Ah. I just need to drop the context from the data type declaration. Sorry, Andrew andrew cooke said:
Hi,
I'm trying to use type classes and suspect I've got confused somewhere down the line, because I've got to an error message that I don't understand and can't remove.
I have a class that works like a hash table, mapping from String to some type. I have two instances, one that is case insensitive for keys. I want to hide these instances from the rest of the code, which should only use the class. This class is called Dictionary.
In addition, for Dictionaries that map Strings to Strings, I have some functions which do substitutions on Strings using their own contents. Possibly the first source of problems is that I can't find a way to express these two classes together without multiple parameter type classes (one parameter for the case/no case and one for the type returned):
class Dictionary d a where add' :: d a -> (String, a) -> d a ...
instance Dictionary DictNoCase a where add' d (k, v) = ...
-- Dict is the underlying tree implementation and Maybe stores the -- value for the null (empty string) key. data DictNoCase a = DictNoCase (Dict a) (Maybe a)
class (Dictionary d String) => SubDictionary d where substitute :: d String -> String -> String ...
instance SubDictionary DictNoCase where substitute d s = ...
All the above compiles and seems correct (is it?).
I also provide an empty instance of the two instance types. For DictNoCase, this is empty = DictCase Empty Nothing where Empty is the empty type constructor for Dict
Now (almost there) elsewhere I want to define a data type that contains two of these Dictionaries. One stores String values. The other stores functions that take this same type and return a result and a copy of the type:
data Context s f = (Dictionary s String, Dictionary f (CustomFn s f)) => Ctx {state :: s String, funcs :: f (CustomFn s f)}
type CustomFn s f = Context s f -> Arg -> IO (Context s f, Result s f)
data Result s f = Attr Name String | Repeat (CustomFn s f) ...
newContext = Ctx empty emptyNC
(where empty is the case-sensitive empty dictionary)
Now THAT doesn't compile:
Template.lhs:60: All of the type variables in the constraint `Dictionary s String' are already in scope (at least one must be universally quantified here) When checking the existential context of constructor `Ctx' In the data type declaration for `Context'
Template.lhs:60: All of the type variables in the constraint `Dictionary f (CustomFn s f)' are already in scope (at least one must be universally quantified here) When checking the existential context of constructor `Ctx' In the data type declaration for `Context'
where line 60 is "data Context...."
And I can't see what I've done wrong. Any help gratefully received.
Cheers, Andrew
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