[Haskell-beginners] How to avoid repeating a type restriction from a data constructor

[Daniel Fischer]

On Wednesday 24 April 2013, 13:25:01, gs wrote:
> Daniel Fischer <daniel.is.fischer <at> googlemail.com> 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.