1. REBOL
Writing Functions

2. Four ways to define functions
To define a function with parameters and local variables: function spec locals actions
Example: plus: function [x y] [sum] [sum: x + y sum]
To define a function with parameters: func spec actions
Example: plus: func [x y] [x + y]
To define a function with no parameters: does actions
Example: sing: does [print "do re mi"]
To define a function with local variables but no parameters: has locals actions
Example: bigrand: has [x y] [x: random 10 y: random 10 max x y]

3. The “spec” and formal parameters
In both function spec locals actions and func spec actions, the spec is a block
The first element of the spec may be a documentation string
The following elements are the names of formal parameters
Each formal parameter may be followed by a block containing the acceptable datatypes for that parameter
There may be refinements (words prefixed by a backslash)
Inside the function, the refinement acts like a boolean variable
There may be local variables following /local
Example:
sum: func ["Sum or average" nums [block!] /average /local total] [ total: foreach num nums [total: total + num] either average [total / (length? nums)][total] ]

4. Return values The actions is a block of expressions to be evaluated
The value returned by a function is the value of the last evaluated action
To return before reaching the end, use return value
To return without a value, use exit

5. Looking at functions
To see if a word is a function, use function? word
To see the source code of a function, use source functionName
To see the documentation string, use help functionName
>> sum: func ["Adds two numbers" x y] [x + y]
>> sum 3 5 == 8
>> help sum USAGE: SUM x y DESCRIPTION: Adds two numbers SUM is a function value. ARGUMENTS: x -- (Type: any) y -- (Type: any)

6. The End