Can't get postgres to recognise input variables in a function - postgresql

Question

What am I doing wrong here?
I want to write a function that returns a table of all the products that we have had dismal
performance for between two dates - where 'dismal' means marketing costs exceeded profits. I
have three tables: products, spend and transactions.
CREATE TABLE products
(id int not null, name varchar(255), primary key (id));
CREATE TABLE spend
(id int not null, spenddate date, spendamount decimal (9, 2));
CREATE TABLE transactions
(id int not null, transactiondate date, profit decimal (9, 2));
What I'd do is union queries between the two tables and then sum them to get a line per
product:
WITH a as (
SELECT products.id, products.name,
sum(transactions.profit) as profit,
sum(0) as spendamount
FROM transactions
WHERE transactiondate BETWEEN startdate AND enddate
GROUP BY products.id, products.name
UNION ALL
SELECT id, sum(0) as profit
sum(amount) as spendamount
FROM spend
WHERE spenddate BETWEEN startdate AND enddate
GROUP BY id)
SELECT products.id, products.name, sum(profit)-sum(spendamount) as loss
FROM a, products
WHERE products.id = a.id
GROUP BY products.id, products.name
HAVING sum(profit)-sum(spendamount) < 0;
But I don't want to keep changing the start and enddate values every time I run my code.
I thought I'd do this:
CREATE FUNCTION report_dismal_products (startdate date, enddate date,
out id int,
out name varchar(255),
out loss decimal(9, 2)
)
RETURNS SETOF RECORD
as $$
SELECT products.id, products.name, sum(profit)-sum(spendamount) as loss
FROM
(
SELECT id,
sum(transactions.profit) as profit,
sum(0) as spendamount
FROM transactions
WHERE transactiondate BETWEEN startdate AND enddate
GROUP BY id
UNION ALL
SELECT id, sum(0) as profit
sum(amount) as spendamount
FROM spend
WHERE spenddate BETWEEN startdate AND enddate
GROUP BY id) a, products
WHERE products.id = a.id
GROUP BY products.id, products.name
HAVING sum(profit)-sum(spendamount) < 0;
$$ Language sql;
But it returns
ERROR: column "startdate" does not exist
LINE 17: WHERE transactiondate BETWEEN startdate AND enddate