[Haskell-cafe] question on types

Stephan Friedrichs deduktionstheorem at web.de
Wed Feb 18 05:08:50 EST 2009

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)



Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.

 - Dieter Nuhr

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