invoking a procedure to print the prompt

[Dave Yost <Dave@xxxxxxxx>]

I need my prompt to contain one thing in one situation and another 
thing in another situation.

It occurs to me that a more clean and powerful way to implement a 
custom prompt would be via a procedure rather than via the 
traditional prompt string variable substitution syntax.

I can't find a way to do this in the zsh man page.

It seems to me that the only reason prompts weren't implemented using 
functions would have to be the historical accident that shell 
functions came many years later.

Dave