Control Flow

Conditionals, loops, and exception handling.

Conditionals

If / Elsif / Else

my int $x = 10;

if ($x > 10) {
    say("Greater than 10");
} elsif ($x == 10) {
    say("Equal to 10");
} else {
    say("Less than 10");
}

Unless

unless is the opposite of if — it executes when the condition is false:

my int $logged_in = 0;

unless ($logged_in) {
    say("Please log in");
}

# With else (no elsif allowed with unless)
unless ($logged_in) {
    say("Please log in");
} else {
    say("Welcome back");
}

# Equivalent to:
if (!$logged_in) {
    say("Please log in");
}

Ternary Operator

my int $age = 20;
my str $status = $age >= 18 ? "adult" : "minor";
say($status);  # "adult"

Loops

While Loop

my int $i = 0;
while ($i < 5) {
    say($i);
    $i++;
}

Until Loop

until is the opposite of while — it loops while the condition is false:

my int $count = 0;
until ($count >= 5) {
    say($count);
    $count++;
}
# Equivalent to: while ($count < 5) { ... }

Do-While Loop

my int $i = 0;
do {
    say($i);
    $i++;
} while ($i < 5);

For Loop

for (my int $i = 0; $i < 5; $i++) {
    say("i = " . $i);
}

Foreach Loop

Iterate over arrays:

my array @fruits = ["apple", "banana", "cherry"];

# With type declaration
foreach my str $fruit (@fruits) {
    say($fruit);
}

# Without type (uses existing variable)
my str $item;
foreach $item (@fruits) {
    say($item);
}

# "for" works like "foreach" with arrays
for my str $fruit (@fruits) {
    say($fruit);
}

Increment/Decrement

my int $x = 5;

$x++;        # Post-increment: returns 5, then x becomes 6
++$x;        # Pre-increment: x becomes 7, returns 7
$x--;        # Post-decrement: returns 7, then x becomes 6
--$x;        # Pre-decrement: x becomes 5, returns 5

# Often used in loops
my int $i = 0;
while ($i < 10) {
    say($i++);  # Print then increment
}

Loop Control

for (my int $i = 0; $i < 10; $i++) {
    if ($i == 3) {
        next;      # Skip to next iteration
    }
    if ($i == 7) {
        last;      # Exit loop
    }
    say($i);
}
# Output: 0, 1, 2, 4, 5, 6

# redo - restart current iteration without re-checking condition
my int $attempts = 0;
while ($attempts < 3) {
    $attempts++;
    my str $input = readline();
    if ($input eq "") {
        say("Input required");
        redo;    # Restart iteration (don't re-check or advance)
    }
}

Labeled Loops

Labels allow breaking out of nested loops:

OUTER: for (my int $i = 0; $i < 5; $i++) {
    INNER: for (my int $j = 0; $j < 5; $j++) {
        if ($j == 2) {
            next INNER;   # Skip to next inner iteration
        }
        if ($i == 3) {
            last OUTER;   # Exit both loops
        }
        say("i=" . $i . " j=" . $j);
    }
}

Functional Array Operations

Strada supports Perl-style map, grep, and sort for powerful array transformations. Use $_ for the current element.

Map (Transform)

my array @nums = (1, 2, 3, 4, 5);

# Transform each element
my array @doubled = map { $_ * 2 } @nums;
# Result: (2, 4, 6, 8, 10)

Map to Hash (Perl Idiom)

Create lookup hashes using the fat arrow (=>):

my array @fruits = ("apple", "banana", "cherry");

# The classic Perl idiom - works in Strada!
my hash %lookup = map { $_ => 1 } @fruits;

# Fast O(1) membership testing
if (exists($lookup{"apple"})) {
    say("Found apple!");
}

Grep (Filter)

my array @nums = (1, 2, 3, 4, 5, 6);

# Keep only even numbers
my array @evens = grep { $_ % 2 == 0 } @nums;
# Result: (2, 4, 6)

Sort

my array @nums = (5, 2, 8, 1, 9);

# Numeric sort (ascending) - use $a, $b with spaceship
my array @asc = sort { $a <=> $b } @nums;

# Descending
my array @desc = sort { $b <=> $a } @nums;

Chaining Operations

# Filter, transform, and sort in one line
my array @result = sort { $a <=> $b } map { $_ * 10 } grep { $_ > 3 } @data;

Exception Handling

Try / Catch / Throw

func divide(int $a, int $b) int {
    if ($b == 0) {
        throw "Division by zero!";
    }
    return $a / $b;
}

func main() int {
    try {
        my int $result = divide(10, 0);
        say("Result: " . $result);
    } catch ($e) {
        say("Error: " . $e);
    }
    say("Program continues...");
    return 0;
}

Nested Try/Catch

try {
    try {
        throw "Inner error";
    } catch ($e) {
        say("Caught inner: " . $e);
        throw "Re-throwing";  # Re-throw or throw new
    }
} catch ($e) {
    say("Caught outer: " . $e);
}

Goto and Labels

For exceptional control flow (use sparingly):

func main() int {
    my int $i = 0;

start:
    say($i);
    $i++;
    if ($i < 5) {
        goto start;
    }

    say("Done");
    return 0;
}

Pattern Matching

Use regex in conditionals:

my str $input = "hello123";

if ($input =~ /[0-9]+/) {
    say("Contains numbers");
}

if ($input !~ /^[0-9]+$/) {
    say("Not purely numeric");
}

# Case-insensitive match
if ($input =~ /HELLO/i) {
    say("Found hello (case-insensitive)");
}

Switch Statement

Use switch for multi-way branching:

my int $day = 3;

switch ($day) {
    case 1 {
        say("Monday");
    }
    case 2 {
        say("Tuesday");
    }
    case 3 {
        say("Wednesday");
    }
    default {
        say("Other day");
    }
}

Switch with Strings

my str $color = "red";

switch ($color) {
    case "red" {
        say("Stop");
    }
    case "yellow" {
        say("Caution");
    }
    case "green" {
        say("Go");
    }
    default {
        say("Unknown signal");
    }
}

Alternative: Hash Dispatch

For dynamic dispatch or function tables, hashes work well:

my hash %day_names = {
    "Mon" => "Monday",
    "Tue" => "Tuesday",
    "Wed" => "Wednesday"
};

my str $day = "Mon";
if (exists(%day_names, $day)) {
    say($day_names{$day});
} else {
    say("Unknown day");
}

Statement Modifiers

Perl-style postfix modifiers let you write concise one-liners:

# Postfix if/unless
say("hello") if $verbose;
say("warning") unless $quiet;

# Postfix while/until
$i = $i + 1 while $i < 10;
$i = $i + 1 until $i >= 10;

# Works with return, last, next, redo
return 0 if $error;
return $val unless $invalid;
last if $done;
next unless $valid;

Short-Circuit Evaluation

# && and || short-circuit
my int $x = 0;

# Second expression not evaluated if first is false
$x && say("x is true");

# Second expression not evaluated if first is true
$x || say("x is false");

# Use for default values
my str $name = "";
my str $display = $name || "Anonymous";