Re: [Haskell-cafe] MultiCase alternative

16 Jun 2017

      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 のメール:
You can record match like so "x@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  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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Re: [Haskell-cafe] MultiCase alternative

Hiromi ISHII