Racket v5.1.1

* The new `racket/stream' library provides `stream-first',
  `stream-rest', a lazy `stream-cons', and so on.  Streams are a
  subtype of sequences, so they work in `for' forms.  Some sequence
  generators, such as `in-range', now produce streams.  A
  `racket/sequence' library replaces the old `racket/stream'
  library.

* The new `racket/syntax' library contains facilities useful for
  writing macros.  The new `syntax/srcloc' and `syntax/location'
  libraries provide support for manipulating source locations.

* The `racket/gui' library now supports multi-column list boxes and
  scrolling panels.

* The new `ffi/file' library is useful for writing foreign library
  bindings that cooperate with Racket's security guard mechanism.

* Generators from the `racket/generator' library can now have formal
  arguments that are used when the generator is fired up.

* Single-precision floating-point support is now enabled by default.
  Single-precision floats print differently from their default
  double-precision counterparts, new primitives convert between the
  two precisions, and new reader syntax supports single-precision
  literals.

* JIT improvements include a small change to aid x86 branch
  prediction on function-call returns, which can speed up some
  programs significantly.

* Typed Racket:
  - The numeric tower has been entirely overhauled.  TR programs can
    now use more precise types than before, and check more numeric
    properties, such as sign or range properties.
  - Fixnum optimizations have been improved and should apply more
    broadly.
  - The performance of the typechecker has been improved.  In
    particular, dispatch on large union types should typecheck much
    faster than before.

* The Stepper can now step through Lazy Racket programs.

* The `racket/future' library includes `fsemaphore' values, the
  `future' primitive no longer freezes futures (so a future can
  spawn new futures), and `future' log messages are more
  informative.

* PLaneT development links are now version-specific.

* The `2htdp/image' library now includes `overlay/align',
  `underlay/align', `overlay/align/offset' and
  `underlay/align/offset'.

* The network protocol for universes in `2htdp/universe' has
  changed, so that v5.1.1 is incompatible with earlier versions.

* The "DrScheme" application (which simply ran DrRacket in the last
  few releases) has been removed.  The "MrEd" GUI executables for
  Windows and Mac OS X have also been removed, although the "mred"
  console executable remains for Unix and Mac OS X to support old
  scripts.

Racket v5.1

The most significant change in version 5.1 is a rewrite of the GUI
library:

  http://blog.racket-lang.org/2010/12/racket-version-5.html

Unix/X users will see the biggest difference with this change,
because DrRacket and all Racket GUI programs now take on the desktop
theme for menus, buttons, and other GUI widgets.

In the long run, Racket GUI programs on all platforms will improve
as a result of the library rewrite.  In the short run, beware that
this first release of a new library will inevitably include a new
set of bugs.

Version 5.1 changes in more detail:

* The `racket/draw' library -- which implements the drawing half the
  GUI toolkit -- can be used independent of the `racket/gui/base'
  library and without a graphics display (e.g., without an X11
  connection).

  The new library has one small incompatibility with the old GUI
  toolbox: 'xor drawing is no longer supported.  The new library has
  many additional features: rotation and general affine
  transformations, PDF and SVG drawing contexts, gradients, and
  alpha-channel bitmaps.

* The GRacket executable is no longer strictly necessary for running
  GUI programs, because the `racket/gui/base' library can be used
  from Racket.  To the degree that a platform distinguishes GUI and
  console applications, however, the GRacket executable still offers
  some additional GUI-specific functionality (e.g., single-instance
  support).

  The new `racket/gui/base' library includes small incompatibilities
  with the old GUI toolbox: the `send-event',
  `current-ps-afm-file-paths', and `current-ps-cmap-file-paths'
  functions have been removed.  The `racket/gui/base' library
  re-exports `racket/draw', so it includes the same drawing
  functionality as before (except for 'xor drawing).

* The new `racket/snip' library can be used independently of
  `racket/gui/base' to work with graphical editor content (e.g.,
  images in student programs).  Like `racket/draw', the
  `racket/snip' library is re-exported by `racket/gui/base'.

* The Web Server includes a backwards incompatible change that
  prevents X-expressions and lists of bytes from being directly
  returned from servlets.  This change will increase performance for
  those types of responses and allow easier experimentation with
  response types.  Please see "collects/web-server/compat/0/README"
  in the installation to learn about porting your servlets forward.
  Don't worry.  It's easy.

* The new `raco demodularize' tool collapses a module's dependencies
  into a single module comprising the whole program.  This
  transformation currently provides no performance improvement, but
  is the basis for cross-module optimization and dead-code
  elimination tools to come.  The transformation is currently useful
  for static analysis of whole Racket programs.

* The picturing-programs teachpack, formerly installed via PLaneT,
  is now bundled with the standard distribution.  Use the teachpack
  with `(require picturing-programs)' instead of `(require
  installed-teachpacks/picturing-programs)'.  The old PLaneT-based
  installation procedure still works, but it now merely installs a
  stub that invokes the bundled version.

* Slideshow picts, `racket/draw' bitmaps, and images created with
  `2htdp/image' can now be used directly in Scribble documents.
  More generally, the new `file/convertible' protocol enables any
  value that is convertible to a PNG and/or PDF stream to be used as
  an image in a Scribble document.

* The Same game sports a new look and an improved scoring system.
  (The current known high score is 12,429; can you beat that?)

Racket v5.0.2

* Typed Racket's optimizer is now turned on by default; error
  messages have been simplified and clarified.

* Contracts: contracts on mutable containers allow functions or
  other higher order values, and contracts are checked when updating
  or dereferencing the containers.  The new contracts are slower
  than the old ones, so the old check-once functionality is still
  available.

  A new dependent function contract combinator, `->i', properly
  assigns blame for contracts that violate themselves and the
  generated wrappers are more efficient than `->d'.  (Although it
  does more checking so your contracts may be faster or slower).

  See the docs for `box/c', `hash/c', `vector/c', `vectorof' and
  `->i' for more details.

* The `when', `unless', `cond', `case', and `match' forms (in
  `racket/base' and derived languages) now allow immediate internal
  definitions.

* Web server: the formlets library provides more HTML form elements;
  `make-xexpr-response' supports a preamble for DTD declarations;
  `serve/servlet' supports stateless servlets.

* New WebSocket implementation, see `net/websocket' for details.

* The new `data' collection contains implementations of several data
  structures, including growable vectors and order-based
  dictionaries.

* `racket/match' is now significantly faster

* The Racket documentations are built in parallel by default.

* The stepper is now compatible with programs using the Universe
  teachpack.

* `2htdp/image': pinholes are now supported in the library (if you
  don't use pinhole primitives you will not see them); a number of
  new triangle functions added; supports conversion of images to
  color lists and back.  Also, cropping has been improved for
  scenes; see the documentation section on the nitty-gritty of
  pixels for details.

* Signatures have been moved to ASL, BSL, BSL+, ISL, and ISL+ (HtDP
  teaching languages) no longer support checked signatures.

* Student languages: one-armed `check-error' in all levels; ASL is
  extended with hash operations, and `define-datatype'.

* DMdA languages: Checking for parametric signatures is now eager.
  This catches errors earlier, but retains the asymptotic complexity
  properties; signatures for record types now have generators;
  `list-of' and `any' signatures are now provided.

Racket v5.0.1

* Datalog is a lightweight deductive database system with Racket
  integration.  It is now available in the `datalog' collection and
  with `#lang datalog'.

* Racklog provides Prolog-style logic programming in Racket, adapted
  from Dorai Sitaram's Schelog package.  It is available in the
  `racklog' collection and now as `#lang racklog'.

* By default `make install' and `raco setup' compile collections in
  parallel on all available processors.  (Use `raco setup -j 1' to
  disable, if necessary.)

* Changes (as part of 5.0) in the `racket' language compared to the
  `scheme' language: constructor-style printing, a `struct'
  alternative to `define-struct' that fits more naturally with
  `match' and constructor-style printing, bytecode-dependency
  management via SHA-1 hashes instead of just timestamps (where the
  `openssl/sha1' library provides the SHA-1 hash function), a
  reorganization of `scheme/foreign' into `ffi/unsafe' and
  associated libraries, and new printing functions `eprintf' and
  `displayln'.  Also, a generator from `racket/generator' is
  required to have the form `(generator () body ...)', which
  supports a planned extension to let a generator accept arguments.

* Changes to the `racket' language (since 5.0): internal-definition
  positions allow mixing expressions with definitions, full
  continuations can escape past a continuation barrier, custodians
  can attempt to terminate subprocesses and subprocess groups (see
  `current-subprocess-custodian-mode', `subprocess-group-enabled'),
  the JIT supports additional unboxing flonum operations and unsafe
  variants, `ffi/unsafe' provides an asychronous-call mechanism to
  deal with foreign threads, a new "." modifier for format string
  directives (e.g., "~.s" and "~.a") limits the respective output to
  `(error-print-width)' characters.

* The core type system of Typed Racket has been substantially
  revised.  In particular, Typed Racket can now follow significantly
  more sophisticated reasoning about the relationships between
  predicates.  Additionally, Typed Racket now allows variable arity
  types in more places, allowing programmers to specify
  variable-arity lists.

* We are working on an optimizing version of Typed Racket that takes
  advantage of type information for certain classes of programs.
  This project is a work in progress.  For those interested, see the
  documentation for `#:optimized'.

* The `web-server/formlets' library adds a `formlet*' form that
  allows dynamic formlet construction, as opposed to `formlet' which
  requires syntactic Xexprs and static formlets.  Several new
  library formlets are added.

* The `syntax/parse' library has new support for matching literals
  at different phases using the `#:phase' argument for literals and
  literal sets.

* RackUnit now includes a GUI test runner as `rackunit/gui'.

* The `2htdp/image' library now includes `flip-vertical' and
  `flip-horizontal' operations that mirror images (vertically and
  horizontally).

PLT is happy to announce the release of Racket, available from

  http://racket-lang.org/

With Racket, you can script command shells and web servers; you can
quickly prototype animations and complex GUIs; regexps and threads are
here to serve you.  To organize your systems, you can mix and match
classes, modules or components.  Best of all, you start without writing
down types.  If you later wish to turn your script into a program, equip
your Racket modules with explicit type declarations as you wish.  And
Racket doesn't just come as a typed variant; you can also write your
modules in a purely functional and lazy dialect.

Racket comes in so many flavors because Racket is much more than a
standard scripting language or a plain programming language.  Racket
supports language extensibility to an unequaled degree.  A Racket
programmer knows that making up a new language is as easy as writing a
new library.

To help you start quickly, Racket includes batteries in all shapes and
sizes, most importantly, extensive documentation and all kinds of
libraries.

Racket occupies a unique position between research and practice.  It
inherits many major ideas from language research, among them type safety
(when the type system says that x is a number, then at runtime it always
is a number) and memory safety (when some memory is reclaimed by the
garbage collector it is impossible to still have a reference to it).  At
the same time, user demand governs rigid adherence to purely theoretical
principles.

Racket, formerly PLT Scheme, is a product of over 15 years of
development.  Although Racket starts with a mature software base and an
established user community, its new name reflects our view that this is
just the beginning of Racket's evolution.