Pair

A pair is two independent values packed into one. The only thing that differentiates pairs in Par, compared to other language, is their sequential syntax. While unusual, it makes pairs applicable to a much wider set of use-cases.

A pair type consists of two types, the first enclosed in round parentheses.

type Pair = (String) Int

If the second type is another pair, we can use syntax sugar to write it more concisely:

type Triple1 = (String) (Int) String
type Triple2 = (String, Int) String
// these two are exactly the same type

For a symmetric pair syntax, it’s idiomatic to use the unit type as the last element.

type SymmetricPair = (String, Int)!

Pairs in their sequential style are frequently used in combination with other types to insert values into bigger structures. The pre-defined List<a> type uses a pair for its .item variant:

type List<a> = recursive either {
  .end!,
  .item(a) self,
}

An infinite stream type may use a pair to produce an element together with the remainder of the stream:

type Stream<a> = iterative choice {
  .close => !,
  .next  => (a) self,
}

Construction

Pair values look the same as their types, with values instead of types in place of elements.

def Example1: Pair    = ("Hello!") 42
def Example2: Triple1 = ("Alice") (42) "Bob"

// `Triple1` and `Triple2` really are the same type
def Example3: Triple1 = ("Alice", 42) "Bob"
def Example3: Triple2 = ("Alice", 42) "Bob"

// notice the `!` at the end
def Example4: SymmetricPair = ("Hello!", 42)!

When embedded in other types, sequential pairs blend in seamlessly:

def Names: List<String> = .item("Alice").item("Bob").item("Cyril").end!
//                             |             |           |_____________
//                             |             |_________________________
//                             |_______________________________________

Destruction

Pairs are deconstructed in patterns on assignments. Those can appear in:

• let-expressions
• function arguments
• case/.case branches

Aside from pairs and whole values, unit types can be matched in patterns, too.

Here are some examples:

def Five: Int =
  let (x) y = (3) 2
  in Int.Add(x, y)

def FiveSymmetrically: Int =
  let (x, y)! = (3, 2)!
  in Int.Add(x, y)

dec AddSymmetricPair : [(Int, Int)!] Int
def AddSymmetricPair = [(x, y)!] Int.Add(x, y)
//                      \_____/<---- pattern here

dec SumList : [List<Int>] Int
def SumList = [list] list.begin.case {
  .end!       => 0,
  .item(x) xs => Int.Add(x, xs.loop),
//     \____/<---- pattern here
}