Hi,
I don't have a solution for you problem (besides using undecidable
instances) but I can explain the "coverage condition".
On 12/19/06, Robert
class CommandFunction f st | f -> st where parseCommand :: String -> f -> CommandParser st commandSyntax :: f -> [Doc]
The functional dependency on this class adds the following axiom (improvement rule) : improve (CommandFunction f st1, CommandFunction f st2) using st1 = st2 Before accepting an instance, an implementation needs to check that the instance will not violate this rule (i.e., the functional dependency is satisfied). In general, this may be difficult to check. The "coverage condition" (CC) is a (conservative) rule that guarantees that the functional dependency is satisfied. It states that an instance is accepted if all type variables in the determined types are mentioned in the determining types. The rule is conservative because (as you have noticed) there are cases when the FD axiom is not violated but the rule rejects an instance. Examples:
instance CommandFunction (Sh st ()) st where parseCommand wbc m str = ...
This instance satisfies the CC because "st" is mentioned in "Sh st ()".
instance CommandFunction r st => CommandFunction (Int -> r) st where ...
This instance does not satisfy the CC because "st" is not mentioned in "Int -> r".
instance CommandFunction r st => CommandFunction (Integer -> r) st where ...
This instance does not satisfy the CC because "st" is not mentioned in "Integer -> r". Hope this helps. -Iavor