Re: Proposal: Data.Bool.implies

I agree with you that not a || b says something a bit different from a `implies` b in a more general logical context. However, I believe that a <= b says exactly the same thing. It's true that it may not be quite obvious to everyone, but anyone thinking enough about logic to *want* an implies function should probably be willing to learn such things. On Mon, Jan 18, 2016 at 1:57 AM, Evan Laforge wrote:

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 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 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 wrote:

...

-1. We already have a `<=` operator.

On Sun, Jan 17, 2016 at 9:17 PM, Niklas Hambüchen 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 `==`. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

---

Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries