GHC rewrite rules for class operations & laws

[George Colpitts]

Do you want me to add a task ticket to remove this restriction that rewrite
rules can't be used for class methods?

On Tue, Nov 22, 2016 at 8:06 AM Simon Peyton Jones via
Glasgow-haskell-users <glasgow-haskell-users at haskell.org> wrote:

> Conal
>
>
>
> Is it possible to apply GHC rewrite rules to class methods?
>
>
>
> Not currently.  See https://ghc.haskell.org/trac/ghc/ticket/11688, esp
> comment:7 which gives links to similar examples.
> https://ghc.haskell.org/trac/ghc/ticket/10528 comment:13  gives more
> background.
>
>
>
> It’d be great if someone wanted to think through all this.
>
>
>
> Simon
>
>
>
> *From:* Glasgow-haskell-users [mailto:
> glasgow-haskell-users-bounces at haskell.org] *On Behalf Of *Conal Elliott
> *Sent:* 17 November 2016 16:40
> *To:* glasgow-haskell-users at haskell.org
> *Subject:* GHC rewrite rules for class operations & laws
>
>
>
> Is it possible to apply GHC rewrite rules to class methods? From what I’ve
> read and seen, class methods get eliminated early by
> automatically-generated rules. Is there really no way to postpone such
> inlining until a later simplifier stage? The GHC Users Guide docs say no
> <https://na01.safelinks.protection.outlook.com/?url=https:%2F%2Fdownloads.haskell.org%2F~ghc%2Flatest%2Fdocs%2Fhtml%2Fusers_guide%2Fglasgow_exts.html%23how-rules-interact-with-class-methods&data=02%7C01%7Csimonpj%40microsoft.com%7C8678611c4c57499f97be08d40f08662a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636149976146128136&sdata=GjkFhlWdNkT6eo85FvcLKkJYoir7Dui9xJ9kMTYKVmU%3D&reserved=0>,
> and suggests instead giving a duplicate vocabulary with somewhat awkward
> names for class methods. I’ve not seen this practice in libraries. I gather
> that we cannot therefore use class laws as optimizations in the form of
> rewrite rules, which seems a terrible loss.
>
> In Control.Category and Control.Arrow, I see rules for class laws but
> also header comments saying “The RULES for the methods of class Arrow may
> never fire e.g. compose/arr; see Trac #10528”.
>
> I’d appreciate a reality check about my conclusions as well as any
> strategies for using class laws in optimization.
>
> Thanks, -- Conal
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