On Wednesday 24 April 2013, 13:25:01, gs wrote:
Daniel Fischer
writes: I meant example code using a GADT for Source, and not DatatypeContexts; and some function where you still need the context.
OK, here are some truncated instances/functions:
data Source v a where Source :: Variable v => {bindings :: v [Binding a] ,var :: v a} -> Source v a
instance Variable v => Variable (Source v) where newVar a = do bindings <- newVar [] v <- newVar a return $ Source bindings v
instance Variable v => Bindable (Source v) where bind (Source bindings var) extract target apply = do let binding = Binding extract target apply --update the new binding a <- readVar var
Removing the `Variable v` context from instance declarations is at least tricky. I don't think you can do it at all here. For the context to become available, you need to pattern-match, but in `newVar`, the `Source` appears only as the result. Therefore you need the `Variable v` context to be able to write v <- newVar a Consequently, you can't have an instance Variable (Source v) where ... without context, and since `Variable b` is a superclass constraint on `Bindable b`, you need something that guarantees that `Source x` (resp. `BindingList x`) has a `Variable` instance.
data BindingList v a where BindingList :: Variable v => {source :: Source v a, list :: v [v a],pos
:: v Int} -> BindingList v a
fromBindingList :: Variable v => BindingList v a -> IO [a] fromBindingList b = do update b readVar (list b) >>= mapM readVar
You're not pattern-matching on the constructor. I wrote that you have to do that to make the context available:
The `Variable v` context becomes available by pattern-matching on the constructor `Source` (but not by using the field names to deconstruct a value of type `Source v a`!).
Pattern-matching, fromBindingList :: BindingList v a -> IO [a] fromBindingList b@(BindingList s l p) = do update b readVar l >>= mapM readVar removes the need to mention the `Variable v` context.