I'm using wl-pprint to output a data type with its fields rendered and separated with spaces. Some of the fields can be Nothing, so I render them as empty. Obviously, I don't want to add an extra space next to an empty string. It was easily achievable with HughesPJ library, as its <+> operator has empty as unit. In wl-pprint the space in <+> is not optional however.
It would be easy to implement myself, given a function to test if Doc is empty. My another question is didn't anyone ever need to check for an empty Doc? Is it considered to be a drawback of my algorithm design?
This answers the "test if Doc is empty" part.
renderCompact gives you a SimpleDoc which you can pattern-match for SEmpty.
It seems sufficiently lazy, e.g.
case ( renderCompact $ vcat $ repeat $ text "foobar" ) of
SEmpty -> True ; _ -> False