A question about run-time errors when class members are undefined

[Carter Schonwald]

Carlos, local scoping for type classes is flat out not gonna happen in the
haskell language standard any time soon.

if you want to make a case for it, demonstrate its utility, this mailing
list isn't for that. Especially for something that fundamentally changes
the programming model of the language in question in a way that isn't
compatible

merry adventures!
-Carter

On Mon, Oct 8, 2018 at 8:47 PM Carlos Camarao <carlos.camarao at gmail.com>
wrote:

> Hi.
>
> > Thanks Carlos. I wish I could say thank you for clarifying, but I'm
> > afraid this is as muddled as all the comments on the two proposals.
> >
> > I don't want to go over it again. I just want to say that my
> > suggestion earlier in the thread is fundamentally different.
> >
> >>    Global instance scope is not ok either: instances should be modular.
> > I just plain disagree. Fundamentally.
>
> Global instance scope is not required for principal typing: a
> principal type is (just) a type of an expression in a given typing
> context that has all other types of this expression in that typing
> context as instances.
>
> (Also: instance modularity is not the central issue.)
>
>     >>> Wadler & Blott's 1988 paper last paragraph had already explained:
> "But
>     >>> there is no principal type! "
>
>     >> There is always a principal type, for every expression.
>     >> Of course the type depends on the context where the expression
> occurs.
>
> > Then it's not a _principal_ type for the expression, it's just a local
> type.
> > http://foldoc.org/principal
>
> A type system has the principal type property if, given a
> term and a typing context, there exists a type for this term in this
> typing context such that all other types for this term in this typing
> context are an instance of this type.
>
> > We arrive at the principal type by unifying the principal types of
> > the sub-expressions, down to the principal types of each atom. W&B
> > are pointing out that without global scope for instances, typing
> > cannot assign a principal type to each method. (They left that as an
> > open problem at the end of the paper. Haskell has resolved that
> > problem by making all instances global. Changing Haskell to modular
> > instances would be a breakage. Fundamentally.)
> >
> > Under my suggestion, we can assign a (global) principal type to each
> > method -- indeed you must, by giving a signature very similar to a
> > class declaration; and that distinguishes overloaded functions from
> > parametric polymorphic functions.
>
> A principal type theorem has been proved: see, for example, Theorem 1 in
> [1].
>
> Kind regards,
>
> Carlos
>
> [1] Ambiguity and Constrained Polymorphism,
>     Carlos Camarão, Lucília Figueiredo, Rodrigo Ribeiro,
>     Science of Computer Programming 124(1), 1--19, August 2016.
>
>
> On Mon, 8 Oct 2018 at 20:03, Anthony Clayden <anthony_clayden at clear.net.nz>
> wrote:
>
>> On Tue, 9 Oct 2018 at 7:30 AM, <camarao at dcc.ufmg.br> wrote:
>>
>> Thanks Carlos. I wish I could say thank you for clarifying, but I'm
>> afraid this is as muddled as all the comments on the two proposals.
>>
>> I don't want to go over it again. I just want to say that my suggestion
>> earlier in the thread is fundamentally different.
>>
>> Em 2018-10-08 06:21, Anthony Clayden escreveu:
>>> > On Mon, 8 Oct 2018 at 8:41 PM, Simon Peyton Jones wrote:
>>> >
>>
>>
>> Strange: Simon's message has not appeared on the forum (he did send to
>> it). I've quoted it in full in my reply, but did break it into separate
>> pieces.
>>
>>
>>>
>>> Global instance scope is not ok either: instances should be modular.
>>
>>
>> I just plain disagree. Fundamentally.
>>
>>
>>> >
>>> > Wadler & Blott's 1988 paper last paragraph had already explained: "But
>>> > there is no principal type! "
>>>
>>> There is always a principal type, for every expression.
>>> Of course the type depends on the context where the expression occurs.
>>
>>
>> Then it's not a _principal_ type for the expression, it's just a local
>> type.
>> http://foldoc.org/principal
>>
>> We arrive at the principal type by unifying the principal types of the
>> sub-expressions, down to the principal types of each atom. W&B are pointing
>> out that without global scope for instances, typing cannot assign a
>> principal type to each method. (They left that as an open problem at the
>> end of the paper. Haskell has resolved that problem by making all instances
>> global. Changing Haskell to modular instances would be a
>> breakage. Fundamentally.)
>>
>> Under my suggestion, we can assign a (global) principal type to each
>> method -- indeed you must, by giving a signature very similar to a class
>> declaration; and that distinguishes overloaded functions from parametric
>> polymorphic functions.
>>
>>
>> AntC
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>> Haskell-prime at haskell.org
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>>
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