Creating a boolean function

by the way you know how to create the following sql function?

I need to return a boolean saying if an object is onloan.

the loan table is as follows:

loans
-----
code_loan
code_user
typetable

i want to check these three fields, the first two fields (the code ones) are
integer and the last a string.

I tried

create function check_loan (text,int,int)
returns bool
as
'select CAST(typetable AS text) from loans where typetable=$1 and
code_user=$2 and code_loan=$3;'
language 'sql';

But I got that the parser cannot identify the =$ operator and requires me to
do the proper casting.

How can I do it? Sorry for the questions, but I am new at creating functions
and I have to hand in this project tomorrow.

Many thanks

Miguel