Manuel M T Chakravarty:
This is problematic as the instance heads are distinguished only by
the
context; ie, both instances are for `Lacks m (N a r)'. Haskell's
instance selection mechanism (with or without associated types)
selects
instances purely by looking at the arguments of the class; hence, you
cannot use instance context as a kind of guard to guide instance
selection.
A pity. Would resolving instance declarations as Horn clauses be
useful to deal with any other examples of overlapping instances?
If you don't mind the code for each new label being linear in the
number of previous labels (so the total code is quadratic in the
global number of labels) the idea can be rescued.
Get rid of (:<:), and keep the same definitions of Constructor,
Contains, Lacks, Disjoint and Empty, and the same instances of 'Lacks
n Empty' and 'Disjoint Empty r'.
For each label 'N', define:
> data N a r = N a r
>
> instance Contains N (N a r) where
> type Project N (N a r) = a
> type Delete N (N a r) = r
> project N (N x t) = x
> delete N (N x t) = t
>
> instance Disjoint r s, Lacks N (Union r s) => Disjoint (N a r) s
where
> type Union (N a r) s = Extend N a (Union r s)
> union (N x t) u = extend N x (union t u)
and for each previous label 'M', define:
> instance Contains M r => Contains M (N a r) where
> type Project M (N a r) = Project M r
> type Delete M (N a r) = N a (Delete M r)
> project M (N x t) = project M t
> delete M (N x t) = N x (delete M t)
>
> instance Lacks M r => Lacks M (N a r) where
> type Extend M b (N a r) = N a (Extend M b r)
> extend M y (N x t) = N x (extend M y t)
>
> instance Lacks N (M a r) where
> type Extend N b (M a r) = N b (M a r)
> extend N y (M x t) = N y (M x t)
This saves the bother of fiddling with (:<:) but means that every
time you import a module which declares labels you would have to
generate code for the interactions between each new label and each
existing one.
At least it's not exponential!
Barney.
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