An ECL procedure for computing the greatest common divisor of two integers according to the Euclidean algorithm could be defined as follows: This is an assignment of a procedure constant to the variable gcd.
The computational components of an ECL program are called forms.
Bind-class LISTED is similar to UNEVAL, but provides a capability similar to that of varargs in C: 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 representations, one for each remaining actual parameter.
ECL has an EVAL built-in function for evaluating an abstract syntax tree; alternatively, there are functions by which programmers can explore the nodes of the abstract syntax tree and process them according to their own logic.
PISEL = Proceedings of the international symposium on Extensible languages, Grenoble, France, 1971, published in ACM SIGPLAN Notices 6:12, December 1971.