NonEmptyArray

An enumerable which is guaranteed to have at least one element. E.g., #first will never fail.

These four methods give non-empty-aware access:

• #first
• #last

• #tail
• #all_but_last

And one method for mutating the list:

• #push

Sometimes I know that an Array isn't empty. Or more accurately, it shouldn't be empty, because otherwise, it means there's a bug in my code. The usual way to handle this is to repeatedly check the array's length, or check for nil, and throw an exception if, for some reason, the Array is empty. We've all been there.

The NonEmptyArray approach saves this work by moving the non-emptyness into the type system, letting Ruby check and prevent misuse. I.e., this class is designed so that it's impossible for it to be empty. It has made illegal states unrepresentable.

And it has accessors like #first and #last which always return an element - they can never fail. While Ruby isn't statically typed, the interpreter does check arity (the number of arguments provided). So we enlist the interpreter's help to ensure a NonEmptyArray is always created with at least one element:

require 'non_empty_array'

a = NonEmptyArray.new()  # => Ruby error - missing parameter

require 'non_empty_array'

a = NonEmptyArray.new('1000')  # Simplest way to create one
a.count()       # => 1
a.first()       # => '1000'
a.push('2000')
a.count()       # => 2

require 'non_empty_array'

a = NonEmptyArray.new(100, [200, 300]).  # Can also create from a head and tail

# Methods from Enumerable
a.count()         # => 3
a.max()           # => 300
a.to_a()          # => [100, 200, 300]

# Methods specific to NonEmptyArray
a.first()         # => 100          Always succeeds - never returns a "no element" error.
a.last()          # => 300          Always succeeds
a.all_but_last()  # => [100, 200]   A normal array, which may indeed be empty.
a.push('400')
a.all_but_last()  # => [100, 200, 300]
a.tail()          # => [200, 300, 400]

Inspired by Haskell's NonEmpty List.