[Haskell-cafe] Re: Is it possible to read existential types?
S. Alexander Jacobson
haskell at alexjacobson.com
Thu Apr 29 12:50:48 EDT 2004
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.
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
> 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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