CONSTANT

The CONSTANT instruction defines a program constant.

Syntax

[PRIVATE|PUBLIC] CONSTANT
 identifier [ data-type ] = literal
 [,...]

identifier is the name of the constant to be defined, that must follow the convention for identifiers.
data-type can be any primitive data type, except complex types like TEXT or BYTE.
literal must be an integer, decimal, string, or date/time, interval literal, or an MDY() expression.
literal cannot be NULL.

Usage

Define constants to name static values that do not change during the program execution.

Constants are similar to variables, except that its value cannot be modified.

Constants can be defined with global, module, or function scope.

By default, module constants are private; they cannot be used by an other module of the program. To make a module constant public, add the PUBLIC keyword before CONSTANT. When a module constant is declared as public, it can be referenced by another module by using the IMPORT instruction.

When declaring a constant, the data type specification can be omitted. The literal value automatically defines the data type:

CONSTANT c1 = "Drink" -- Declares a STRING constant 
CONSTANT c2 = 4711    -- Declares an INTEGER constant

However, in some cases, you may need to specify the exact data type for a constant:

CONSTANT c1 SMALLINT = 12000 -- Would be an INTEGER by default

Constants can be used in variable, records, and array definitions:

CONSTANT n = 10
DEFINE a ARRAY[n] OF INTEGER

Constants can be used at any place in the language where you normally use literals:

CONSTANT n = 10
FOR i=1 TO n
  ...

Constants can be passed as function parameters, and returned from functions.

Define public constants in a module to be imported by others:

PUBLIC CONSTANT pi = 3.14159265

For date time constants, the value must be specified as an MDY() literal, DATETIME literal or INTERVAL literal:

CONSTANT my_date DATE = MDY(12,24,2011)
CONSTANT my_datetime DATETIME YEAR TO SECOND
              = DATETIME(2011-12-24 11:22:33) YEAR TO SECOND
CONSTANT my_interval INTERVAL HOUR(5) TO FRACTION(3)
              = INTERVAL(-54351:50:24.234) HOUR(5) TO FRACTION(3)

A constant cannot be used in the ORDER BY clause of a static SELECT statement, because the compiler considers identifiers after ORDER BY as part of the SQL statement (i.e. column names), not as constants:

CONSTANT pos = 3
-- Next line will produce an error at runtime
SELECT * FROM customers ORDER BY pos

Automatic data type conversion can take place in some cases:

CONSTANT c1 CHAR(10) = "123"
CONSTANT c2 CHAR(10) = "abc"
DEFINE i INTEGER
FOR i = 1 TO c1 -- Constant "123" is converted to 123 integer
   ...
FOR i = 1 TO c2 -- Constant "abc" is converted to zero!
   ...

Character constants defined with a string literal that is longer than the length of the data type are truncated:

CONSTANT s CHAR(3) = "abcdef"
DISPLAY s  -- Displays "abc"

The compiler throws an error when an undefined symbol is used in a constant declaration:

CONSTANT s CHAR(c) = "abc"
-- Compiler error: c is not defined.

The compiler throws an error when a variable is used in a constant declaration:

DEFINE c INTEGER
CONSTANT s CHAR(c) = "abc"
-- Compiler error: c is a variable, not a constant.

The compiler throws an error when you try to assign a value to a constant:

CONSTANT c INTEGER = 123
LET c = 345
-- Runtime error: c is a constant.

The compiler throws an error when the symbol used is not defined as an integer constant:

CONSTANT c CHAR(10) = "123"
DEFINE s CHAR(c)
-- Compiler error: c is a not an integer constant.

You typically define common special characters with constants:

CONSTANT c_esc  = '\x1b'
CONSTANT c_tab  = '\t'
CONSTANT c_cr   = '\r'
CONSTANT c_lf   = '\n'
CONSTANT c_crlf = '\r\n'