Add ifThenElse and (?) to Data.Bool

[winter]

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 at 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 at gmail.com <mailto:david.feuer at 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 at qq.com <mailto:drkoster at qq.com>> wrote:
>> It seems this’s a very old request, see https://wiki.haskell.org/If-then-else <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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> 

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