The containers package uses the awkward double name approach. See, for
example, the way that Data.Map and Data.Sequence fuse (indexed) maps and
indexed) traversals. I know that Edward Kmett is very much opposed to
class-based rules as found in Control.Arrow because non-law-abiding
instances will behave differently when optimized.
On Nov 17, 2016 11:40 AM, "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..., 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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