Data.Fixed Missing E4, E5, etc

[Erik Hesselink]

That sounds like a quick win. Do you want me to send a patch? Or does
this need more discussion/approval?

Erik

On 4 August 2016 at 10:01, Edward Kmett <ekmett at gmail.com> wrote:
> Despite appearances the above code doesn't need overlapping instances, as
> the KnownNat n constraint forces (n :: Nat), while the existing instances
> are all in *.
>
> On Thu, Aug 4, 2016 at 4:00 AM, Edward Kmett <ekmett at gmail.com> wrote:
>>
>> This could be done in a backwards compatible way by making the
>> HasResolution class polykinded.
>>
>> {-# LANGUAGE PolyKinds, ScopedTypeVariables, DataKinds #-}
>> import Data.Proxy
>>
>> instance KnownNat n => HasResolution n where
>>   resolution _ = natVal (Proxy :: Proxy n)
>>
>> Then we could just keep the existing Exx for backwards compatibility or
>> for folks who want portable code, and folks who want the fancier API can
>> have it. Not only that but with `reflection` they could safely generate
>> precision types dynamically at runtime if it was an unknown. Not a bad
>> return on investment for 3 lines of code!
>>
>> -Edward
>>
>>
>> On Thu, Aug 4, 2016 at 3:38 AM, Erik Hesselink <hesselink at gmail.com>
>> wrote:
>>>
>>> If Data.Fixed in base is changed, might it make more sense to switch
>>> to type level naturals and KnownNat as a class? It implements exactly
>>> the same functionality but in a more readable and general way.
>>>
>>> Erik
>>>
>>> On 3 August 2016 at 19:46, Andrew Martin <andrew.thaddeus at gmail.com>
>>> wrote:
>>> > Thanks for your response. I'm actually already doing you recommended in
>>> > my
>>> > library. It does work, but I still think it would be better to have
>>> > this in
>>> > base.
>>> >
>>> > -Andrew Martin
>>> >
>>> > On Wed, Aug 3, 2016 at 11:46 AM, Merijn Verstraaten
>>> > <merijn at inconsistent.nl>
>>> > wrote:
>>> >>
>>> >> Hi Andrew,
>>> >>
>>> >> Data.Fixed actually already provides all the tools you need to support
>>> >> any
>>> >> arbitrary precision. As such, I don't see the need to add obscure
>>> >> precisions
>>> >> to the library in addition to the existing common ones. Using
>>> >> Data.Fixed
>>> >> with a 4 digit precision like you want can be achieved in 3 lines of
>>> >> code in
>>> >> you geolite library. Simply define:
>>> >>
>>> >> data MyPrecision
>>> >> instance HasResolution MyPrecision where
>>> >>     resolution _ = 4
>>> >>
>>> >> and use "Fixed MyPrecision" as type in your library. As you can see in
>>> >> the
>>> >> haddocks, the Num, Real, RealFrac, etc. instances work for any
>>> >> arbitrary
>>> >> instance of HasResolution.
>>> >>
>>> >> Cheers,
>>> >> Merijn
>>> >>
>>> >> > On 3 Aug 2016, at 13:57, Andrew Martin <andrew.thaddeus at gmail.com>
>>> >> > wrote:
>>> >> >
>>> >> > I've been working on a library for parsing geolite's GeoIP csv file
>>> >> >
>>> >> >
>>> >> > (http://hackage.haskell.org/package/geolite-csv-0.2/docs/Geolite-Types.html).
>>> >> > In this file, the latitude and longitude are always given to four
>>> >> > decimal points of precision. It seems like the Fixed data type (from
>>> >> > Data.Fixed in base) is the best choice for representing this.
>>> >> >
>>> >> > However, the precision levels provided are:
>>> >> >
>>> >> > - E0
>>> >> > - E1
>>> >> > - E2
>>> >> > - E3
>>> >> > - E6
>>> >> > - E9
>>> >> > - E12
>>> >> >
>>> >> > I would like to propose adding all of the missing ones into
>>> >> > Data.Fixed
>>> >> > as well. Even though needed a four-decimal-point-precision number is
>>> >> > uncommon, it's not unheard of. Admittedly, the precision offered by
>>> >> > E11
>>> >> > seems unlikely to ever be needed, but I think it would be nice for
>>> >> > completeness. I would be happy to PR this if others agree that it's
>>> >> > a
>>> >> > good idea.
>>> >> >
>>> >> > -Andrew Martin
>>> >> >
>>> >> > _______________________________________________
>>> >> > Libraries mailing list
>>> >> > Libraries at haskell.org
>>> >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>> >>
>>> >
>>> >
>>> >
>>> > --
>>> > -Andrew Thaddeus Martin
>>> >
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>>
>>
>