[Haskell-cafe] A question about constraints

[Luke Palmer]

On Thu, Oct 2, 2008 at 4:11 AM, Bulat Ziganshin
<bulat.ziganshin at gmail.com> wrote:
> finally, ghc now includes some form of active patterns which may be
> used to define your own way to decompose values. but their syntax
> isn't compatible with constructors so you can't define complex type
> which mimicks simple ones, and in particular you can replace simple
> type with complex one w/o rewriting all the client code
>
> imho it's serious lack in haskell support for Abstract Data Types

Not that this hasn't probably been discussed at length elsewhere, I
wholeheartedly agree.

The ability to mimic a simple type with a complex one is precisely
what moves views from "syntax sugar", which does little more than to
make code easier on the eyes, into a proper abstraction mechanism that
actually adds engineering value.

Luke