automatic type cast to numeric ?

Gretings!

I try to change from using of float8 to numeric... For quantitative
calculations and storage the float8 wasn't the best choice;)
For some reasons i need automatic or explicit type cast from text to
numeric and vice versa.
But:
test=# select '123'::text::numeric;
ERROR:  Cannot cast type 'text' to 'numeric'
;(
test=# CREATE FUNCTION numeric(text) as 'SELECT float8($1)::numeric;'
LANGUAGE 'SQL';
ERROR:  parser: parse error at or near "numeric"
;(((
The another side (numeric to text) works...

I had some more clumsy shots with the type '_numeric', but i don't now
enough about the working of parametric types...

So i need Help;)

--
 thanx
 nek.

i use 7.0.2/7.0.3 i386.rpm-s