DEF Keyword

DEF FNname(param, ...)
DEF PROCname(param, ...)
    ...
ENDPROC

Defines a function (FN) or procedure (PROC). The parameter list is optional.

• A function must end with a line that begins with =; the expression after = is the return value.
• A procedure ends with ENDPROC (it does not return a value).

Remarks

There is no space between FN/PROC and the identifier - in both definitions and calls.

Examples: DEF FNadd(A,B), PRINT FNadd(1,2), DEF PROCbox(M$), PROCbox("Hi")

GLOBAL is only used to pass variables into child programs spawned with

CHAIN.

It does not declare "global variables" for program-wide scope inside a single program.

Use parameters and LOCAL to manage scope within your own code.

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Calling functions and procedures

• Call a function within an expression:

  RESULT = FNadd(10, 32)

• Invoke a procedure as a statement:

  PROCbox("HELLO")

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Example: function (returns a value)

DEF FNadd(A, B)
    SUM = A + B
= SUM
 
PRINT FNadd(1, 2)    : REM prints 3

The final line starts with = and supplies the return value of the function.

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Example: procedure (no return value)

DEF PROCbox(MSG$)
    PRINT "**************"
    PRINT "* "; MSG$; " *"
    PRINT "**************"
ENDPROC
 
PROCbox("HELLO")

ENDPROC marks the end of the procedure body.

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Example: locals within a definition

DEF FNhypot(X, Y)
    LOCAL XSQ = X * X
    LOCAL YSQ = Y * Y
= SQRT(XSQ + YSQ)

LOCAL creates temporary variables scoped to the definition.

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Rules and behaviour

• Parameters
  The list may be empty. Parameters are available by name in the body.
  Use LOCAL for additional temporaries.
• Function result
  Inside a DEF FN... block, the last line must start with = followed by an expression.
  Using a leading = outside a function is an error.
• Procedure termination
  Procedures must end with ENDPROC. Using ENDPROC outside a procedure is an error.
• Errors
  Calling an undefined FN/PROC, or passing the wrong number of arguments, raises a runtime error
  (catchable with ON ERROR).

See also:
FN · PROC · ENDPROC · LOCAL · CHAIN · Variable Naming · Parameter types