| Is this really the compiler’s job here? After all, the programmer would be
able to
| write
|
| deriving cNT' :: NT (Foo a) (Foo b) -> NT (C a) (C b)
| cNT :: NT a b -> NT (C a) (C b)
| cNT = cNT' . fooNT
|
| and expose just cNT to his users, so no expressiveness is lost by not
providing
| automatic support here.
True. But it could get pretty inconvenient.
data Foo a = F1 [a] (Tree [a]) (Maybe a) [Maybe a]
We could restrict you (only ) to saying
deriving fooNT
:: NT [a] [b]
-> NT (Tree [a]) (Tree [b])
-> NT (Maybe a) (Maybe b)
-> NT [Maybe a] [Maybe b]
-> NT (Foo a) (Foo b)
but the excitement would wear off pretty fast :-). It'd be nicer to write
deriving fooNT :: NT a b -> NT (Foo a) (Foo b)
and have the compiler use the other lexically-in-scope NT values to solve the
problem. This is, after all, precisely what the type-class inference machinery
does.
There's something a bit strange about looking for all NT values that are
lexically in scope; it all amounts to something pretty similar to named
type-class instances.
Simon
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