Pattern Matching

Pattern matching lets you destructure and inspect values.

Pattern Types

Literal Patterns

Match exact values:

match n
  0 -> "zero"
  1 -> "one"
  2 -> "two"
  _ -> "many"
end

Works with: Int, Float, Bool, String

Wildcard Pattern

Match anything:

match x
  0 -> "zero"
  _ -> "not zero"
end

Binding Patterns

Capture matched value:

match n
  0 -> "zero"
  x -> "got {x}"
end

Tuple Patterns

match point
  (0, 0) -> "origin"
  (x, 0) -> "on x-axis"
  (0, y) -> "on y-axis"
  (x, y) -> "({x}, {y})"
end

List Patterns

match items
  [] -> "empty"
  [x] -> "one: {x}"
  [x, y] -> "two: {x}, {y}"
  [first, ...rest] -> "first: {first}, rest: {length(rest)}"
end

With rest capture:

match items
  [first, second, ...rest] -> # first two elements, rest in list
  [...rest, last] -> # all but last in rest, last separately
end

Record Patterns

match user
  User(name: "Admin", ..) -> "administrator"
  User(name: n, active: true, ..) -> "active: {n}"
  User(active: false, ..) -> "inactive"
end

Enum Patterns

Simple enum:

match status
  Pending -> "waiting"
  Active -> "running"
  Done -> "complete"
end

With data:

match result
  Ok(value) -> "success: {value}"
  Err(msg) -> "error: {msg}"
end

Type Patterns

Check type in unions:

match value
  n: Int -> "int: {n}"
  s: String -> "string: {s}"
  b: Bool -> "bool: {b}"
end

Guard Patterns

Add conditions:

match n
  x if x < 0 -> "negative"
  x if x == 0 -> "zero"
  x if x < 10 -> "small positive"
  _ -> "large positive"
end

Or Patterns

Match multiple patterns:

match c
  "a" | "e" | "i" | "o" | "u" -> "vowel"
  _ -> "consonant"
end

Nested Patterns

match result
  Some(Ok(value)) -> "success: {value}"
  Some(Err(msg)) -> "failed: {msg}"
  None -> "nothing"
end

Match Expression

Match returns a value:

let label = match status
  Pending -> "pending"
  Active -> "active"
  Done -> "done"
end

Each arm must return the same type.

Match Statement

Match can be used as a statement:

match status
  Pending -> print("still waiting")
  Active -> print("now active")
  Done -> print("completed")
end

Destructuring in let

Patterns work in let bindings:

let (a, b) = (1, 2)
let [first, ...rest] = [1, 2, 3, 4]
let Point(x, y) = point
let User(name: n, ..) = user

Destructuring in for

for (index, item) in enumerate(items)
  print("{index}: {item}")
end

for [key, value] in pairs
  print("{key} = {value}")
end

Pattern Matching Rules

1. Patterns are tried in order — First match wins
2. Matches must be exhaustive — Use _ for fallback
3. Guards are evaluated after match — Pattern must match first
4. Bindings must be unique — Can't bind same name twice

Examples

HTTP Status

cell status_message(code: Int) -> String
  match code
    200 -> "OK"
    201 -> "Created"
    204 -> "No Content"
    400 -> "Bad Request"
    401 -> "Unauthorized"
    403 -> "Forbidden"
    404 -> "Not Found"
    500 -> "Internal Server Error"
    c if c >= 200 and c < 300 -> "Success"
    c if c >= 400 and c < 500 -> "Client Error"
    c if c >= 500 -> "Server Error"
    _ -> "Unknown"
  end
end

JSON Navigation

cell get_string(json: Json, key: String) -> String | Null
  match json
    obj: map[String, Json] ->
      match obj[key]
        s: String -> s
        _ -> null
      end
    _ -> null
  end
end

Expression Evaluator

enum Expr
  Num(value: Float)
  Add(left: Expr, right: Expr)
  Sub(left: Expr, right: Expr)
  Mul(left: Expr, right: Expr)
  Div(left: Expr, right: Expr)
end

cell eval(expr: Expr) -> Float
  match expr
    Num(n) -> n
    Add(l, r) -> eval(l) + eval(r)
    Sub(l, r) -> eval(l) - eval(r)
    Mul(l, r) -> eval(l) * eval(r)
    Div(l, r) -> eval(l) / eval(r)
  end
end

Best Practices

1. Order from specific to general — Put _ last
2. Use guards for complex conditions — Keep patterns simple
3. Be exhaustive — Cover all cases or use _
4. Name bindings meaningfully — x vs user_id

Next Steps

• Declarations — Records, enums, cells
• Error Handling — Using result types