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

Ertugrul Söylemez ertesx at gmx.de
Sat Apr 25 18:43:23 UTC 2015

> 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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