Pattern Matching

The match expression compares a value against a series of patterns and evaluates the branch of the first pattern that matches. It is an expression, so it produces a value that can be assigned to a variable.

Basic Match

Match a value against literal patterns:

let label = match x {
    1 => "one"
    2 => "two"
    3 => "three"
    _ => "other"
}

Each arm consists of a pattern followed by => and a result expression. Arms are evaluated in order and the first match wins.

Wildcard Pattern

The _ pattern matches any value. It serves as the default or catch-all case and is typically placed last:

let description = match status {
    200 => "ok"
    404 => "not found"
    500 => "server error"
    _   => "unknown"
}

Guard Expressions

Match arms can include a guard clause using if to add a condition. The arm only matches when both the pattern and the guard are satisfied:

match x {
    1 if y > 0 => "positive one"
    1 if y < 0 => "negative one"
    1 => "one with zero y"
    _ => "other"
}

Guards are useful when the same pattern needs different results depending on other state.

Match as Expression

Since match is an expression, it can be used anywhere a value is expected:

// Assign the result to a variable
let category = match age {
    0 => "newborn"
    12 => "child"
    18 => "teenager"
    _ => "adult"
}

// Assign match result then use in a template string
let status_code = match code {
    200 => "ok"
    _   => "error"
}
let msg = `Status: ${status_code}`

// Return from a function
function classify(n) {
    return match n % 2 {
        0 => "even"
        _ => "odd"
    }
}

Multiple Statements in Arms

When an arm needs to execute multiple statements, wrap them in braces. The value of the arm is the last expression in the block:

let result = match input {
    0 => 0
    _ => {
        let doubled = input * 2
        doubled + 1
    }
}

Practical Examples

Command Dispatch

let output = match command {
    "help"    => show_help()
    "version" => show_version()
    "run"     => run_script(args)
    _         => error("unknown command: %s", command)
}

Type-Based Handling

let description = match type(value) {
    "int"    => "an integer"
    "string" => "a string"
    "list"   => "a list"
    "map"    => "a map"
    _        => "something else"
}

Range Classification

function http_status(code) {
    if (code >= 500) {
        "server error"
    } else if (code >= 400) {
        "client error"
    } else if (code >= 300) {
        "redirect"
    } else if (code >= 200) {
        "success"
    } else {
        "informational"
    }
}

Collections

Built-in Functions

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