PROPOSAL: toBoundedIntegral: an adaptation of fromIntegral that respects bounds

[David Feuer]

Er... Obviously, I mean give this a FiniteBits *context*.

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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>>
>
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