Thank you very much — it works! [and many new useful functions discovered along the way :-)].
I'm fairly happy with this solution (using readConstr as suggested by Niklas, and fromConstrB, as per above). The only wrinkle now is with this term in the definition of f :
dataTypeOf $ B 1
We provide a value here (B 1) — is there a way to make it take the constructor (B) instead, or, alternatively, make f aware of the signature of the constructor (Int -> D) and/or the resulting data type (D) somehow? I've hoogled it and also looked through Data.Data with no luck…
It's just that constructing a full-blown value of type D might be non-trivial if D is complex, but it seems a bit wasteful as we are after the outer constructor alone. And of course, if D or types D depends on change we need to modify f…
Thank you, S.
The idiomatic way to handle this kind of stuff normally is to pass an abstract proxy that carries the type as a type argument, rather than passing a value of that type itself, eg.:
data Proxy a = Proxy
dataTypeOf :: Data a => Proxy a -> DataTypeOf
or even a more polymorphic version:
dataTypeOf :: Data a => f a -> DataTypeOf
which can be instantiated at any ‘f’, including Proxy, [], Maybe or others. Unfortunately, it does not seem that Data.Data.Data has gone for this route, so my feedback is a bit useless. You could still provide your own wrapper function (that uses ‘undefined’ internally, which we know here to be safe even though it's ugly), though.