# ECL programming language

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Extensible programming language system

Not to be confused with [ECL (data-centric programming language)](/source/ECL_(data-centric_programming_language)).

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The **ECL** programming language and system were an [extensible](/source/Extensible_programming) high-level [programming language](/source/Programming_language) and [development environment](/source/Integrated_Development_Environment) developed at [Harvard University](/source/Harvard_University) in the 1970s. The name 'ECL' stood for 'Extensible Computer Language' or 'EClectic Language'. Some publications used the name 'ECL' for the system as a whole and **EL/1** (Extensible Language) for the language.

ECL was an interactive system where programs were represented within the system; there was a compatible [compiler](/source/Compiler) and [interpreter](/source/Interpreter_(computing)). It had an [ALGOL](/source/ALGOL)-like syntax and an extensible [data type](/source/Data_type) system, with data types as [first-class citizens](/source/First-class_citizen). Data objects were values, not references, and the calling conventions gave a choice between [call by value](/source/Call_by_value) and [call by reference](/source/Call_by_reference) for each argument.

ECL was primarily used for research and teaching in [programming language design](/source/Programming_language_design), [programming methodology](/source/Programming_methodology) (in particular programming by [transformational refinement](/source/Program_transformations)), and [programming environments](/source/Integrated_Development_Environment) at Harvard, though it was said to be used at some government agencies as well. It was first implemented on the [PDP-10](/source/PDP-10), with a later (interpreted-only) implementation on the [PDP-11](/source/PDP-11) written in [BLISS](/source/BLISS)-11 and cross-compiled on the PDP-10.

## Procedures and bind-classes

An ECL procedure for computing the [greatest common divisor](/source/Greatest_common_divisor) of two integers according to the [Euclidean algorithm](/source/Euclidean_algorithm) could be defined as follows:

 gcd <-
   EXPR(m:INT BYVAL, n: INT BYVAL; INT)
   BEGIN
     DECL r:INT;
     REPEAT
       r <- rem(m, n);
       r = 0 => n;
       m <- n;
       n <- r;
     END;
   END

This is an assignment of a *procedure constant* to the variable gcd. The line

EXPR(m:INT BYVAL, n: INT BYVAL; INT)

indicates that the procedure takes two parameters, of type INT, named m and n, and returns a result of type INT. (Data types are called *modes* in ECL.) The *bind-class* BYVAL in each parameter declaration indicates that that parameter is passed [by value](/source/Call_by_value). The computational components of an ECL program are called *forms*. Some forms resemble the expressions of other programming languages and others resemble statements. The execution of a form always yields a value. The REPEAT ... END construct is a loop form. Execution of the construct

r = 0 => n

when the form r = 0 evaluates to TRUE causes execution of the loop to terminate with the value n. The value of the last statement in a block (BEGIN ... END) form becomes the value of the block form. The value of the form in a procedure declaration becomes the result of the procedure call.

In addition to the bind-class BYVAL, ECL has bind-classes SHARED, LIKE, UNEVAL, and LISTED. Bind-class SHARED indicates that a [parameter](/source/Parameter) is to be passed [by reference](/source/Call_by_reference). Bind-class LIKE causes a parameter to be passed [by reference](/source/Call_by_reference) if possible and [by value](/source/Call_by_value) if not (e.g., if the actual parameter is a pure value, or a variable to which a type conversion must be applied). Bind-class UNEVAL specifies that an [abstract syntax tree](/source/Abstract_syntax_tree) for the actual parameter is to be passed to the formal parameter; this provides extraordinary flexibility for programmers to invent their own notations, with their own evaluation semantics, for certain procedure parameters. Bind-class LISTED is similar to UNEVAL, but provides a capability similar to that of [varargs](/source/Varargs) in [C](/source/C_(programming_language)): the LISTED bind-class can only appear in the last formal parameter of the procedure, and that formal parameter is bound to a list of [abstract syntax tree](/source/Abstract_syntax_tree) representations, one for each remaining actual parameter. ECL has an EVAL built-in function for evaluating an [abstract syntax tree](/source/Abstract_syntax_tree); alternatively, there are functions by which programmers can explore the nodes of the [abstract syntax tree](/source/Abstract_syntax_tree) and process them according to their own logic.

## See also

- [Fexpr](/source/Fexpr)

## References

PISEL = *Proceedings of the international symposium on Extensible languages*, Grenoble, France, 1971, published in *ACM SIGPLAN Notices* **6**:12, December 1971.

- Benjamin M. Brosgol, "An implementation of ECL data types", PISEL, pp. 87–95.

- [Thomas E. Cheatham, Jr.](https://en.wikipedia.org/w/index.php?title=Thomas_E._Cheatham,_Jr.&action=edit&redlink=1), Glenn H. Holloway, Judy A. Townley, "Program refinement by transformation", *Proceedings of the 5th international conference on Software engineering*, 1981, pp. 430–437. [ISBN](/source/ISBN_(identifier)) [0-89791-146-6](https://en.wikipedia.org/wiki/Special:BookSources/0-89791-146-6)

- Glenn H. Holloway, "Interpreter/compiler integration in ECL", PISEL, pp. 129–134.

- Charles J. Prenner, "The control structure facilities of ECL", PISEL, pp. 104–112.

- Ben Wegbreit, "An overview of the ECL programming system", PISEL, pp. 26–28.

- Ben Wegbreit, "Studies in extensible programming languages." Technical Report ESD-TR-70-297. Harvard University, Cambridge, Massachusetts, May 1970.

- Glenn Holloway, Judy Townley, Jay Spitzen, Ben Wegbreit, "ECL Programmer's Manual", Report 23-74, Center for Research in Computing Technology, Harvard University, December 1974.

- Larry Denenberg, "The implementation of PDP-11 ECL", Technical Report 29-77, Center for Research in Computing Technology, Harvard University, June 1977.

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Adapted from the Wikipedia article [ECL programming language](https://en.wikipedia.org/wiki/ECL_programming_language) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/ECL_programming_language?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.