List.Nonempty for Elm

A list that is known, at compile-time, to be nonempty. This means head and tail are guaranteed to succeed and you don't have to carry Maybes throughout your program.

import List.Nonempty exposing (..)

one : Nonempty Int
one = fromElement 2

two : Nonempty Int
two = cons 4 one

toList two --> [4, 2]

head two --> 4

tail two --> [2]

toList (reverse two) --> [2, 4]

toList (dropTail two) --> [4]

member 4 two --> True

foldl1 (+) two --> 6

For actual usage, I recommend import List.Nonempty as NE exposing (Nonempty) to just import the type.

Testing

npm install -g elm-test
npm install -g elm-verify-examples
elm verify-examples && elm test # From project root. Will require downloading packages on the first run.

Upgrading

4.0.0, for Elm 0.19, removed the following functions: (:::), scanl, and scanl1. sample uses the new PRNG found in elm-lang/random.

The only breaking change from 2.x to 3.0.0 is that andMap has arguments reversed to allow chaining with |> instead of backticks.

The only breaking change from 1.x to 2.0.0 is that sample returns a generator rather than managing seeds.