Add List.first!/1 and List.last!/1

[pierrelegall]

Summary

See discussion about it.

Add List.first!/1 and List.last!/1 functions that raise an ArgumentError when the list is empty.

• List.first!/1 - returns the first element or raises if empty
• List.last!/1 - returns the last element or raises if empty

Motivation

The existing List.first/2 and List.last/2 functions return a default value when the list is empty. The new bang variants provide an alternative when an empty list indicates a bug, following the pattern established by List.keyfind!/3, Map.fetch!/2, and Enum.fetch!/2.

Examples

iex> List.first!([1, 2, 3])
1

iex> List.first!([])
** (ArgumentError) trying to get the first element of an empty list

iex> List.last!([1, 2, 3])
3

iex> List.last!([])
** (ArgumentError) trying to get the last element of an empty list

Test plan

• Added unit tests for first!/1 and last!/1
• Added doctests with examples
• All existing List tests pass

[whatyouhide]

Nit:

Suggested change

	def first!(list)
	def first!(list)

[pierrelegall]

Already merged.

Is there a convention? Is there is, should we add this as a formatting rule?

[josevalim]

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