CPL (programming language)

CPL
Paradigm	Multi-paradigm: procedural, imperative, structured, functional
Designed by	Christopher Strachey et al.
First appeared	1963; 58 years ago
Influenced by
	ALGOL 60
Influenced
	BCPL, POP-2

CPL (Combined Programming Language) is a multi-paradigm programming language, that was developed in the early 1960s. It is an early ancestor of the C language via the BCPL and B languages.

Design

CPL^[1] was developed initially at the Mathematical Laboratory at the University of Cambridge as the "Cambridge Programming Language" and later published jointly between Cambridge and the University of London Computer Unit as the "Combined Programming Language". Christopher Strachey, David Barron and others were involved in its development. (CPL was also nicknamed by some as "Cambridge Plus London"^[2] or "Christopher's Programming Language"^[3]). The first paper describing it was published in 1963, while it was being implemented on the Titan Computer at Cambridge and the Atlas Computer at London.

It was heavily influenced by ALGOL 60, but instead of being extremely small, elegant and simple, CPL was intended for a wider application area than scientific calculations and was therefore much more complex and not as elegant as ALGOL 60. CPL was a big language for its time. CPL attempted to go beyond ALGOL to include industrial process control, business data processing and possibly some early command line games.^[4] CPL was intended to allow low-level programming and high level abstractions using the same language.

However, CPL was only implemented very slowly. The first CPL compiler was probably written about 1970,^[5] but the language never gained much popularity and seems to have disappeared without trace sometime in the 1970s.

BCPL (for "Basic CPL", although originally "Bootstrap CPL") was a much simpler language based on CPL intended primarily as a systems programming language, particularly for writing compilers;^[6] it was first implemented in 1967, prior to CPL's first implementation. BCPL then led, via B, to the popular and influential C programming language.

Example

The function MAX as formulated by Peter Norvig:^[5]

Max(Items, ValueFunction) = value of
§ (Best, BestVal) = (NIL, -∞)
while Items do §
(Item, Val) = (Head(Items), ValueFunction(Head(Items)))
if Val > BestVal then (Best, BestVal) := (Item, Val)
Items := Rest(Items) §⃒
result is Best §⃒

(The closing symbol to match the opening symbol "§" is a "§" with a vertical stroke through it. That symbol can be composed in Unicode as "§⃒", which is § (U+00A7, SECTION SIGN) composed with ⃒ (U+20D2, COMBINING LONG VERTICAL LINE OVERLAY), but it may not display correctly on your browser.)

Implementations

It is thought that CPL was never fully implemented in the 1960s, existing as a theoretical construct with some research work on partial implementations.^[7]

Peter Norvig has written (for Yapps, a Python compiler-compiler) a simple CPL to Python translator for modern machines.^[5]^[8]

References

^ Barron, D. W.; Buxton, J. N.; Hartley, D. F.; Nixon, E.; Strachey, C. (1963-08-01). "The Main Features of CPL". The Computer Journal. 6 (2): 134–143. doi:. ISSN 0010-4620.
^ "Clive Feather on CPL and BCPL". Lysator.liu.se. Retrieved 2013-08-18.
^ Jensen, Richard (December 9, 2020). ""A damn stupid thing to do"—the origins of C". Ars Technica. Retrieved December 10, 2020.
^ Yadav, P. (2005). Computer and Languages. Discovery Publishing House. ISBN 978-81-8356-041-2.
^ ^a ^b ^c Norvig, Peter (August 23, 2011). "Prescient but Not Perfect: A Look Back at a 1966 Scientific American Article on Systems Analysis". Scientific American Blog Network. Retrieved December 10, 2020.
^ Mitchell, John C.; Apt, Krzysztof (2003). Concepts in Programming Languages. Cambridge University Press. ISBN 978-0-521-78098-8.
^ Coulouris, G. F. (1 January 1968). "The London CPL1 compiler". The Computer Journal. 11: 26–30. doi:.
^ Norvig, Peter. "Complete Annotated Checkers Program". norvig.com. Retrieved 11 June 2021.

Bibliography

Collected papers of Christopher Strachey, section pertaining to CPL, archived at the Bodleian Library, Oxford; CSAC 71.1.80/C.136-C.184
D. W. Barron, J. N. Buxton, D. F. Hartley, E. Nixon, and C. Strachey. "The main features of CPL" The Computer Journal 6:2:134-143 (1963), available online.
J. Buxton, J. C. Gray, and D. Park. CPL Elementary Programming Manual, Edition II (Cambridge).
University of London Institute of Computer Science and The Mathematical Laboratory, Cambridge. CPL Working Papers.

[1] Barron, D. W.; Buxton, J. N.; Hartley, D. F.; Nixon, E.; Strachey, C. (1963-08-01). "The Main Features of CPL". The Computer Journal. 6 (2): 134–143. doi:. ISSN 0010-4620.

[2] "Clive Feather on CPL and BCPL". Lysator.liu.se. Retrieved 2013-08-18.

[3] Jensen, Richard (December 9, 2020). ""A damn stupid thing to do"—the origins of C". Ars Technica. Retrieved December 10, 2020.

[4] Yadav, P. (2005). Computer and Languages. Discovery Publishing House. ISBN 978-81-8356-041-2.

[peter_norvig-5] Norvig, Peter (August 23, 2011). "Prescient but Not Perfect: A Look Back at a 1966 Scientific American Article on Systems Analysis". Scientific American Blog Network. Retrieved December 10, 2020.

[6] Mitchell, John C.; Apt, Krzysztof (2003). Concepts in Programming Languages. Cambridge University Press. ISBN 978-0-521-78098-8.

[7] Coulouris, G. F. (1 January 1968). "The London CPL1 compiler". The Computer Journal. 11: 26–30. doi:.

[8] Norvig, Peter. "Complete Annotated Checkers Program". norvig.com. Retrieved 11 June 2021.

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