Re: [Haskell-beginners] Re: [Haskell-cafe] About "Fun with type functions" example

Then you can write fromInt 1 :: Zero, and it will type check. /J On 19 November 2010 04:42, Bastian Erdnüß wrote:

On Nov 19, 2010, at 1:07, Daniel Peebles wrote:

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The best you can do with fromInt is something like Int -> (forall n. (Nat n) => n -> r) -> r, since the type isn't known at compile time.

Or you put it in the type class as fromInt :: Int -> n and do in Zero: fromInt _ = Zero and in the other: fromInt _ = Succ $ fromInt (undefined :: n). Shouldn't that work, too?

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On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly

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Thanks a lot, that works perfectly fine! Did not know this one... BTW, I would be interested in the fromInt too.

Arnaud

On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink wrote:

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On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly wrote:

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Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly:

[snip]

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instance (Nat n) => Nat (Succ n) where toInt _ = 1 + toInt (undefined :: n)

[snip]

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And here is the error:

Naturals.hs:16:18: Ambiguous type variable `n' in the constraint: `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 Probable fix: add a type signature that fixes these type variable(s)

You need to turn on the ScopedTypeVariables extension (using {-# LANGUAGE ScopedTypeVariables #-} at the top of your file, or -XScopedTypeVariables at the command line). Otherwise, the 'n' in the class declaration and in the function definition are different, and you want them to be the same 'n'.

Erik

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