Class op rules

[Carter Schonwald]

so i did some poking around see eg
https://gitlab.haskell.org/ghc/ghc/blob/4898df1cc25132dc9e2599d4fa4e1bbc9423cda5/compiler/basicTypes/BasicTypes.hs#L1187-1207
, and at the moment, the simplifier phase number ordering  internally (in
order from first to last)

"Initial phase" --- essentially positive infinity
.... -- currently we can add new phases here
2
1
0

-------
This actually surprised me, as i've always (at least from how rules are
usually written in eg vector) thought it was counting UP!

@Christiaan  ... so we'd need a Pre initial phase count? that happens
before Initial phase?



On Fri, Mar 6, 2020 at 10:22 AM Christiaan Baaij <christiaan.baaij at gmail.com>
wrote:

> Hello,
>
> The other day I was experimenting with RULES and got this warning:
>
> src/Clash/Sized/Vector.hs:2159:11: warning: [-Winline-rule-shadowing]
>     Rule "map Pack" may never fire
>       because rule "Class op pack" for ‘pack’ might fire first
>     Probable fix: add phase [n] or [~n] to the competing rule
>      |
> 2159 | {-# RULES "map Pack" map pack = id #-}
>
> The warning seems to suggests two things:
> 1. "Class op" -> "dictionary projection" are implemented as rewrite rules
> and executed the same way as other user-defined RULES
> 2. These rules run first, and you cannot run anything before them
>
> Now my question is, is 1. actually true? or is that warning just a (white)
> lie?
> If 1. is actually true, would there be any objections to adding a "-1"
> phase: where RULES specified to start from phase "-1" onward fire before
> any of the Class op rules.
> I'm quite willing to implement the above if A) Class op rules are actually
> implemented as builtin RULES; B) there a no objections to this "-1" phase.
>
> Thanks,
> Christiaan
>
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