* 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'
* 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
* 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
* JIT improvements include a small change to aid x86 branch
prediction on function-call returns, which can speed up some
* 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
- 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
* PLaneT development links are now version-specific.
* The `2htdp/image' library now includes `overlay/align',
`underlay/align', `overlay/align/offset' and
* 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