behaviours

[Thomas Lindgren]

--- Per Bergqvist <per@REDACTED> wrote:
> Still don't see why one would like to use a
> -behaviour if there 
> is no underlying support for it. It will basically
> be a no-op except
> for the warning. Am I missing something ?

As far as I know, all behaviours work that way: they
just verify that the module exports the right
functions.

I think, however, that user-defined behaviours are an
excellent idea: behaviours are useful because they
formalize an expected interface and provide the basis
for further documentation. A bit like Javas
interfaces, say.

(With user-defined behaviours, one also has the gotcha
that the behaviour files should be compiled before any
of those who use the behaviour.)

If a module has several behaviours, which I *do* think
should be possible, a linter should probably check
that their required functions do not overlap. In that
case, something is likely to be wrong.

Okay, that's the main reply. I have been thinking a
bit about behaviours, so here are some more things to
ponder:

* Should we keep around the fact that a module has a
behaviour, so that we can check this at runtime?
(e.g., gen_server can check that the callback module
has -behaviour(gen_server)). module_info/1 can keep
track of this easily.

(Strictly speaking, this is *not* the same thing as
the module exporting the functions required by
gen_server :-)

* Should behaviours also *require* some functions (ie,
yield an error rather than a warning if f/n is not
exported)?

* Should we have versioning of behaviours, beyond the
versioning of SASL? (Several versions of foo_b in the
system?)

* Finally, I would like a mechanism for composing
behaviours: roughly speaking, that 
    B = compose(B1,B2,B3) 
yields at runtime a "module" B that behaves like
modules B1 and B2 and B3. (Usable for any number of
Bi, preferrably)

There is a straightforward way of doing this --
dynamically generate and compile a new module -- but I
think that's a bit too heavyweight and might not quite
do the right thing.

Best,
Thomas


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