Re: Data.Fixed Missing E4, E5, etc

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 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

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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@inconsistent.nl> wrote:

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Hi Andrew,

Data.Fixed actually already provides all the tools you need to support

any

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arbitrary precision. As such, I don't see the need to add obscure

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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

On 3 August 2016 at 19:46, Andrew Martin wrote: precisions the

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haddocks, the Num, Real, RealFrac, etc. instances work for any arbitrary instance of HasResolution.

Cheers, Merijn

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On 3 Aug 2016, at 13:57, Andrew Martin 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

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-- -Andrew Thaddeus Martin

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