Proposal: Data.Bool.implies

[Evan Laforge]

I have only very rarely needed 'implies' and when I do I'm happy with
'if a then b else False'.  But if I needed to do that a lot, I would
write my own function and certainly never use (<=), because I wouldn't
guess that (<=) has the same logic as implies.  And even if it does,
isn't the means more important than the end?  For the same reason I
wouldn't write 'not a || b' because when I want to say implies I'll
just directly say implies.

On Mon, Jan 18, 2016 at 11:44 AM, David Feuer <david.feuer at gmail.com> wrote:
> I think changing the Ord instance for Bool *may* be the way to go.
> Yes, the lazy one is likely a bit less efficient in some cases, but I
> somehow doubt many are relying on it to be fast. Moreover, people
> expect most Bool operations to short-circuit (I even seem to have lost
> that fight wrt the Data.Bits operations, which I firmly believe should
> be *strict*).
>
> instance Ord Bool where
>   False <= _ = True
>   True <= x = x
>
>   True >= _ = True
>   False >= x = not x
>
>   False < x = x
>   True < _ = False
>
>   False > _ = False
>   True > x = not x
>
>
> On Mon, Jan 18, 2016 at 12:23 AM, Jon Purdy <evincarofautumn at gmail.com> wrote:
>> +1. “<=” has the wrong strictness. In “a `implies` b”, “b” should not be
>> evaluated if “a” is false.
>>
>> As a strawman, I’d propose that the Ord instance for Bool be changed—but
>> this is more likely to break existing code, however slightly.
>>
>> On Sun, Jan 17, 2016 at 8:12 PM, David Feuer <david.feuer at gmail.com> wrote:
>>>
>>> -1. We already have a `<=` operator.
>>>
>>> On Sun, Jan 17, 2016 at 9:17 PM, Niklas Hambüchen <mail at nh2.me> wrote:
>>> > I propose to add to Data.Bool:
>>> >
>>> >     -- | Boolean implication.
>>> >     implies :: Bool -> Bool -> Bool
>>> >     implies True  x = x
>>> >     implies False _ = True
>>> >
>>> >     infix 4 `implies` -- same as (==)
>>> >
>>> > The request for this is quite old (see e.g.
>>> >
>>> > http://neilmitchell.blogspot.de/2007/02/logical-implication-in-haskell.html).
>>> >
>>> > I believe that by not trying to use an operator for it, and keeping it
>>> > in Data.Bool, we can avoid doing anything wrong.
>>> >
>>> > A quick superficial search on Stackage Hoogle suggests that adding this
>>> > function should create no breakage (4 packages define their own function
>>> > called `implies`, none of them import Data.Bool).
>>> >
>>> > `infix` instead of `infixl` or `infixr` to force you to bracket it; for
>>> > the same reason it has the same precedence as `==`.
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>>
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