[Haskell-cafe] Hello and type-level SKI

[Andras Slemmer]

> 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).


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.
>
>
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