IF

Conditional expression that evaluates one of two branches based on a boolean condition.

Syntax

IF condition THEN consequent ELSE alternative

Purpose

IF expressions select between two values based on a condition. Unlike imperative if-statements, L4's IF is an expression that always returns a value.

Examples

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 (using #CHECK since they use ASSUME-like params)
#CHECK `is adult` WITH `person age` IS 21
#CHECK `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

Basic If-Then-Else

IF TRUE THEN "yes" ELSE "no"

GIVEN age IS A NUMBER
DECIDE status age IS
  IF age >= 18 THEN "adult" ELSE "minor"

Nested Conditions

GIVEN score IS A NUMBER
DECIDE grade score IS
  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"

Multi-line Format

GIVEN x IS A NUMBER
DECIDE absolute x IS
  IF x >= 0
  THEN x
  ELSE 0 MINUS x

Boolean Expressions in Condition

GIVEN age IS A NUMBER
GIVEN hasLicense IS A BOOLEAN
DECIDE canDrive IS
  IF age >= 16 AND hasLicense
  THEN TRUE
  ELSE FALSE

DECIDE ... IF Form

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

GIVEN age IS A NUMBER
DECIDE isEligible IF age >= 18

This is shorthand for:

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

AND - Logical conjunction in conditions
OR - Logical disjunction in conditions
CONSIDER - Pattern matching alternative
CONTROL-FLOW - All control flow keywords

Note: THEN and ELSE are part of the IF syntax, not separate keyword pages.

IF