Programs and Bodies

Program units

A program-unit consists of a sequence of definitions and expressions.

program-unit ::= library-definition+ [statements]
  | statements
statements ::= statement+
statement ::= definition | expression | (begin statement* )

Typically a program-unit corresponds to a single source file (i.e.a named file in the file system). Evaluating a program-unit first requires the Kawa processor to analyze the whole program-unit to determine which names are defined by the definitions, and then evaluates each statement in order in the context of the defined names. The value of an expression is normally discared, but may be printed out instead, depending on the evaluating context.

The read-eval-print-loop (REPL) reads one or more lines until it gets a valid program-unit, and evaluates it as above, except that the values of expression are printed to the console (as if using the display function). Then the REPL reads and evaluates another program-unit, and so on. A definition in an earlier program-unit is remembered and is visible in a later program-unit unles it is overridden.

A comment in the first 2 lines of a source file may contain an encoding specification. This can be used to tell the reader what kind of character set encoding is used for the file. This only works for a character encoding that is compatible with ASCII (in the sense that if the high-order bit is clear then it’s an ASCII character), and that are no non-ASCI characters in the lines upto and including the encoding specification. A basic example is:


In general any string that matches the following regular expression works:



A program-unit may contain library-definitions. In addition, any statements in program-unit comprise an implicit library, in that it can be given a name, and referenced from other libraries. Certain names defined in the program-unit an be exported, and then they can be imported by other libraries For more information see Module classes.

It is recommended but not required that:

Following these recommendations makes it easier to locate and organize libraries. However, having multiple libraries in a single program-unit is occasionally useful for source distribution and for testing.


The body of a lambda, let, let*, let-values, let*-values, letrec, or letrec* expression, or that of a definition with a body consists of zero or more definitions or expressions followed by a final expression. (Standard Scheme requires that all definitions precede all expressions.)

body ::= statement*

Each identifier defined by a definition is local to the body. That is, the identifier is bound, and the region of the binding is the entire body. Example:

(let ((x 5))
  (define foo (lambda (y) (bar x y)))
  (define bar (lambda (a b) (+ (* a b) a)))
  (foo (+ x 3)))
⇒ 45

When begin, let-syntax, or letrec-syntax forms occur in a body prior to the first expression, they are spliced into the body. Some or all of the body, including portions wrapped in begin, let-syntax, or letrec-syntax forms, may be specified by a macro use.

An expanded body containing variable definitions can be converted into an equivalent letrec* expression. (If there is a definition following expressions you may need to convert the expressions to dummy definitions.) For example, the let expression in the above example is equivalent to

(let ((x 5))
  (letrec* ((foo (lambda (y) (bar x y)))
            (bar (lambda (a b) (+ (* a b) a))))
    (foo (+ x 3))))