OPAL (OPtimized Applicative Language) is a functional programming language first developed at the Technical University of Berlin. Example program This is an example OPAL program, which calculates the GCD recursively. Signature file (declaration) SIGNATURE GCD FUN GCD: nat ** nat -> nat Implementation file (definition) IMPLEMENTATION GCD IMPORT Nat COMPLETELY DEF GCD(a,b) == IF a % b = 0 THEN b ELSE IF a-b < b THEN GCD(b,a-b) ELSE GCD(a-b,b) FI FI External links The OPAL Home Page OPAL Installation Guide Functional languages