Expression-oriented programming languages

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An expression-oriented programming language is a programming language where (nearly) every construction is an expression and yields a value. Macro definitions, preprocessor commands, and declarations are often treated as statements in expression-oriented languages. Some expression-oriented languages introduce a void return type to be yielded by expressions that have only side-effects.

ALGOL 68 is an example of an expression-oriented language. Pascal is not. All functional programming languages are expression-oriented.

Expression-orientation can be confusing in imperative programming languages, as many commands used normally as statements are in fact expressions. For example, assignment in the C programming language is an expression, not a statement. This allows for the following confusion:

if (x = 1) {
 ...
} else {
  /* this branch is never executed */
}

The condition of the if is the result of the expression x = 1, which is 1. Thus the if will always execute the true branch. The above is often confused with:

if (x == 1) {
  ... /* executed if x is 1 */
} else {
  ... /* executed if x is not 1 */
}

For this reason, many programmers write the constant first in conditionals, as 1 = x is a syntax error, and mistyping = for == will not result in a bug.

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