Racket v5.0 is the first release under the name "Racket". The Racket
core was formerly named "MzScheme" and the overall package was
formerly named "PLT Scheme".

The Racket language is slightly different from the PLT Scheme language
of v4.x:

 * The preferred form for creating new structure types is now
   `struct', the `struct' form is like `define-struct', but it does
   not bind a constructor with a `make-' prefix; instead, the
   structure-type name doubles as its constructor:

     > (struct a (x y))
     > (a 1 2)
     #<a>

 * The default printing mode now tries print values as expressions
   that would produce the value, instead of text for which `read'
   would produce the value.

   For transparent structures, the printed form looks like a use of
   the constructor. For symbols, lists, vectors, prefab structure
   types, etc., the printed form uses a quote mark if possible or
   constructors like `list' otherwise. Opaque values, such as
   procedures, print using #<...> notation in either quoted or
   unquoted positions.

     > (list 1 2 3)
     '(1 2 3)
     > (lambda (x) x)
     #<procedure>
     > (list (lambda (x) x))
     '(#<procedure>)

     > (struct a (x y))
     > (a 1 2)
     #<a>
     > (list (a 1 2) (a 3 4))
     '(#<a> #<a>)

     > (struct b (x y) #:transparent)
     > (b 1 2)
     (b 1 2)
     > (list (b 1 2) (b 3 4))
     (list (b 1 2) (b 3 4))

The old PLT Scheme language is still provided by the libraries
`scheme', `scheme/base, etc. The new Racket language is provided by
`racket', `racket/base', etc.  The `mzscheme' executable starts
`racket' with `scheme/init' as the start-up language instead of
`racket/init'.

To improve compatibility between Racket and v4.x programs and libraries,

 * the `racket' language includes the old `define-struct' form, and

 * the `define-struct' form of `scheme' binds the type name as a
   constructor, in addition to binding a `make-' prefixed name.

Beware, however, that `scheme/unit' (and therefore `scheme') exports a
`struct' form for use in signatures that is different from the
`struct' now exported by `racket'.

The `slideshow' and `scribble/base' languages are now derived from
`racket' and `racket/base' instead of `scheme' and `scheme/base'.

The preferred file extension for Racket sources is ".rkt". To smooth
the transition to the new file extension, a ".ss" file extension in a
module reference is automatically converted to ".rkt". At the same
time, when loading a module through a path that ends in ".rkt", a
".ss" file is substituted if it exists while the ".rkt" version
doesn't exist.
