I don't think this works. I just tried it with:
main = print $ lookupRead 1 [(1,("Integer","100"))]
How would Haskell know that typ actually does
equal typeOf?
-Alex-
_________________________________________________________________
S. Alexander Jacobson mailto:[EMAIL PROTECTED]
tel:917-770-6565 http://alexjacobson.com
On Wed, 28 Apr 2004, Chung-chieh Shan wrote:
> On 2004-04-28T15:12:03-0400, S. Alexander Jacobson wrote:
> > Ok, but it sounds like you need to know the list
> > of possible types in advance. Is it possible
> > to have a lib take a filepath and type name as an
> > arbitrary string, and read the instance in?
>
> I don't think you need to know the list of possible types in advance.
> Here is some (only slightly tested) code:
>
> import Data.Dynamic
> import Maybe
> import Monad
>
> lookupRead :: (Eq key, Read val, Typeable val) =>
> key -> [(key, (String, String))] -> Maybe val
> lookupRead key list = ret
> where ret = case lookup key list of
> Just (typ, val) ->
> if typ == show (typeOf (fromJust ret))
> then case reads val of
> [(v,"")] -> Just v
> _ -> Nothing
> else Nothing
> _ -> Nothing
>
> The question is how to get the result of lookupRead memoized, so that
> the call to reads happens at most once for each entry in the list.
>
> --
> Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig
> BBC News: Universities face week of protest
> http://news.bbc.co.uk/1/hi/education/3508209.stm
>
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