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4.2.7 Exception handling

syntax: guard ¶

(guard (⟨variable⟩
       ⟨cond clause1⟩ ⟨cond clause2⟩ …)
   ⟨body⟩)

Syntax: Each ⟨cond clause⟩ is as in the specification of cond.

Semantics: The ⟨body⟩ is evaluated with an exception handler that binds the raised object (see raise in Exceptions) to ⟨variable⟩ and, within the scope of that binding, evaluates the clauses as if they were the clauses of a cond expression. That implicit cond expression is evaluated with the continuation and dynamic environment of the guard expression. If every ⟨cond clause⟩’s ⟨test⟩ evaluates to #f and there is no else clause, then raise-continuable is invoked on the raised object within the dynamic environment of the original call to raise or raise-continuable, except that the current exception handler is that of the guard expression.

See Exceptions for a more complete discussion of exceptions.

(guard (condition
         ((assq 'a condition) => cdr)
         ((assq 'b condition)))
  (raise (list (cons 'a 42))))
⇒ 42

(guard (condition
         ((assq 'a condition) => cdr)
         ((assq 'b condition)))
  (raise (list (cons 'b 23))))
⇒ (b . 23)