Class op rules

Carter Schonwald carter.schonwald at
Fri Mar 6 19:11:21 UTC 2020

so i did some poking around see eg
, 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

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>

> 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
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the ghc-devs mailing list