Pattern matching on List

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"