Example: I wish to define a structured container type, let's call it a "RatsNest", that is type-variable in two ways:
(a) it is parameterized by a type of some class, let's call it "RatsTail", such that a RatsNest is a structure of things that have common properties. Certain operations between RatsNest values (e.g. a merge) are valid only if they are containers for the same kind of RatsTail. Such operations would not be different operations due to variation of thye contained type.
(b) it can be implemented in various ways; e.g. a simple version is an in-memory data structure, while another may be implemented in database storage. This corresponds to the Haskell idea of overloading, or classes.
This seems to call for a type-parameterized class specification, e.g.
[[
class (Eq n, RatsTail t) => (RatsNest t) n where
merge :: ( (RatsNest t1) n1, (RatsNest t1) l2) => n1 -> n2 -> n2
]]But this, as far as I can tell, is not valid Haskell. Is there a way to achieve this effect?
What I can't figure out is a way to impose that the merge operation is not valid between *any* instances of RatsNest, but only those that are based on the same member element type, but *can* be used between different implementations (e.g. disk-based and memory-based).
#g
------------------- Graham Klyne <[EMAIL PROTECTED]> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E
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