Control Flow

Learning Echo from scratch?

Start with Lesson 3 — Making decisions and Lesson 4 — Repeating things. This page is the full control-flow surface.

if / switch / while / for / foreach, plus break and continue. Conditions use Echo truthiness. switch is value dispatch (literals, type arms, destructuring) — not error handling.

Truthiness

Used by if, while, &&, ||, !, and default().

Falsy values: false, null, 0, 0.0, "", [], {}.

Every other value is truthy.

Full contract: Language Semantics — Truthiness.

Basic Example

for i: int in 0..5 {
    if i == 2 {
        continue;
    }

    if i == 4 {
        break;
    }

    say(i);
}

Output

0
1
3

How It Works

x: int = 1;
condition: bool = true;
items: list = ["a", "b"];

if condition {
    say("yes");
} else {
    say("no");
}

switch x {
    0 { say("zero"); }
    1 { say("one"); }
    else { say("other"); }
}

i: int = 0;
while i < 1 {
    say("loop");
    i = i + 1;
}

for n: int in 0..2 by 1 {
    say(n);
}

foreach item: str in items {
    say(item);
}

Output

yes
one
loop
0
1
2
a
b

if, else, and else if

else if is supported in source form.

while

Runs while the condition stays truthy.

for

• .. means inclusive end
• ... means exclusive end
• by sets the step
• by 0 aborts (E2702)
• loop variable type must be int
• The same .. / ... / by spelling is a list of int in expression position (xs: list = 0...5;)

foreach

Each item is checked against the declared loop variable type at runtime. A range expression is a list, so foreach over 0...3 iterates that list.

Common Mistakes

Using the wrong loop variable type in for — E1008

for i: str in 0..10 {
    say(i);
}

Using break outside a loop — E1004

break;

That is a semantic error (E1004), not a syntax error.

Forgetting braces

Echo does not support implicit blocks.

for ... by 0 — E2702

Aborts (E2702). It does not loop forever.

Related

• Loops Reference
• Operators
• Language Semantics
• Errors and Troubleshooting