Hello,
Usually, you can program such things by using super-classes. Here is
how you could encode your example:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances #-}
class HeaderOf addr hdr | addr -> hdr
class HeaderOf addr hdr => AddressOf hdr addr | addr -> hdr
data IPv4Header = C1
data IPv4 = C2
data AppAddress = C3
data AppHeader = C4
instance AddressOf IPv4Header IPv4
instance HeaderOf IPv4 IPv4Header
{- results in error:
instance AddressOf AppHeader AppAddress
instance HeaderOf AppAddress [AppHeader]
-}
Hope that this helps,
Iavor
On Sat, Jan 3, 2009 at 7:22 AM, Thomas DuBuisson
Cafe, I am wondering if there is a way to enforce compile time checking of an axiom relating two separate type families.
Mandatory contrived example:
type family AddressOf h type family HeaderOf a
-- I'm looking for something to the effect of: type axiom HeaderOf (AddressOf x) ~ x
-- Valid: type instance AddressOf IPv4Header = IPv4 type instance HeaderOf IPv4 = IPv4Header
-- Invalid type instance AddressOf AppHeader = AppAddress type instance HeaderOf AppAddress = [AppHeader]
So this is a universally enforced type equivalence. The stipulation could be arbitrarily complex, checked at compile time, and must hold for all instances of either type family.
Am I making this too hard? Is there already a solution I'm missing?
Cheers, Tom _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe