Re: [Haskell-beginners] Question about define my own typeclass

Thanks for your reply. Yes, I understand that if I specify the 'b' to 'Prelude.Bool', it should work, but what if I also want use the typeclass Conversion to convert other types other than MyBool, like MyInt, or MyString? --Ke On Thu, Mar 27, 2014 at 11:36 AM, Brandon Allbery wrote:

On Thu, Mar 27, 2014 at 11:28 AM, ke dou wrote:

...

class Conversion a where conversion :: a -> b

b is completely unspecified here, since it's not defined as part of the typeclass. The literal meaning of this is that "the caller can request any type it pleases, and you have no way of knowing what it is". So the only possible result of `conversion` is bottom (e.g. `undefined`).

This is key: it does NOT mean that `conversion` gets to specify the result type! You can't do that, except by specifying the type in the type signature.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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