Collections

Sometimes we need to deal with multiple values. Crochet has two primary types for this: Lists and Records.

Lists

A list is a sequence of similar items—where “similar” really means “we can treat these uniformly”, and often boils down to having the same type.

Lists are written in the square brackets syntax:

[1, 2, 3, 4, 5]

The expression above creates a list that contains 5 elements, which are the integers from 1 to 5, in that exact order.

An empty list is written as follows:

[]

Lists can be peeked into, transformed, sorted, combined, and a so on. The standard library defines a rich set of ways to manipulate lists, and similar collections, such as sets.

Records

A record is an unordered bag of values that can be referenced with a key. Each key references exactly one value in a record, and all values have a key.

We can write records using the bracket-with-arrows syntax:

[
  name -> "Alice",
  first-appeared-in -> "Alice in Wonderland",
]

This is a bag with two key and value pairs. The name key references the value "Alice". And the name first-appeared-in references the value "Alice in Wonderland".

An empty record is written as follows, to avoid confusion with empty lists:

[->]

We can project values out of records in a similar fashion to typed data, using the dot operator with the key:

let Point = [x -> 1, y -> 2];
Point.x;

Security considerations

Records are somewhat of a weaker form of Typed Data in Crochet— we have a structure with some values we can use, but we choose to neither attach a unique meaning or security policies to it. This means that records can only be used for public and non-sensitive information, and are best kept restricted to very local uses— where we hold on to the record for a short amount of time, and represent bags of information we’ll properly store in typed data soon.