Deprecations

• Calling -zip with two arguments now emits a warning. This long-discouraged calling convention remains supported, but the caller is now referred to the equivalent -zip-pair instead (Stefan Monnier, #400).
• Calling -zip-pair with less than or more than two arguments is now deprecated, and can be replaced with the equivalent call to -zip-lists instead.

Fixes

• Fixed a regression from 2.18 in -take that caused it to prematurely signal an error on improper lists (#393).

• The function -pad can now be called with zero lists as arguments.

• The functions -union, -intersection, and -difference now return proper sets, without duplicate elements (#397).

• The functions -same-items? and -permutations now work on multisets (lists with duplicate elements) (#390, #397, #399).

  For example:

  (-same-items? '(1 1 2 3) '(3 1 2)) ; => t
  (-permutations '(1 1 2)) ; => '((1 1 2) (1 2 1) (2 1 1))

• Several functions which are documented as returning a fresh, mutable object (such as a copy of one of their arguments) are no longer marked as pure. Pure functions called with constant arguments are evaluated during byte-compilation; the resulting value is an immutable constant, and thus unsafe to modify destructively. The functions in question are: -clone, -cons*, -drop-last, -interleave, -interpose, -iota, -non-nil, -repeat, -slice, -snoc, -split-at, -take, -take-last.

New features

• The function -contains? now returns the matching tail of the list instead of just t, similarly to member (#397).
• New function -frequencies that takes a list and counts how many times each distinct element occurs in it (suggested by @ebpa, #209, #214, #399).
• New functions -zip-lists-fill and -unzip-lists which are better-behaved versions of -zip-fill and -unzip, respectively (#400).