Thanks, this explanation is what I was looking for. Wikipeidia has an explanation on it also:<br><br><a href="http://en.wikipedia.org/wiki/System_F#System">http://en.wikipedia.org/wiki/System_F#System</a><br><br>daryoush<br>
<br><div class="gmail_quote">On Wed, Feb 18, 2009 at 2:08 AM, Stephan Friedrichs <span dir="ltr"><<a href="mailto:deduktionstheorem@web.de">deduktionstheorem@web.de</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div><div></div><div class="Wj3C7c">Daryoush Mehrtash wrote:<br>
> Is there a way to define a type with qualification on top of existing<br>
> type (e.g. prime numbers)? Say for example I want to define a<br>
> computation that takes a prime number and generates a string. Is there<br>
> any way I can do that in Haskell?<br>
<br>
</div></div>Haskell's type system is decidable, so you can't let the type system<br>
check arbitrary properties. It probably is possible in C++ by some<br>
template hack (C++ templates are Turing complete), but not in Haskell.<br>
But, as mentioned in the other responses, you can<br>
<br>
- use a representation that makes it impossible to use wrong values<br>
(-> Ketil's n-th prime representation)<br>
<br>
- check values at runtime (-> Luke's repsonse)<br>
<br>
//Stephan<br>
<font color="#888888"><br>
--<br>
<br>
Früher hieß es ja: Ich denke, also bin ich.<br>
Heute weiß man: Es geht auch so.<br>
<br>
- Dieter Nuhr<br>
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