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

Chung-chieh Shan ccshan at post.harvard.edu
Thu Apr 29 02:19:53 EDT 2004

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

    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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