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Subject: Re: [boost] An alternative approach to TypeErasure
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-06-24 18:20:16

on Sun Jun 24 2012, Pyry Jahkola <> wrote:

> More importantly, I'm still kind of missing the point of why the
> feature of operator overloading is really needed. In essence: How
> should binary operators behave? Steven's TypeErasure defines binary
> operators such that only when the wrapped types of a and b are the
> same, can you sum them up: "a + b".
> But where would you use this kind of an "any", where only some of the
> instances can be added up, and others cause undefined behavior?
> Wouldn't you actually do the addition on the side where you know the
> types, and not on the type-erased side?
> Do we really have a real world use case that would prompt us to
> implement features like operator overloading?

Well, it's interesting; this type erasure can be viewed/used two
different ways:

1. Runtime polymorphism
2. Static polymorphism with separate compilation of generic functions

Approach 1 admits a runtime type error when the types don't match in
a+b. Approach 2 doesn't. Haskell actually takes approach 2.

  Prelude> :t (+)
  (+) :: (Num a) => a -> a -> a

That's why, for example, it isn't possible to make a list of a given
type class and store two different types that conform to that type class
in the list. *Your* type erased things do runtime polymorphism, so
they're very different from Haskell's typeclasses in that sense.

For dynamic polymorphism in Haskell, you need something like
> Before releasing more worms from the can of type-erased operators, I
> must confess that I know still too little about the possible uses for
> type erasure / expression problem / what you name it. What I propose
> is we should look into how e.g. Haskell and Clojure programmers use
> their type classes and protocols.

It's probably a good idea to learn as much as possible from other
languages, but that said, we *do* have plenty of prior art in C++ for
type erasure. Just making those use cases better is a worthy goal on
its own.

Dave Abrahams
BoostPro Computing

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