problem with function in prompt

[Stephen Prater <stephenp@agrussell.com>]

I have the following function in a sourced file..

function free_mem() {
   FM=$(ruby -e '
     x = `vm_stat`.each_line.to_a[0..4].join.scan(/([0-9]*?) 
\./).flatten.map { |i| (i.to_i * 4096) / 1000000 };
     y = `sysctl hw.memsize`.split(/:\s/).last.to_i / 1000000;
     puts "%F{82}#{x[0]}%f/%F{yellow}#{x[2]}%f/%F{69}#{y}%f";')
   echo $FM
}

and then in my prompt i have this:

$(free_mem)

The problem is that it never re-executes the function in the prompt ----

setopt prompt_subst

PROMPT_INFO="$(draw f21 ul hbar) $(host_name)- %L
$(draw f21 ll hbar) $(dir_name) $(free_mem)"
DEF_PROMPT="${(e)${PROMPT_INFO}} %F{cyan} [ %f"
INS_MODE_PROMPT="${(e)${PROMPT_INFO}} %F{red} [ %f"


The value that appears in the prompt is always the same as the one  
when the shell was initially executed.

I tried placing it in a variable within precmd and then using that  
variable in the prompt like this:

precmd() {
	FREE_MEM=$(free_mem)
}
PROMPT="$FREE_MEM"

but that didn't work either

If I call free_mem directly from the prompt (or echo FREE_MEM) it  
outputs the correct string... so what am I missing?

stephen