Hi Pedro,
I'm not sure there is a nice solution - reflection at the type level (using _|_ at the value level), combined with strictness at the value level, has limitations. It may be that the reflection machinery in SYB can be tweaked to either alert the user in advance (i.e. by getting the strictness of various fields), or providing some operation combining gmapQ and fromConstr which isn't strict. To see my use case take a look at "contains" in:
http://www.cs.york.ac.uk/fp/darcs/uniplate/Data/Generics/PlateData.hs
I'm not sure there's an easy solution either. As you say, the problem here seems to be caused by the strictness. Getting the strictness of each field would require changes to the representation types and to the deriving mechanism.
Would your problem be solved if you used fromConstrB instead of simply fromConstr and built an entirely determined (without bottoms) value?
I might be able to use fromConstrB, but it requires a lot more work - initialising lots of things and creating lots of dummy values. I'll look into it. I also note that the documentation for fromConstrB seems to have disappeared. See: http://haskell.org/ghc/docs/latest/html/libraries/syb/doc-index.html - the entry is still there but the link is gone. (I'm also aware that the Hoogle documentation for it is missing, but hope to fix that this weekend - I've had issues trying to build things) Thanks Neil