[Haskell-cafe] question on types

[Lennart Augustsson]

Also, if you are using ghc you can turn on the extension that allows
undecidable instances and make the type system Turing complete.

  -- Lennart

On Wed, Feb 18, 2009 at 10:08 AM, Stephan Friedrichs
<deduktionstheorem at web.de> wrote:
> Daryoush Mehrtash wrote:
>> Is there a way to define a type with qualification on top of existing
>> type (e.g.  prime numbers)?   Say for example I want to define a
>> computation that takes a prime number and generates a string.   Is there
>> any way I can do that in Haskell?
>
> Haskell's type system is decidable, so you can't let the type system
> check arbitrary properties. It probably is possible in C++ by some
> template hack (C++ templates are Turing complete), but not in Haskell.
> But, as mentioned in the other responses, you can
>
>  - use a representation that makes it impossible to use wrong values
>   (-> Ketil's n-th prime representation)
>
>  - check values at runtime (-> Luke's repsonse)
>
> //Stephan
>
> --
>
> Früher hieß es ja: Ich denke, also bin ich.
> Heute weiß man: Es geht auch so.
>
>  - Dieter Nuhr
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