carbon-lang/docs/design/control_flow/conditionals.md

Conditionals

Table of contents

• Overview
• Alternatives considered
• References

Overview

if and else provide conditional execution of statements. Syntax is:

if (boolean expression ) { statements }

[ else if ( boolean expression ) { statements } ] ...

[ else { statements } ]

Only one group of statements will execute:

• When the first if's boolean expression evaluates to true, its associated statements will execute.
• When earlier boolean expressions evaluate to false and an else if's boolean expression evaluates to true, its associated statements will execute.
  • ... else if ... is equivalent to ... else { if ... }, but without visible nesting of braces.
• When all boolean expressions evaluate to false, the else's associated statements will execute.

When a boolean expression evaluates to true, no later boolean expressions will evaluate.

Note that else if may be repeated.

For example:

if (fruit.IsYellow()) {
  Print("Banana!");
} else if (fruit.IsOrange()) {
  Print("Orange!");
} else if (fruit.IsGreen()) {
  Print("Apple!");
} else {
  Print("Vegetable!");
}
fruit.Eat();

This code will:

• Evaluate fruit.IsYellow():
  • When True, print Banana! and resume execution at fruit.Eat().
  • When False, evaluate fruit.IsOrange():
    • When True, print Orange! and resume execution at fruit.Eat().
    • When False, evaluate fruit.IsGreen():
      • When True, print Apple! and resume execution at fruit.Eat().
      • When False, print Vegetable! and resume execution at fruit.Eat().

Alternatives considered

• Optional braces
• Optional parentheses
• elif

References

• Proposal #285: if and else
• Proposal #623: Require braces