GHC rewrite rules for class operations & laws

[Conal Elliott]

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://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#how-rules-interact-with-class-methods>,
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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