Guarded commands

The Guarded Command Language (GCL) is a language defined by Edsger Dijkstra for predicate transformer semantics. It combines programming concepts in a compact way, before the program is written in some practical programming language. Its simplicity makes proving the correctness of programs easier, using Hoare logic.

The guarded command is the most important element of the guarded command language. In a guarded command, just as the name says, the command is "guarded". The guard is a proposition, which must be true before the statement is executed. At the start of that statement's execution, one may assume the guard to be true. Also, if the guard is false, the statement will not be executed. The use of guarded commands makes it easier to prove the program meets the specification. The statement is often another guarded command.

A guarded command is a statement of the form G → S, where

If G is true, the guarded command may be written simply S.

At the moment G is encountered in a calculation, it is evaluated.

Skip and Abort are very simple as well as important statements in the guarded command language. Abort is the undefined instruction: do anything. The abort statement does not even need to terminate. It is used to describe the program when formulating a proof, in which case the proof usually fails. Skip is the empty instruction: do nothing. It is used in the program itself, when the syntax requires a statement, but the programmer does not want the machine to change states.

Assigns values to variables.

or

where

Statements are separated by one semicolon (;)

The selection (often called the "conditional statement" or "if statement") is a list of guarded commands, of which one is chosen to execute. If more than one guard is true, one statement is nondeterministically chosen to be executed. If none of the guards are true, the result is undefined. Because at least one of the guards must be true, the empty statement "skip" is often needed.

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