[Haskell-cafe] question on types
lennart at augustsson.net
Wed Feb 18 05:12:59 EST 2009
Also, if you are using ghc you can turn on the extension that allows
undecidable instances and make the type system Turing complete.
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)
> Früher hieß es ja: Ich denke, also bin ich.
> Heute weiß man: Es geht auch so.
> - Dieter Nuhr
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