On Tue, May 25, 2010 at 07:59:24PM +0100, Stephen Tetley wrote:
Hi Eugeny
Its not that GHC thinks you're doing something weird, but that there is no relation between the type parameters in the Storage class. You could use either functional dependencies or type families to introduce a relation / relations, but personally I would look at doing something simpler - for instance why do you need a map type that is polymorphic on shape?
Currently I am creating set of objects from name-value pairs, and I decided to use Map for keeping relations between an object id and record with the id. So I will be able to parse the parameter like param_1_propname=value then take the object with ID=1 from Map, and update it's property 'propname' with value, and put it back into the Map. But I faced several cases when a set of name-value pairs describes 2 or even more kinds of objects. And I want to be able to parse them all at one pass, so I would need 2 or more maps. And I simply tried to generalize the solution. Probably I should think in different way. May be a chain of Writer monads or something similar. -- Eugene Dzhurinsky