Destructuring binds

Tuples and data types can be decomposed withpattern matching,but Unison also provides several ways to more concisely bind variable names to the corresponding parts of a data structure.

Destructuring tuples

Variables can be assigned to the constituent parts of a tuple by surrounding the variable definition with parentheses and separating the variables with commas.

tuple = (1, 2)
let
  (first, second) = tuple
  ()

🎨If a tuple is passed as an argument to alambda,you can use thecaseskeyword as a way to destructure the tuple.

List.map (cases (first, second) -> second) [(1, 2), (3, 4)]
[2, 4]

Destructuring data types

The fields of a data type can also be bound to variables by destructuring theirdata constructors

Say you have a simpleBoxtype:

unique type Box a
unique type Box a = Box a

You could write a function that adds twoBox Nattypes together by accessing theNatelements like this:

addBox : Box Nat -> Box Nat -> Nat
addBox boxA boxB =
  use Nat +
  (Box a) = boxA
  (Box b) = boxB
  a + b

So destructuring a data type takes the form(DataConstructor field1 field2 … fieldN) = instanceOfDataType

Usage notes

Currently, Unison does not support decomposition interm definitions,so a function which accepts a tuple or data type as a parameter cannot be destructured when the function parameter is first named. This means atermDeclarationlikeaddTwo : (Nat, Nat) -> Natcan notbe implemented with a term definition which starts withaddTwo (one,two) = one + two.The decomposition of the tuple would have to be done on a separate line.

In some languages you can use list constructors likeList.+:or:+to name separate elements in the list. We do not currently support that feature for term definitions, but youcanuse them in pattern matching; seelist pattern matchingfor more details.

-- you can't do this
list = [1,2,3,4]
one +: tail = list
list = [1, 2, 3, 4]

(one +: tail) = list