> >
> > data T a b c = A a | B b | C c | D c  -- existing type
> >
> > data Which a b c = A' a | B' b | CD Bool c
> >
> > which :: T a b c -> Which a b c
> > which (A a) = A' a
> > which (B b) = B' b
> > which (C c) = CD False c
> > which (D c) = CD True  c
> >
> > then
> >     case which $ x of
> >       A' a ->
> >       B' b ->
> >       CD _ c -> ...
> >
> > If you want to merge the C and D cases often, I like this approach,
> > otherwise the
> >     C c -> f c
> >     D c -> f c
> >     where f c = ...
> > approach is better.
> >
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----- 石井 大海 ---------------------------
konn.jinro at gmail.com
筑波大学数理物質科学研究科
数学専攻 博士後期課程二年
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