How PG parser search (build-in) function?

Hi,

I have a question... How PG parser select (build-in) function?
Check parser function's arguments datetypes only or check returns value 
datetype too (for function searching in system cache ..etc)?

I have function to_number(text, text) this function returns numeric 
datetype. Is possible (effective) write this function for more 
datatypes than for numeric? The function is always "to_number(text, text)",
possible difference is in a returned datetype. 

(The function extract number from formated string and this number 
return, but how datetype we want return?)
                        Karel

PS. Sorry if my problem description is a litle mazy..

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Karel Zak <zakkr@zf.jcu.cz>              http://home.zf.jcu.cz/~zakkr/

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