(= (+ Kawa Derby) Fun)

A Kawa + Derby Tutorial


Kawa is an implementation of Scheme (and other languages) that compiles to Java byte code. Derby (also known as Cloudscape) is a SQL database engine written in Java which, as in this tutorial, can be run as an in-process server. Together, they are a complementary combination of elegant, functional programming (Scheme), an extensive library of utility code (Java's runtime libraries) and a powerful SQL database (Derby) that can be deployed with just a few JAR files.

  1. Download the Kawa .jar file and the Derby .jar files and put them in your CLASSPATH. Then you can play with Kawa's Scheme REPL like this:
    C:\harold>java kawa.repl
    #|kawa:1|# (+ 1 2)
    #|kawa:2|# (exit)
  2. As a first program, write a simple "Hello, World" program, compile it, and run it:
    C:\harold>cat > hello.scm
    (display "Hello, World!")
    C:\harold>java kawa.repl --main -C hello.scm
    (compiling hello.scm)
    C:\harold>java hello
    Hello, World!
  3. Then, try a simple database example to get started:


    ; Load the Derby JDBC driver
    (java.lang.Class:forName "org.apache.derby.jdbc.EmbeddedDriver")
    ; Connect, creating DB if needed
    (define uri "jdbc:derby:testdb;create=true")
    (define conn (java.sql.DriverManager:getConnection uri))
    ; Create a table
    (define stmt (*:createStatement conn))
    (*:execute stmt "create table t (c1 int)")
    ; Disconnect
    (*:close conn)
  4. Which, when run, works as expected (1):
    C:\harold>java kawa.repl --main -C dbtest.scm
    (compiling dbtest.scm)
    dbtest.scm:10:14: warning - no accessible method 'createStatement' in java.lang.Object
    dbtest.scm:11:1: warning - no accessible method 'execute' in java.lang.Object
    dbtest.scm:14:1: warning - no accessible method 'close' in java.lang.Object
    C:\harold>java dbtest
    C:\harold>java dbtest
    SQL Exception: Table/View 'T' already exists in Schema 'APP'.
    ... (exception stack) ...
  5. Here is some helper code, to get things going:
    ;;; ======================================
    ;;; Use Java Classes
    ;;; ======================================
    ; Set up some Kawa namespaces for invoking static methods
    ; SQL
    (define-namespace Class <java.lang.Class>)
    (define-namespace DrMgr <java.sql.DriverManager>)
    (define-namespace Conn  <java.sql.Connection>)
    (define-namespace Stmt  <java.sql.Statement>)
    ; Regular Expressions
    (define-namespace Pattern <java.util.regex.Pattern>)
    ;;; ======================================
    ;;; Globals
    ;;; ======================================
    ; Current connection
    (define conn :: <java.sql.Connection> #!null)
    ;;; ======================================
    ;;; Functions
    ;;; ======================================
    ; Connect, creating the DB if needed
    (define (connect)
        (Class:forName "org.apache.derby.jdbc.EmbeddedDriver")
        (set! conn (DrMgr:getConnection "jdbc:derby:testdb;create=true")))
    ; Disconnect from DB
    (define (disconnect)
        (*:close conn)
        (set! conn #!null))
    ; Execute a list of SQL statements with no results
    (define (runSQL . sqlStrings)
        (let ((stmt (*:createStatement conn)))
          (map (curry *:execute stmt) sqlStrings)))
    ; Execute a single SQL statement and return the result
    (define (getSQL sqlString)
        (let ((stmt (*:createStatement conn)))
          (*:executeQuery stmt sqlString)
          (*:getResultSet stmt)))
    ; Run a function across the result sets of a query
    (define (map-rs fn rs)
        (letrec ((loop (lambda ()
                         (if (*:next rs) 
                             (begin (fn rs) (loop)) #t))))
    (define (curry fn . args)
        (lambda moreArgs
          (apply fn (append args moreArgs))))
    (define (displayn . args)
        (apply display args)
    (define (regex pattern)
        (let ((pattern (Pattern:compile pattern)))
          (lambda (string)
            (let ((matcher (*:matcher pattern string)))
              (if (*:find matcher 0) matcher #f)))))
  6. Now it is even easier to write a nice program to create a table if it doesn't exist, insert a row, and query the table:
    ; Create the table if it does not already exist
     (begin (runSQL "create table t1 (tabname char(20), created date)")
            (displayn "Table T1 Created..."))
     (e <java.sql.SQLException>
        (displayn "Table T1 Exists...")))
    ; Insert a row
    (runSQL "insert into t1 values ('Hello', current date)")
    ; Print out the contents of the table
    (map-rs (lambda (rs) 
              (display (*:getString rs 1))
              (display " - ")
              (display (*:getString rs 2))
            (getSQL "select * from t1"))


1: The warnings mean that Scheme's dynaming typing cannot guarantee the method exists on the object you reference. You can use the :: <Java Type> syntax to give a variable a type, but it seems to misbehave if you use an interface (like ResultSet) as the type instead of a class.