Maps

Disco has a built-in type of maps, or dictionaries, that map keys to values. The type of a map is written Map(K, V) where K is the type of the keys and V is the type of the values. For example, Map(N, List(Char)) is the type of maps which associate natural number keys to string values.

• The most basic way to construct a Map is to use the built-in function map : Set(k * a) -> Map(k, a). That is, map takes as input a set of key-value pairs and constructs a corresponding Map.

  Disco> map({})
  map({})
  Disco> map({(7, "hi"), (5, "there")})
  map({(5, "there"), (7, "hi")})
  

  Notice that map is also used when printing out values of type Map.

• To look up the value associated with a particular key, you can use lookup : k * Map(k, a) -> (Unit + a). That is, lookup takes a pair of a key and a Map, and looks up that key, returning either a left(unit) if the key was not found, or right(a) with the value a corresponding to the key if it was found.

  Disco> m : Map(N, List(Char))
  Disco> m = map({(5, "there"), (7, "hi")})
  Disco> lookup(5, m)
  right("there")
  Disco> lookup(6, m)
  left(unit)
  

• To insert a new key/value pair into a map, or to change the value associated with an existing key, you can use the function insert : k * a * Map(k, a) -> Map(k, a). This function takes three arguments: a key, a value, and an existing Map, and returns an updated Map where the given key is now associated to the given value.

  Disco> insert(5, "you", m)
  map({(5, "you"), (7, "hi")})
  Disco> insert(9, "you", m)
  map({(5, "there"), (7, "hi"), (9, "you")})
  

  Note that calling insert did not actually change m; it simply returned a new map.

• To turn a map back into a set of key/value pairs, use mapToSet : Map(k, a) -> Set(k * a).

  Disco> mapToSet(m)
  {(5, "there"), (7, "hi")}