-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 8/27/10 22:58 , Greg wrote:
I'm still having a hard time finding a way to make this work, even given the fine suggestions from Tobias and Jürgen. I suspect there's some piece of information that the compiler can't make sense of that I'm just not seeing-- a case of it insisting on doing what I say instead of what I mean... =)
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)
In addition, I'm having a hard time understanding the errors I'm getting from constructs like this:
data Foo a = Foo a
class TwoPi a where div2pi :: (Floating b) => a -> b
You were told about this already: because "b" is only mentioned in the result of div2pi, it must be able to return *any* "b" that has a Floating instance. But then you say
instance (Floating a) => TwoPi (Foo a) where div2pi (Foo x) = x / (2*pi)
An instance applies to a *specific* type; thus, this instance declaration forces a *specific* type on the result of div2pi, when the class declaration says it must be able to return *any* type. Expanding the types, the class declaration says
div2pi :: forall b. (Floating a, Floating b) => a -> b
but the instance declares
div2pi :: (Floating a) => a -> a
The instance doesn't conform to the class definition; it includes a constraint that the class does not, as the class insists that the type of the result must be independent of the type of the argument, while the instance insists that they must be identical. Perhaps the correct question is "what exactly are you trying to do?" - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.10 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkx4f0IACgkQIn7hlCsL25WexQCfTWxZxqchxvAB0Dm1wWwYQIrq sBQAoLeaz8Lq6ydO5WJPu679WyNEu8w0 =BnBu -----END PGP SIGNATURE-----