Re: Proposal: Data.Bool.implies

I disagree. The equivalence between `implies` and (<=) depends on the ordering chosen for Bool, which is completely arbitrary, and is not something people thinking about logic should be expected to know or learn. For example, the Coq standard library happens to choose the other order, with True < False. -Brent On Mon, Jan 18, 2016 at 1:04 AM David Feuer wrote:

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.

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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:

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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:

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+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:

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

On Sun, Jan 17, 2016 at 9:17 PM, Niklas Hambüchen

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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 ).

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I believe that by not trying to use an operator for it, and keeping

it

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in Data.Bool, we can avoid doing anything wrong.

A quick superficial search on Stackage Hoogle suggests that adding

On Mon, Jan 18, 2016 at 1:57 AM, Evan Laforge wrote: this

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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

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