Re: Haskell, anyone?

[Mark Carroll <mtbc@bcs.org>]

On 15 Nov 2015, Nasser M. Abbasi wrote:
(snip)
> On Ada, One of the best features is the ability to define subtypes very
> easily. I have not seen any other language with this feature. (one has
> to make Class in other languages, and define all the operations each time).
>
> This is for me what makes Ada for me. One can define a type
> that matche the range of the physical quantity being modeled.
> This helps catch many errors.
>
> Can one do this in Haskell?  Say define a new integer subtype that
> can only take values from only 1...20.  And have the compiler
> and run time check for this?

One could do subrange types back with Modula-3 too! (Possibly also
earlier Pascal-like languages.) I /can't/ think off-hand how to do this
easily with Haskell (at least without unusual extensions) but I /think/
that Idris, that I'd mentioned as illuminating a possible way forward,
would probably make you work a bit harder for subtypes, in the theorem
proving for dependent types, but with the payback of allowing a rather
wider range of kinds of subtype constraint. Unfortunately the leading
edge of compiler-checked correctness still makes one sweat for that
final thumbs-up.

-- Mark