#nonempty

A list with at least one element.

NonEmpty a is a first element and a tail, so it cannot be empty. That makes head, maximum, and minimum total: they return the element itself rather than an Option. Build one with singleton, fromList, or the NonEmpty constructor.

Import the module by name, import nonempty, and call nonempty.head and the rest qualified, since the names overlap with list's.

#NonEmpty

data NonEmpty a
  = NonEmpty a (List a)

A first element and the rest of the list.

Instances: Mappable, Foldable, Traversable, Semigroup, Eq, Debug, Display

#Construction

#singleton

singleton : a -> NonEmpty a
singleton x

A list holding one element.

> toList (singleton 9)
[9]

#fromList

fromList : List a -> Option (NonEmpty a)

A non-empty list holding the elements of a list, or None when the list is empty.

The inverse of toList.

> fromList [1, 2, 3]
Some (NonEmpty [1, 2, 3])
> fromList ([] : List Int)
None

#Accessing elements

#head

head : NonEmpty a -> a

The first element.

> head (NonEmpty 7 [8, 9])
7

#maximum

maximum : Ord a => NonEmpty a -> a

The largest element.

> maximum (NonEmpty 3 [1, 4, 1, 5])
5

#minimum

minimum : Ord a => NonEmpty a -> a

The smallest element.

> minimum (NonEmpty 3 [1, 4, 1, 5])
1

#Instances

#Mappable NonEmpty

impl Mappable NonEmpty

map applies a function to every element.

> toList (map (n => n * 2) (NonEmpty 1 [2, 3]))
[2, 4, 6]

#Foldable NonEmpty

impl Foldable NonEmpty

The Foldable methods visit the elements in order, first element first. toList gives back the plain list.

> toList (NonEmpty 1 [2, 3])
[1, 2, 3]

#Semigroup (NonEmpty a)

impl Semigroup (NonEmpty a)

++ concatenates two non-empty lists.

> toList (append (NonEmpty 1 [2]) (NonEmpty 3 [4]))
[1, 2, 3, 4]

#Display (NonEmpty a)

impl Display (NonEmpty a) requires Display a

display renders a non-empty list as NonEmpty [x, ...].

> display (NonEmpty 1 [2, 3])
"NonEmpty [1, 2, 3]"