On Wed, Feb 18, 2009 at 2:08 AM, Stephan Friedrichs <deduktionstheorem@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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