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<p class="MsoNormal">Greetings CLC;<br>
<br>
I'm writing this email to propose a change to `Data.Fixed`. Full credit for this idea goes to Bhavik Mehta (@b-mehta on GitHub), who implemented it in this
<a href="https://github.com/dmcclean/exact-pi/pull/8">PR</a> for `exact-pi`.<br>
<br>
In `Data.Fixed` there are several `E`-prefixed datatypes used to represent a certain number of digits of precision in fixed-precision arithmetic. For example, `E1` has 1 decimal place, `E12` has 12. Each of them, `E{0,1,2,3,6,9,12}` is hardcoded. If more precision
types are to be provided, they have to be hardcoded as well, and all of these types resemble each other. I think there is room for improvement here.<br>
<br>
Instead of having<br>
<br>
```<br>
data E0<br>
<br>
instance HasResolution E0 where<br>
resolution _ = 1<br>
```<br>
<br>
and repeating it as many times as there are `E` datatypes, I propose to add the following type:<br>
<br>
```<br>
{-# LANGUAGE DataKinds #-}<br>
{-# LANGUAGE KindSignatures #-}<br>
<br>
import GHC.TypeLits (Nat, KnownNat, natVal)<br>
<br>
data E (n :: Nat)<br>
```<br>
<br>
and then do<br>
<br>
```<br>
instance KnownNat n => HasResolution (E n) where<br>
resolution _ = 10^natVal (undefined :: E n)<br>
```<br>
<br>
just once, replacing `data E0` with `type E0 = E 0` (and the same for the rest of them) to continue reexporting these types. `E` should also be exported.<br>
<br>
I have created a Trac feature request <a href="https://ghc.haskell.org/trac/ghc/ticket/15622">
ticket</a> with the same contents as this email, and made a <a href="https://github.com/ghc/ghc/pull/196">
PR</a> to GHC’s repository on GitHub.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">To finalize, there are a few topics I’d like to raise regarding this change.<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal">1. Does the community find this change beneficial in general?<br>
2. Does the community approve of using DataKinds in a mundane section of the base package?<br>
3. Does everyone accept a small breaking change of E0, E1, etc. from a data type to a type synonym? Or should we go the conservative way and just add E without refactoring E0, E1, etc.?<br>
<br>
<o:p></o:p></p>
<p class="MsoNormal">Regards,<br>
<br>
Alexandre<o:p></o:p></p>
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