In most programming languages, a fact like "Foreach loops have a variable binding, a collection to iterate through, and a body" is guaranteed by the parser, and

In most programming languages, a fact like "Foreach loops have a variable binding, a collection to iterate through, and a body" is guaranteed by the parser, and the rest of the language implementation can assume that the foreach loops are well-formed and concentrate on lexical and type checks. An S-Expression parser will accept (loop for in do), and the rest of the language implementation has to deal with detecting that there is no variable, no collection and no action.