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DECIDE

Defines a named value or function with a body. The primary way to create computations in L4.

Syntax

DECIDE name IS expression
DECIDE name MEANS expression
name MEANS expression

Forms

Named Values

DECIDE pi IS 3.14159
DECIDE greeting IS "Hello, World!"

Functions with GIVEN

GIVEN x IS A NUMBER
DECIDE double x IS x TIMES 2

Using MEANS

MEANS introduces the body of a function:

GIVEN n IS A NUMBER
DECIDE factorial n MEANS
  IF n EQUALS 0
  THEN 1
  ELSE n TIMES factorial (n MINUS 1)

Omitting DECIDE

For functions, DECIDE can be omitted:

GIVEN x IS A NUMBER
double x MEANS x TIMES 2

Examples

Example file:

-- DECIDE keyword examples

-- Simple Named Values

DECIDE `the answer` IS 42
DECIDE `approximate pi` IS 3.14159
DECIDE `welcome greeting` IS "Hello, World!"
DECIDE `is enabled` IS TRUE

#EVAL `the answer`
#EVAL `welcome greeting`

-- Functions with DECIDE and MEANS

-- Simple function using MEANS
GIVEN x IS A NUMBER
`double a number` x MEANS x TIMES 2

#EVAL `double a number` 21

-- Function with DECIDE ... IS
GIVEN n IS A NUMBER
DECIDE `square of` n IS n TIMES n

#EVAL `square of` 7

-- Conditional Decisions

GIVEN age IS A NUMBER
`age category for` age MEANS
  IF age < 13 THEN "child"
  ELSE IF age < 20 THEN "teenager"
       ELSE IF age < 65 THEN "adult"
            ELSE "senior"

#EVAL `age category for` 25
#EVAL `age category for` 10

-- Boolean Decisions with IF

ASSUME `person age` IS A NUMBER
ASSUME `has valid ID` IS A BOOLEAN
DECIDE `can enter venue` IF
  `person age` >= 21 AND `has valid ID`

#CHECK `can enter venue`

-- Decisions with WHERE Clauses

GIVEN radius IS A NUMBER
DECIDE `area of circle with radius` radius IS
  `local pi` TIMES radius TIMES radius
  WHERE
    `local pi` MEANS 3.14159

#EVAL `area of circle with radius` 10

-- Recursive Decisions

GIVEN num IS A NUMBER
GIVETH A NUMBER
`factorial of` num MEANS
  IF num EQUALS 0
  THEN 1
  ELSE num TIMES `factorial of` (num MINUS 1)

#EVAL `factorial of` 5

-- Multi-parameter Functions

GIVEN a IS A NUMBER
      b IS A NUMBER
`maximum of two numbers` a b MEANS IF a > b THEN a ELSE b

#EVAL `maximum of two numbers` 10 25

Simple Values

DECIDE answer IS 42
DECIDE isEnabled IS TRUE
DECIDE message IS "Welcome"

Conditional Decisions

GIVEN age IS A NUMBER
DECIDE isAdult age IS
  IF age >= 18
  THEN TRUE
  ELSE FALSE

With WHERE Clauses

DECIDE circleArea radius IS
  pi TIMES radius TIMES radius
  WHERE
    pi MEANS 3.14159

Boolean Decisions with IF

For boolean-returning functions, use DECIDE ... IF:

GIVEN age IS A NUMBER
GIVEN income IS A NUMBER
DECIDE isEligible IF
  age >= 18 AND income > 30000
  • MEANS - Introduces the function body
  • GIVEN - Introduces parameters
  • GIVETH - Specifies return type
  • WHERE - Local definitions
  • IF - Boolean condition form

Tutorials