[Haskell-cafe] Class-like features for explicit arguments

Ertugrul Söylemez ertesx at gmx.de
Sat Apr 25 20:06:02 UTC 2015

> Here yah go https://hackage.haskell.org/package/reflection
> It exploits how dictionary passing works in a pretty robust way that
> ghc is likely to at some point codify officially.

Oh, that one.  Of course I'm familiar with it, but it's less an
optimisation than a clever way to implement implicit configurations.

>> > Could that specialization be accomplished today using eds reflection
>> > pkg? I guess not quite in terms of that pre apply pattern.
>> I'm not familiar with that package and couldn't find it on Hackage by a
>> quick search.  But I believe that it can only be done with compiler
>> support, although with enough hackery you can probably get an ugly
>> version of it using TH.
>> > This is interesting.  And it's a good example of a larger problem of
>> > not enough support for composable specialization with good sharing
>> > across the use sites that doesn't require egregious Inlining.  At
>> > least for code that isn't Type class driven.
>> Indeed.  Specialisation is a really good way to get very fast code
>> without making your executable size explode.  I believe that support for
>> more fine-grained specialisation should and will improve.  I'm not sure
>> how to make it more composable though.
>> >> At the definition and instantiation sites I mostly miss defaults.  At
>> >> the application sites I would love to have specialisation for certain
>> >> arguments.  For example I would like to be able to tell GHC that I
>> >> would like to have a version of my function `f` with a certain
>> >> argument inlined.  Note that I don't want to inline `f` itself.
>> >> Rather I'd like to preapply certain arguments:
>> >>
>> >>     f :: X -> Y -> Z
>> >>
>> >>     {-# SPECIALISE f SomeX #-}
>> >>     {-# SPECIALISE f SomeOtherX #-}
>> >>
>> >> This would generate two specialised versions of `f` with exactly the
>> >> given arguments inlined.  That way I can get a very efficient `f`
>> >> without having to inline it at the application sites.  And as long as
>> >> `f` is INLINABLE I can put those pragmas pretty much everywhere.  I
>> >> believe this is exactly what happens for type class dictionaries.
>> >>
>> >> This can (and probably should) be a separate feature though.  For some
>> >> of my applications I need to inline a huge chunk of code multiple times
>> >> to compensate for the lack of this feature.
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