On Mon, Mar 10, 2014 at 10:47:26PM +0000, gb wrote:
Great! But how do I recover the actual constructor? E.g.,
f :: String -> Constr f s = fromMaybe (error "error in f") $ readConstr (dataTypeOf $ B 1) s
gives me back Data.Data.Constr (not D). I was hoping for something along
the lines
f "A" $ 1
to get back a value
A 1
of type D, etc.
Many thanks, S.
Here's one way:
import qualified Data.Generics.Builders as B import Data.Generics.Aliases
fromConstrB (B.empty `extB` (12::Int)) (f "B")::D
B 12
fromConstrB B.empty (f "B")::D
B 0
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. -- Семен Тригубенко http://trygub.com