[Haskell-beginners] instances of different kinds

[Brandon S Allbery KF8NH]

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On 8/27/10 22:58 , Greg wrote:
> I guess the problem I'm having is finding a way to treat parametric and
> non-parametric types interchangeably.  The syntax doesn't seem to exist that
> will allow me to say:
> 
> div2pi :: (Floating a) => a -> a   -- for non parametric types (ie. Float)
> and
> div2pi :: (Floating b) => a b -> b  -- for parametric types (ie. Foo Float)

It occurs to me that what you really want *may* be:

> class (Floating a, Floating b) => TwoPi a b | a -> b where
>   div2pi :: a -> b

which says that, given a specific type a, the compiler can infer a specific
type b corresponding to it, without specifying what that type is (leaving
the instance declaration to do so).

- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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