This is for historical reasons (the List module is copied from Belt).

For example:

• List: let forEach: (t<'a>, 'a => 'b) => unit
• Others: let forEach: (t<'a>, 'a => unit) => unit

or

• List: let mapWithIndex: (t<'a>, (int, 'a) => 'b) => t<'b>
• Others: let mapWithIndex: (t<'a>, ('a, int) => 'b) => t<'b>

or

• List: getBy
• Others: find