Pattern Matching

Pattern matching with match is the primary way to inspect and destructure values in Almide. Every match must be exhaustive — the compiler verifies that all possible cases are covered.

Run this, then delete the West arm and run it again — the compiler names the case you dropped instead of failing at runtime:

Almide Run

main.almd

type Direction = | North | South | East | West

fn describe(d: Direction) -> String =
  match d {
    North => "up",
    South => "down",
    East  => "right",
    West  => "left",
  }

fn main() -> Unit = {
  for d in [North, South, East, West] {
    println(describe(d))
  }
}

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Basic match

match direction {
  North => "up",
  South => "down",
  East => "right",
  West => "left",
}

match is an expression: it returns a value. All arms must produce the same type.

Arms are separated by commas. Each arm has the form Pattern => expr.

Pattern types

Wildcard

_ matches any value without binding it:

match status {
  200 => "ok",
  404 => "not found",
  _ => "other",
}

Identifier (binding)

A bare name binds the matched value to a variable:

match list.first(items) {
  some(x) => println("first: ${x}"),
  none => println("empty"),
}

Literal

Match against specific values:

match command {
  "start" => run(),
  "stop" => halt(),
  "status" => report(),
  _ => err("unknown command"),
}

Supports Int, Float, String, and Bool literals.

match n {
  0 => "zero",
  1 => "one",
  _ => "other",
}

Negative literal patterns are not supported yet — the exhaustiveness checker treats -1 as a wildcard (#897). Use a guard instead:

match n {
  0 => "zero",
  n if n < 0 => "negative",
  _ => "positive",
}

Option patterns

some(pattern) and none destructure Option[T]:

match map.get(config, "port") {
  some(port) => println("port: ${port}"),
  none => println("using default port"),
}

Result patterns

ok(pattern) and err(pattern) destructure Result[T, E]:

match int.parse(input) {
  ok(n) => n * 2,
  err(e) => {
    println("parse error: ${e}")
    0
  },
}

Constructor patterns

Destructure variant type cases:

type Shape =
  | Circle(Float)
  | Rect{ width: Float, height: Float }
  | Point

fn describe(s: Shape) -> String =
  match s {
    Circle(r) => "circle with radius ${float.to_string(r)}",
    Rect{ width, height } => "${float.to_string(width)}x${float.to_string(height)}",
    Point => "point",
  }

Unit constructors match without parentheses. Tuple constructors use CaseName(patterns...). Record constructors use CaseName{ fields... }.

Record patterns

Destructure record variant cases with field names:

match shape {
  Rect{ width, height } => width * height,
  _ => 0.0,
}

The .. syntax allows partial matching, ignoring fields you don’t need.

Tuple patterns

Destructure tuples directly:

match point {
  (0, 0) => "origin",
  (x, 0) => "on x-axis",
  (0, y) => "on y-axis",
  (x, y) => "at (${int.to_string(x)}, ${int.to_string(y)})",
}

Guards

Add if condition after a pattern for additional constraints:

match n {
  x if x > 0 => "positive",
  x if x < 0 => "negative",
  _ => "zero",
}

When a guard is false, the next arm is tried:

fn classify(score: Int) -> String =
  match score {
    n if n >= 90 => "A",
    n if n >= 80 => "B",
    n if n >= 70 => "C",
    _ => "F",
  }

Guards can use any boolean expression:

match item {
  some(s) if string.len(s) > 0 => process(s),
  some(_) => err("empty string"),
  none => err("missing"),
}

Exhaustiveness

The compiler ensures every possible value is covered. Missing cases are compile errors:

type Color = Red | Green | Blue

match color {
  Red => "red",
  Green => "green",
  // Compile error: non-exhaustive match, missing: Blue
}

Use _ as a catch-all when you don’t need to handle every case individually:

match color {
  Red => "red",
  _ => "not red",
}

Pipe into match

The pipe operator can feed a value directly into match:

list.get(args, 1) |> match {
  some(cmd) => cmd,
  none => "help",
}

Nested patterns

Patterns can be nested to match complex structures:

match result {
  ok(some(value)) => use(value),
  ok(none) => default(),
  err(e) => handle(e),
}

Next steps

• Types & Values — variant types and records
• Error Handling — matching on Result and Option
• Control Flow — if-then-else and guard