On 17 Nov 2016, at 12:03, Edward Kmett <ekmett@gmail.com> wrote:

(?) <$> fFlag <$> flagEnabled <*> flagDisabled

You probably don't want this. This performs both effects (!), regardless of the flag, but only keeps one result.

-Edward

On Wed, Nov 16, 2016 at 11:01 PM, winter <drkoster@qq.com> wrote:

And here’s some other stuff i can came up with it(without obscured readability IMHO):

...

   -- replacement for ifM in various package, similar to (>>= when)

   

   (mFlag >>= (?)) flagEnabled        -- mFlag :: Monad m => m Bool

                 $ flagDisabled

  

   -- nicer if-then-else in applicative style

   (?) <$> fFlag <$> flagEnabled

                 <*> flagDisabled

   -- compose with predicates to define your own if

   ifLower =  (?) . isLower

   ifLower ‘X’ lower

               upper 

...

Basically it's a good if-then-else replacement if you’re comfortable with point-free style.

On 17 Nov 2016, at 11:16, winter <drkoster@qq.com> wrote:

I’m totally aware of the existence of bool, i suppose (?) is mainly used in fully application to get a different style than if-then-else syntax, say,

...

    isGoo <- checkGoo

    isGoo ? goo

          $ woo

...

But like what the wiki suggested, (?) can be used in some high-order situations. I like this operator because the mnemonic of questioning meaning.

On 17 Nov 2016, at 11:03, David Feuer <david.feuer@gmail.com> wrote:

If ifThenElse is good for RebindableSyntax, then I'm +1 on that (but I've never played with that extension, so I don't really know). I'm -1 on (?). We already have bool, which tends to be rather more useful when partially applied.

On Nov 16, 2016 9:43 PM, "winter" <drkoster@qq.com> wrote:

It seems this’s a very old request, see https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a
ifThenElse True  x _ = x
ifThenElse False _ y = y

infixr 1 ?
(?) :: Bool -> a -> a -> a
(?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax.  

+ Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes.

+ In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~

Winter

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