PLT Scheme

It's a family of Scheme implementations, including MzScheme and DrScheme.

It supports the creation of new programming languages through a rich, expressive syntax system.

This is the last version known as PLT Scheme. All new versions were rebranded as Racket.

What is new in this release:

• The unit test framework for the teaching languages provides `check-member-of' and `check-range' for checking "random functions", i.e., "functions" that may produce several different results for one and the same argument.
• The `scheme/vector' library provides common vector operations (also reprovided by `scheme').
• The `scheme/promise' library provides several new kinds of promises with alternatives execution strategies.
• New port-reading utilities: `in-port', `port->list', `file->list'.
• A new require-macro, `path-up', for requiring a file that is higher in the directory tree.

What is new in version 4.2.1:

• This is the last release that includes ProfessorJ. As of the next release, Kathy Gray who created and maintained the Professor will move the code to planet and maintain only at a minimal level.
• Typed Scheme 2.0 extends the type system significantly, making it more expressive. For example, predicates applied to selectors, such as (number? (car x)), are meaningful to the type system.
• Faster installation of Planet packages that trigger install of other Planet packages, because the documentation index is updated only once after a group of packages is installed.
• The `syntax/parse' library provides macro writers with an enhanced syntax pattern matcher that reports errors based on the patterns' declared classes of syntax.
• Identifier mappings following the v4 dictionary interface and naming conventions are available from the `syntax/id-table' library.
• Redex: added `define-relation' and generalized patterns that appear in "where" clauses to use the full Redex pattern matcher. (This is a backwards incompatible change, but one often requested; see the Redex release notes for details.)
• The Web Server's serializable closures are now available for other purposes through the web-server/lang/serial-lambda library.
• Teachpacks: small changes to universe portion of the "universe.ss" API, plus the addition of a form for launching many (communicating) worlds simultaneously. Bug fixes concerning conversion to strings.
• It is now possible to create custom scribble readers with a command characters different than `@', see `make-at-reader/inside' and `make-at-reader'