Re: Are all functions multi-methods?

[Bruce Hoult <address@hidden>]

In article <3d22f179_1@news.iprimus.com.au>,
 "Steven Shaw" <steven_shaw@iprimus.com.au> wrote:

> In Dylan are all functions multimethods or can you define normal function,
> too?

Sure.  Just define only one function with that name and the compiler 
will do the right thing :-)


OK, that's not quite the whole story.  In early Dylan there was "define 
generic" and "define method", which are declarations at toplevel that 
create generic functions and add individual methods to them.  Then there 
was also the "method() ... end" expression, which creates a naked, 
anonymous, function that doesn't have multimethod dispatch.  People 
started to want to use these in named contexts because they can be a tad 
more efficient than generic functions containing a single method (at 
least with crude implementations), and also as a kind of 
documentation/promise that there would only ever be a single function 
implementing that particular method.  So they started doing things like:

  define constant foo = method(a, b) ... end;

After a while this became tedious, so someone created a "define 
function" macro that expended to the "define constant" above.  This 
isn't in the DRM, but it's in all current Dylan implementations.


Also, you'll find a number of the built-in functions in Dylan are 
actually specified to be "functions" rather than "generic functions" 
e.g. "<" is a generic function (which the programmer can extend for new 
data types), while ">" is a simple function (which just calls "<" with 
the arguments reversed).  This dates back to the Lisp-syntax version of 
the language.

-- Bruce