-- record_patterns.ail - Record Pattern Matching Examples

-- Demonstrates record destructuring in pattern matching (v0.6.2+)

-- =======================

-- BASIC SHORTHAND PATTERNS

-- =======================

-- {name, ...} binds field "name" and accepts extra fields via open-record syntax.

-- Pre-v0.22.0 a closed-record pattern {name} accepted records with extra fields

-- by accident (over-polymorphic schemes hid the field-count mismatch). Now use

-- {name, ...} explicitly to accept records that carry additional fields.

let person = {name: "Alice", age: 30} in

match person {

{name, ...} => name

}

-- Output: "Alice"

-- Multiple field shorthand

let user = {name: "Bob", age: 25, active: true} in

match user {

{name, age} => "${name} is ${show(age)}"

}

-- Output: "Bob is 25"

-- =======================

-- RENAMING PATTERNS

-- =======================

-- {name: n} binds field "name" to variable "n"

let person = {name: "Charlie", age: 35} in

match person {

{name: n, age: a} => "${n} (${show(a)})"

}

-- Output: "Charlie (35)"

-- Mix shorthand and renaming

let config = {host: "localhost", port: 8080} in

match config {

{host, port: p} => "${host}:${show(p)}"

}

-- Output: "localhost:8080"

-- =======================

-- NESTED RECORD PATTERNS

-- =======================

-- Match nested records (open-record syntax accepts extra fields like 'email')

let data = {

user: {name: "Diana", email: "[email protected]"}

} in

match data {

{user: {name, ...}, ...} => name

}

-- Output: "Diana"

-- Deeper nesting with renaming

let company = {

ceo: {

profile: {name: "Eve", title: "CEO"}

}

} in

match company {

{ceo: {profile: {name: n, title: t}}} => "${n} - ${t}"

}

-- Output: "Eve - CEO"

-- =======================

-- PATTERN WITH REST

-- =======================

-- {name, ...} matches name field, ignores rest

let big_record = {name: "Frank", age: 40, city: "NYC", active: true} in

match big_record {

{name, ...} => name

}

-- Output: "Frank"

-- =======================

-- FUNCTION RETURNING EXTRACTED FIELD

-- =======================

-- Extract field using record pattern in match. The lambda parameter takes

-- the SHAPE inferred from the pattern, so the call site must pass a record

-- with exactly that shape. Open-record patterns inside lambdas don't yet

-- propagate row polymorphism through the lambda parameter — pass matching

-- shapes when using lambda + match together, or use top-level `match` for

-- open-record patterns over records with extra fields.

let getName = \obj.

match obj {

{name} => name

}

in

getName({name: "Grace"})

-- Output: "Grace"

-- Extract nested field (matching shape exactly)

let getEmail = \obj.

match obj {

{user: {email: e}} => e

}

in

getEmail({user: {email: "[email protected]"}})

-- Output: "[email protected]"

-- =======================

-- MULTIPLE CASES WITH RECORDS

-- =======================

-- Multiple match arms (use COMMA, not pipe!)

type Status = Active | Inactive

let getPersonStatus = \person.

match person {

{name, status: Active} => "${name} is active",

{name, status: Inactive} => "${name} is inactive"

}

in

getPersonStatus({name: "Henry", status: Active})

-- Output: "Henry is active"

-- =======================

-- RECORDS WITH ADT FIELDS

-- =======================

-- Record containing Option value

type Option[a] = Some(a) | None

let maybeUser = {id: 1, email: Some("[email protected]")} in

match maybeUser {

{id, email: Some(e)} => "User ${show(id)}: ${e}",

{id, email: None} => "User ${show(id)}: no email"

}

-- Output: "User 1: [email protected]"