List

The List monad lets you manipulate an Array as a monad, with fmap/bind plus a set of functional helpers. It also includes Indexable and Iterator, so it behaves like a normal Crystal collection where convenient.

Creating a list

Use the [] macro or pass an array to new:

Monads::List[1, 6, 4, 2]
Monads::List.new([1, 6, 4, 2]) # equivalent

Transforming

fmap maps over every element; bind maps each element to a List and concatenates the results:

Monads::List[1, 2, 3].fmap(->(x : Int32) { x * 2 }) # => List[2, 4, 6]

Monads::List[1, 2, 3].bind(->(x : Int32) { Monads::List[x, x] })
# => List[1, 1, 2, 2, 3, 3]

head and tail

head returns the first element wrapped in a Maybe, so an empty list yields Nothing:

Monads::List[1, 2, 3].head # => Just(1)
Monads::List(Int32).new([] of Int32).head # => Nothing(Int32)

tail returns the list without its first element:

Monads::List[1, 2, 3].tail # => List[2, 3]

Other helpers

Monads::List[1, 2, 3] + Monads::List[4, 5] # => List[1, 2, 3, 4, 5]
Monads::List[3, 1, 2].sort                 # => List[1, 2, 3]
Monads::List[1, 2, 3].reverse              # => List[3, 2, 1]
Monads::List[1, 2, 3].join(", ")           # => "1, 2, 3"

Monads::List[1, 2].subsequences # => List[List[], List[1], List[2], List[1, 2]]
Monads::List[1, 2].permutations # => List[List[1, 2], List[2, 1]]

Because List is Indexable and Comparable, indexing, size, empty?, and comparison all work as expected:

list = Monads::List[1, 2, 3]
list[0]       # => 1
list.size     # => 3
list.empty?   # => false

See the List API reference for the complete method list.