Part of what makes genre challenging is that it has two moments, which we
might call by the old-fashioned terms "form" and "function". One can attempt
a formalist definition of, say, the survey article in medicine or the blues
song. Or one can attempt a functionalist definition, looking at how the
survey article in medicine is well-fitted to the interests and strategies
of both authors and readers. Obviously, neither a formalist nor a functionalist
definition will cut it empirically. A genre might arise through functional
adaptation (for example by appropriating or modifying or hybridizing or
opposing oneself to an existing genre), but then once people get a sense of
a stable form, that form might take on a life of its own -- it will be copied
and codified, students will be told "this is a good one; do it like this",
a critic will come along and establish norms and canons, diversions from the
settled pattern will take on significance, and so on. Sometimes these forces
will amount to a homeostatic mechanism, and sometimes they will have some
other overall effect. And when functional needs really do change, genres
often do change too, eventually. So genres hover in suspension between
formalist and functionalist sorts of concerns.
Types of form. I'll bet that others here can provide a much better survey
than I of the range of types of form that have been mentioned in attempts to
characterize particular genres. Sequential organization, register, common
or rare syntactic forms, rhetorical and logical organization, modes of
intertextuality, graphic devices like bullets and typefaces, etc. Geoff
Nunberg tells me that if you want to build a computer to automatically
recognize register, one of the most powerful discriminating variables is
punctuation, e.g., semicolons. Perhaps the best approach is not to think
of "genre" as one more category of form, but rather to think of genres as
ensembles of different types of form (this kind of narrative structure,
that kind of relationship to other texts, the other kind of diction, short
paragraphs and no semicolons, certain recurring tropes, etc).
Phil Agre