PROPOSAL: toBoundedIntegral: an adaptation of fromIntegral that respects bounds

[Edward Kmett]

Yes, it would.

The use of bitSizeMaybe is deliberate.

That way it doesn't have to bound itself when converting an infinite type.

The whole point of this is that it works for infinite cases.

-Edward

On Thu, Nov 20, 2014 at 6:08 PM, David Feuer <david.feuer at gmail.com> wrote:

> That bitSizeMaybe makes me a bit nervous. Would it actually hurt anyone to
> make this thing a subclass of FiniteBits?
>
> On Thu, Nov 20, 2014 at 4:46 PM, Sean Leather <sean.leather at gmail.com>
> wrote:
>
>> On Wed, Nov 12, 2014 at 9:14 AM, Sean Leather wrote:
>>
>>> Inspired by conversations recent [1] and not-so-recent [2] and by my own
>>> past wish for this, I propose adding the following function to base:
>>>
>>> toBoundedIntegral :: (Integral a, Integral b, Bounded b) => a -> Maybe b
>>>
>>
>> [...]
>>
>> Discussion period: ~10 days if there is a strong desire to get it in GHC
>>> 7.10. (I would certainly like to see it get in.)
>>
>>
>> Almost over. Here are some follow-ups on Trac and Phab.
>>
>> GHC Trac: https://ghc.haskell.org/trac/ghc/ticket/9816
>> Phab Diff: https://phabricator.haskell.org/D512
>>
>> On Mon, Nov 17, 2014 at 11:10 AM, Sean Leather wrote:
>>
>>> So is there any reason not to settle on 'toIntegralBounded' and
>>> 'toIntegralSized'?
>>>
>>
>> I decided to drop 'toIntegralBounded' go with only 'toIntegralSized' in
>> my proposal. I realized that the former was not handling Int<->Word-like
>> cases optimally and would require special attention to them while the
>> latter provides them for free.
>>
>> Herbert: It was nice that 'intCastMaybe' was an easy copy-paste into
>> another module. I did a bit of rewriting the comments and code formatting.
>> Hope you don't mind, or bash me on Phab if you prefer. ;) Also, it would be
>> nice to have your tests in the library. Any thoughts on how to do that? I
>> haven't looked into it, yet.
>>
>> about the RULEs... do we really need them for `toBoundedIntegral`? If
>>>> you make `toBoundedIntegral` INLINEABLE, shouldn't you be able to
>>>> inherit the ones from 'fromIntegral' (which is what `intCastMaybe`
>>>> exploits)?
>>>>
>>>
>>> I didn't try that. I'll give it a shot.
>>>
>>
>> I did try it, and I'm not sure how to make that work. 'toBoundedIntegral'
>> splits the 'toInteger' and 'fromInteger' and puts the bounds check in the
>> middle, so 'fromIntegral' is not used. If it were used, then I believe
>> there would be redundant 'toInteger' conversions for types that weren't
>> covered by RULEs.
>>
>>
>>> > Where should the functions go? intCastMaybe/toSizedIntegral could go in
>>>> > Data.Bits. What about toBoundedIntegral?
>>>>
>>>> There's also
>>>>
>>>>   http://hackage.haskell.org/package/base-4.7.0.1/docs/Numeric.html
>>>>
>>>> as a candidate
>>>>
>>>
>>> Works for me. We could rename the section “Miscellaneous” to
>>> “Conversion” and put both functions there. Also, we should probably revise
>>> the module description since there is already a lot more than “functions
>>> for reading and showing RealFloat-like kind of values.”
>>>
>>
>> I decided on Data.Bits since it seemed to be the most obvious place for
>> 'toIntegralSized' alone. Also, I think there might be an import cycle if I
>> put it into Numeric. But if others have a preference and want to make it
>> work somewhere else, that's fine with me.
>>
>> Regards,
>> Sean
>>
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