T (programming language)

T's purpose is to test the thesis developed by Guy L. Steele, Jr. and Gerald Jay Sussman in their series of papers about Scheme: that Scheme may be used as the basis for a practical programming language of exceptional expressive power, and that implementations of Scheme could perform better than other Lisp systems, and competitively with implementations of programming languages, such as C and BLISS, which are usually considered to be inherently more efficient than Lisp on conventional machine architectures. Much of this occurs via an optimizing compiler named Orbit.

T contains some features that modern Scheme lacks. For example, T is object-oriented, and it has first-class environments, called locales, which can be modified non-locally and used as a module system. T has several extra special forms for lazy evaluation and flow control, and an equivalent to Common Lisp's setf. T, like Scheme, supports call-with-current-continuation (call/cc), but it also has a more limited form called catch. From the T manual, a hypothetical implementation of cons could be:

 (define-predicate pair?)
 (define-settable-operation (car pair))
 (define-settable-operation (cdr pair))
 (define (cons the-car the-cdr)
         (object nil
                 ((pair? self) t)
                 ((car self) the-car)
                 ((cdr self) the-cdr)
                 (((setter car) self new-car) (set the-car new-car))
                 (((setter cdr) self new-cdr) (set the-cdr new-cdr))))

This example shows that objects in T are intimately related to closures and message-passing. A primitive called join puts two objects together, allowing for something resembling inheritance.

T was ported to many hardware platforms and operating systems, including:[2]

• MIPS: DEC 3100 (pmax), SGI IRIS
• Motorola 68000 (m68k): Apollo Domain/OS, HP/UX, Mac/AUX, NeXT, SunOS 3
• NS320xx (n32k): Encore Multimax
• SPARC: SunOS 4 and above, Solaris, Unix on Connection Machine 5
• VAX: Ultrix

• Official website
• History of T, by Olin Shivers
• T manual (PDF) from ReadScheme, via the Internet Archive