Thanks, this explanation is what I was looking for. Wikipeidia has an explanation on it also: http://en.wikipedia.org/wiki/System_F#System daryoush 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.
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