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Polymorphic Variants
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Date: 2007-01-18 (12:23)
From: Tom <tom.primozic@g...>
Subject: Re: [Caml-list] Polymorphic Variants
> Both reasons have practical impact. For the first one, using erasure
> semantics means that the programmer also can discard types when
> understanding the runtime behaviour of a program.

Actually, what I had in mind is exclusivelly compile-time overloading, which
causes no overheed at runtime.

> > Hm... Actually, what I had in mind is nominal subtyping... similar to
> > objects, in fact, objects in C++-like languages, just that they have no
> > class methods.
> Reading the description below, this all looks nice, independently of
> the semantics limitation described above. However, you can kiss
> farewell to type inference. With such an extensive overloading, you
> would need type annotations all over the place.

For the second one,
> you can write code that is maximally polymorphic, without too much
> fear about the impact of performance (equality is overloaded, so
> it still matters...) or strange ambiguity-related error messages.

I believe that "strange ambiguity-related error messages" are the result of
stupid programmers, not of the language. If one is careful to design a sane
library, most ambiguities never happen! Besides, incorporating whole-program
analysis (in the spirit of MLton), one can postpone such errors until the
whole application has been written, so for example local ambiguities would
be resolved by examining the uses (if a function is always called with
arguments of single type, there is no ambiguity).

> Not for records, but for objects. From a type-theoretic point of view
> they are just equivalent.

> By the way, this all looks likes the "used this feature in C++"
> syndrome. Sure C++ is incredibly powerful. But it is built on rather
> shaky theoretical fundations. So you can't expect to bring everything
> from C++ to a new language. Why not think about new ways to solve
> problems :-)

Well, I am... but just as you are in your message mentioning the practical
impact, so am I. And I think that every way to implement subtyping of
records/objects (=named tuples) other than the C++ way has an important
practical consequence - it's just slow. I guess that C++ like index-based
access is much faster than name-based access neccessary for structural
subtyping. Thou, please prove me that I am wrong :) I want structural
subtyping, the OCaml way, just very fast.

- Tom