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IF

IF … THEN … ELSE chooses between two values by a BOOLEAN condition. It is an expression: it always gives a value, so the ELSE is required.

Syntax

IF condition THEN value-if-true ELSE value-if-false

IF condition
THEN value-if-true
ELSE value-if-false

Both branches must have the same type (Branch type mismatch). An IF without ELSE is a syntax error (IF without ELSE).

Example file:

-- IF keyword examples

-- Basic If-Then-Else

-- Simple conditional
DECIDE `basic if example` IS IF TRUE THEN "yes" ELSE "no"

-- With variable
GIVEN x IS A NUMBER
`check if positive` x MEANS IF x > 0 THEN "positive" ELSE "not positive"

#EVAL `basic if example`
#EVAL `check if positive` 5
#EVAL `check if positive` -3

-- Multi-line Format

GIVEN n IS A NUMBER
`absolute value of` n MEANS
  IF n >= 0
  THEN n
  ELSE 0 MINUS n

#EVAL `absolute value of` 5
#EVAL `absolute value of` -5

-- Nested If-Else (Chained Conditions)

GIVEN score IS A NUMBER
`letter grade for` score MEANS
  IF score >= 90 THEN "A"
  ELSE IF score >= 80 THEN "B"
       ELSE IF score >= 70 THEN "C"
            ELSE IF score >= 60 THEN "D"
                 ELSE "F"

#EVAL `letter grade for` 95
#EVAL `letter grade for` 75
#EVAL `letter grade for` 50

-- Complex Conditions

GIVEN age IS A NUMBER
      `has valid ID` IS A BOOLEAN
      `is member` IS A BOOLEAN
`can enter venue` age `has valid ID` `is member` MEANS
  IF (age >= 21 AND `has valid ID`) OR `is member`
  THEN "Welcome"
  ELSE "Access denied"

#EVAL `can enter venue` 25 TRUE FALSE
#EVAL `can enter venue` 18 TRUE FALSE
#EVAL `can enter venue` 18 FALSE TRUE

-- DECIDE ... IF Form (Boolean Functions)

-- Shorthand for boolean-returning functions
GIVEN `person age` IS A NUMBER
DECIDE `is adult` IF `person age` >= 18

-- Equivalent long form:
GIVEN `person age 2` IS A NUMBER
DECIDE `is adult 2` IS IF `person age 2` >= 18 THEN TRUE ELSE FALSE

-- Both work the same way; an input can also be given by name with WITH
#EVAL `is adult` 21
#EVAL `is adult 2` WITH `person age 2` IS 15

-- Returning Different Types

-- Returning numbers
GIVEN a IS A NUMBER, b IS A NUMBER
`maximum number` a b MEANS IF a > b THEN a ELSE b

-- Returning lists
GIVEN condition IS A BOOLEAN
`list if true otherwise empty` condition MEANS
  IF condition
  THEN LIST 1, 2, 3
  ELSE EMPTY

#EVAL `maximum number` 10 25
#EVAL `list if true otherwise empty` TRUE
#EVAL `list if true otherwise empty` FALSE

-- If in Complex Expressions

GIVEN n IS A NUMBER
GIVETH A NUMBER
`fibonacci number` n MEANS
  IF n <= 1
  THEN n
  ELSE `fibonacci number` (n MINUS 1) PLUS `fibonacci number` (n MINUS 2)

#EVAL `fibonacci number` 10

Examples

GIVEN age IS A NUMBER
GIVETH A STRING
`age group of` age MEANS IF age >= 18 THEN "adult" ELSE "minor"

GIVEN x IS A NUMBER
GIVETH A NUMBER
`absolute value of` x MEANS
  IF x >= 0
  THEN x
  ELSE 0 MINUS x

The condition can be any BOOLEAN expression, built with AND, OR, NOT:

GIVEN age IS A NUMBER
      `has a licence` IS A BOOLEAN
GIVETH A STRING
`driving status` age `has a licence` MEANS
  IF age >= 16 AND `has a licence`
  THEN "may drive"
  ELSE "may not drive"

Several conditions

An IF can go inside the ELSE of another. Each inner IF must be indented further than the one it belongs to:

GIVEN score IS A NUMBER
GIVETH A STRING
`grade for` score MEANS
  IF score >= 90 THEN "A"
  ELSE IF score >= 80 THEN "B"
       ELSE IF score >= 70 THEN "C"
            ELSE "F"

Lining up every ELSE IF in one column is an indentation error from the second one on. For more than two conditions, BRANCH is flatter.

A rule that is only a condition, such as "a person is eligible if …", is better written DECIDE … IF; see DECIDE IF.