Not long ago I was searching the Internet on the topic "combining eager and
lazy evaluation", and have run over a paper which I obviously dismissed as
"not interesting enough", yet now I have realized that it could indeed be
useful, but am unable to find it.

I know it was talking about a useful primitive, I do not know how exactly it
was named, which checked whether values passed as arguments to functions
were lazy (blocks to be evaluated) or eager (already evaluated), and using
it some functions, e.g. map (this example was present in the paper) could be
implemented to be both eager and lazy at the same time, depending on the
arguments.

Does anyone recognize this description?

 - Tom