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.cama...@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_clay...@clear.net.nz> > wrote: > >> On Tue, 9 Oct 2018 at 7:30 AM, <cama...@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 >> _______________________________________________ >> Haskell-prime mailing list >> Haskell-prime@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime >> > _______________________________________________ > Haskell-prime mailing list > Haskell-prime@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime >
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