Release Announcement for Version 6.0.1

Racket v6.0.1

* A new `racket/undefined` library exports `undefined` as the value
  currently produced by

    (letrec ([x x]) x)

  This library anticipates a future where that expression will raise
  an exception. The `racket/undefined` library will continue to offer
  the `undefined` value as a bridge between versions and as a last
  resort.

* The drawing and GUI libraries provide improved support for
  high-resolution bitmaps and their use on Retina displays. For
  example, `read-bitmap` includes a `#:[email protected]?` option to trigger
  substitutions through the usual "@2x" naming convention.

* Check Syntax cooperates with Typed Racket to show arrows and other
  Check Syntax highlighting even when there is a type error.

* Functions provided via contract-out that have first-order contracts
  perform better.

* The contract boundary between typed/untyped modules is much less
  expensive. Typed Racket now avoids generating contracts for places
  where contracts failures cannot happen.

* Occurrence typing now works better with when/unless. Example:

    (let ((x (read)))
      (unless (number? x) (error 'bad-input))
      (add1 x))

* Types in Typed Racket are now pretty-printed.

* Function types can now be written in prefix style, which is now
  preferred and is used for printing. Infix function types are still
  accepted for backwards compatibility.

* A new `->*' type constructor is used for writing types for functions
  with optional and keyword arguments. The notation is similar to the
  matching contract combinator.

* Typed Racket forms do not have a `:' suffix by default now. For
  example, the `struct' form replaces `struct:'. The suffixed versions
  are all provided for backwards compatibility.

* Typed Racket now has preliminary support for classes and
  objects. However, it is still experimental and the APIs are subject
  to change.

* Type aliases in Typed Racket now support recursion and mutual
  recursion. For example, `(define-type (MyList X) (U Null (Pair X
  (MyList X))))' is now a valid type alias.

* Plot correctly renders intersecting 3D graphs and non-grid-aligned
  3D rectangles.

* Elements in plots output in PDF/PS format have the same relative
  scale as in other formats. In particular, it is not necessary to
  adjust `plot-font-size` to make PDF plots look the same as PNG.