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This patch is joint work of Alexis King and Simon PJ. It does some significant refactoring of the type-class specialiser. Main highlights: * We can specialise functions with types like f :: Eq a => a -> Ord b => b => blah where the classes aren't all at the front (#16473). Here we can correctly specialise 'f' based on a call like f @Int @Bool dEqInt x dOrdBool This change really happened in an earlier patch commit 2d0cf625 Author: Sandy Maguire <sandy@sandymaguire.me> Date: Thu May 16 12:12:10 2019 -0400 work that this new patch builds directly on that work, and refactors it a bit. * We can specialise functions with implicit parameters (#17930) g :: (?foo :: Bool, Show a) => a -> String Previously we could not, but now they behave just like a non-class argument as in 'f' above. * We can specialise under-saturated calls, where some (but not all of the dictionary arguments are provided (#17966). For example, we can specialise the above 'f' based on a call map (f @Int dEqInt) xs even though we don't (and can't) give Ord dictionary. This may sound exotic, but #17966 is a program from the wild, and showed significant perf loss for functions like f, if you need saturation of all dictionaries. * We fix a buglet in which a floated dictionary had a bogus demand (#17810), by using zapIdDemandInfo in the NonRec case of specBind. * A tiny side benefit: we can drop dead arguments to specialised functions; see Note [Drop dead args from specialisations] * Fixed a bug in deciding what dictionaries are "interesting"; see Note [Keep the old dictionaries interesting] This is all achieved by by building on Sandy Macguire's work in defining SpecArg, which mkCallUDs uses to describe the arguments of the call. Main changes: * Main work is in specHeader, which marched down the [InBndr] from the function definition and the [SpecArg] from the call site, together. * specCalls no longer has an arity check; the entire mechanism now handles unders-saturated calls fine. * mkCallUDs decides on an argument-by-argument basis whether to specialise a particular dictionary argument; this is new. See mk_spec_arg in mkCallUDs. It looks as if there are many more lines of code, but I think that all the extra lines are comments!4291bdda
