[Haskell-cafe] Re: Is it possible to read existential types?

[S. Alexander Jacobson]

But isn't the point of this code that you don't
need that type signature?  If I knew in advance
that it was an Integer then I wouldn't need to
passs "Integer" in the list.

-Alex-


On Thu, 29 Apr 2004, Chung-chieh Shan wrote:

> On 2004-04-28T23:33:31-0400, S. Alexander Jacobson wrote:
> > I don't think this works.  I just tried it with:
> >   main = print $ lookupRead 1 [(1,("Integer","100"))]
>
> This fails for the same reason
>
>     print $ read "100"
>
> fails.  You need to give a type signature to avoid type-class instance
> ambiguity:
>
>     main = print $ (lookupRead 1 [(1,("Integer","100"))] :: Maybe Integer)
>
> On GHCi 6.2.1, the above yields "Just 100" for me.
>
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S. Alexander Jacobson                  mailto:me at alexjacobson.com
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