[Haskell-cafe] Hello and type-level SKI

[Andras Slemmer]

> I don't know if I'd put TypeFamilies on there today due to
https://ghc.haskell.org/trac/ghc/ticket/9562
Huh! you learn something everyday:D

On 16 September 2014 19:29, Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk>
wrote:

> On 09/16/2014 06:26 PM, Andras Slemmer wrote:
> >> I am still not sure how far modifications to the type inferencer/checker
> > affect the code produced, beyond whether any code at all is output.
> >
> > Most extensions are safe, there are a few that are known to cause (or to
> > have caused) trouble.
> >
> > Safe (type-system) extensions include: MultiParamTypeClasses,
> > FunctionalDependencies, FlexibleContexts, FlexibleInstances,
> > LiberalTypeSynonyms, DataKinds, ConstraintKinds, GADTs,
> > ExistentialQuantification, RankNTypes, GeneralizedNewtypeDeriving(since
> ghc
> > 7.8), TypeFamilies. I would also put UndecidableInstances here, as it
> will
> > not produce incorrect code, it's just that the compiler may give up on
> type
> > checking. There are a couple of more, this is just to give you an idea..
> >
> > Safe but never to be used extensions(except if you know -exactly- what
> > you're doing): OverlappingInstances, IncoherentInstances
> >
> > Unsafe extensions: GeneralizedNewtypeDeriving(before ghc 7.8), with which
> > one was able to implement coerce :: a -> b
> >
> > Regarding inference: if a type is not inferable that doesn't mean the
> term
> > in question is incorrect. For example if you switch on RankNTypes very
> > simple terms can inhabit several types. For example take f = (\x -> x).
> You
> > may think f must have type :: a -> a, but a completely different type, ::
> > (forall a. a) -> b also works (although it's not very useful).
> >
> >
>
> I don't know if I'd put TypeFamilies on there today due to
> https://ghc.haskell.org/trac/ghc/ticket/9562
>
> > On 16 September 2014 04:37, Jay Sulzberger <jays at panix.com> wrote:
> >
> >>
> >>
> >>
> >> On Sat, 13 Sep 2014, Tslil Clingman <tslil.clingman at gmail.com> wrote:
> >>
> >>  Right, so to be fair a more accurate description would have been: `It
> >>> can be shown that GHC with numerous extensions gives rise to a type
> >>> system which is Turing Complete', my mistake. I certainly didn't mean
> to
> >>> mislead anyone.
> >>>
> >>> All too often I conflate Haskell (2010 or so) and the capabilities of
> >>> GHC -- this is a trap which ensnares many, I suspect.
> >>>
> >>> --
> >>> Yours &c.,
> >>> Tslil
> >>>
> >>
> >> Dear Tslil, my exclamation was with tongue part way in cheek, so
> >> no worry as far as I am concerned.  I do not know much about
> >> Haskell, and less about GHC, but I am slowly learning.  I am
> >> still not sure how far modifications to the type
> >> inferencer/checker affect the code produced, beyond whether any
> >> code at all is output.  My guess today is that that most of the
> >> time, no matter what extensions are invoked, if the source
> >> type-checks, then we get the same Core output.
> >>
> >> I am still smiling at your S, K, I, and Andras's Eval.
> >>
> >> oo--JS.
> >>
> >>
> --
> Mateusz K.
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