Pattern matching part 2

Your first Unison data type

Let's take a look at a pre-made data type, Either. Either is used to represent situations in which a value can be one type or another.

In Unison, Either is defined in the base library like this (we'll break down what these parts mean shortly):

structural type Either a b

structural type Either a b
  = lib.base.Either.Right b
  | lib.base.Either.Left a

type Box a

type Box a = docs.languageReference.destructuringBinds.Box.Box a

On the right hand side of the equals are the data constructors of the type. We use data constructors to create a value of the type being described, so to create an Either we have two options: Either.Left or Either.Right. They're separated by pipes |. When you see the pipe think "or."

structural type Either a b

structural type Either a b
  = lib.base.Either.Right b
  | lib.base.Either.Left a

We'll return to explore more in-depth about data types later.

Decomposing data types with pattern matching

With our whirlwind intro to the parts of a data type behind us, we'll return to how to pattern match on the different data constructors of a given type.

Let's say we wanted a function to tell us which utensils should be paired with a lunch order. We'll use the following types:

type Lunch

type Lunch
  = docs.fundamentals.controlFlow.patternMatching.Lunch.Soup Text
  | docs.fundamentals.controlFlow.patternMatching.Lunch.Salad Text
  | docs.fundamentals.controlFlow.patternMatching.Lunch.Mystery
      Text Boolean

type Utensil

type Utensil
  = docs.fundamentals.controlFlow.patternMatching.Utensil.Fork
  | docs.fundamentals.controlFlow.patternMatching.Utensil.Knife
  | docs.fundamentals.controlFlow.patternMatching.Utensil.Spoon

Our function should take in a type Lunch as an argument and return a List of type Utensil. We know that there are only three ways to make a value of type Lunch, so we match on the data constructor name followed by the number of fields that the constructor contains.

placeSetting : Lunch -> [Utensil]
placeSetting = cases
  Soup soupName   -> [Spoon]
  Salad saladName -> [Fork, Knife]
  _               -> [Spoon, Fork, Knife]

Pattern matching on the data constructors of the type enables us to inspect and make use of the values they contain. In the example above we don't end up using the variables that are bound to the fields in the data, so we could have also represented them as underscores, like Soup _ -> [Spoon], but we can imagine a function where that would become important:

placeSetting : Lunch -> [Utensil]
placeSetting = cases
  Soup "Hearty Chunky Soup"   -> [Fork, Spoon]
  Soup _                      -> [Spoon]
  Salad _                     -> [Fork, Knife]
  Mystery mysteryMeal isAlive ->
    use Text ==
    if (mysteryMeal == "Giant Squid") && isAlive then [Knife]
    else [Spoon, Fork, Knife]

The first case is an example of how to combine a literal pattern match with a data constructor, and the second and third cases are an example of how to match on any value that Lunch.Soup or Salad data constructor might enclose. Our last case extracts the values being provided to the Mystery data constructor as pattern match variables for use on the right.

Note, the underscores above represent the fact that the value being provided to the data constructor isn't important for the logic of our expression on the right. The underscores do, however, need to be present. Every parameter to the data constructor needs to be represented in the pattern either by a variable, as in our Mystery case, or by an underscore, otherwise Unison will return a pattern arity mismatch error.

Head and tail pattern matching

You can use pattern matching to scrutinize the first (left-most) element of a list with the +: syntax.

match ["a", "b", "c"] with
  head +: tail -> head
  _            -> "empty list"
⧨
"a"

The +: is unpacking the first element head to its text value "a" while keeping the remaining elements tail as a List. The underscore will match the "empty list" case. We could have also expressed _ -> "empty list" as [] -> "empty list" or List.empty -> "empty list". All three are valid ways of testing for the empty list case.

You can also pattern match on the last (right-most) element of a list in Unison:

match ["a", "b", "c"] with
  firsts :+ last -> last
  _              -> "empty list"
⧨
"c"

Put together, you can even pattern match on both ends of the list:

match ["a", "b", "c"] with
  first +: midSection :+ last -> first Text.++ last
  _                           -> "fallback"
⧨
"ac"

However, let's say you wanted to pattern match on the first 2 elements of a given list. It might be tempting to do a multi-item pattern match with successive +: operators, but recall that function application for operators always starts at the leftmost sub-expression.

🙅🏻‍♀️ This will not work:

match ["a","b","c"] with
  first +: second +: remainder -> "nope"
  _ -> "empty list"

first +: second is expecting second to be a type List, but what we're trying to express is that it's the unwrapped second element. Instead, if you want to pattern match on a particular list segment length or list segment values, you can use the [_] list constructor syntax.

Our example above can be rewritten:

match ["a", "b", "c"] with
  [first, second] ++ remainder -> first Text.++ " yes!"
  _                            -> "fallback"
⧨
"a yes!"

Or if we don't care about binding the list elements to variables, we can use underscores to pattern match on any list that has exactly [_] elements:

match ["a", "b", "c"] with
  [_, _] ++ remainder -> "list has at least two elements"
  _                   -> "fallback"
⧨
"list has at least two elements"