PROPOSAL: toBoundedIntegral: an adaptation of fromIntegral that respects bounds

[David Feuer]

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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