scott@ORA92> -- lookup table that function selects values from:
scott@ORA92> DESC lookup_table
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 LOOKUP_ID                                                      NUMBER
 LOOKUP_VALUE                                                   VARCHAR2(5)

scott@ORA92> SELECT * FROM lookup_table
  2  /

 LOOKUP_ID LOOKU
---------- -----
         1 TEST1
         2 TEST2
         3 TEST3
         4 TEST4
         5 TEST5

scott@ORA92> -- function:
scott@ORA92> CREATE OR REPLACE FUNCTION <b>test_function</b>
  2    (p_lookup_id IN lookup_table.lookup_id%TYPE)
  3    RETURN VARCHAR2
  4  AS
  5    v_lookup_value lookup_table.lookup_value%TYPE;
  6  BEGIN
  7    SELECT lookup_value
  8    INTO   v_lookup_value
  9    FROM   lookup_table
 10    WHERE  lookup_id = p_lookup_id;
 11    RETURN v_lookup_value;
 12  EXCEPTION
 13    WHEN NO_DATA_FOUND THEN RETURN NULL;
 14  END test_function;
 15  /

Function created.

scott@ORA92> SHOW ERRORS
No errors.

scott@ORA92> -- contents of text file test.dat that has values used in loading data:
1 
2 
3 
4
5 

scott@ORA92> -- empty table to load data into
scott@ORA92> DESC test_table
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 TEST_VALUE                                                     VARCHAR2(5)

scott@ORA92> SELECT * FROM test_table
  2  /

no rows selected

scott@ORA92> -- contents of SQL*Loader control file test.ctl:
LOAD DATA
INFILE 'test.dat'
REPLACE
INTO TABLE test_table
FIELDS TERMINATED BY WHITESPACE
TRAILING NULLCOLS
  (test_value "<b>test_function</b> (:test_value)")

scott@ORA92> -- load data:
scott@ORA92> host sqlldr scott/tiger control=test.ctl log=test.log

scott@ORA92> -- results:
scott@ORA92> SELECT * FROM test_table
  2  /

TEST_
-----
TEST1
TEST2
TEST3
TEST4
TEST5