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

Carter Schonwald carter.schonwald at gmail.com
Sat Apr 25 19:23:04 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.

On Saturday, April 25, 2015, Ertugrul Söylemez <ertesx at gmx.de> wrote:

> > 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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