ANSI Common Lisp 5 Data and Control Flow 5.3 Dictionary of Data and Control Flow
- Arguments and Values:
name - a symbol.
form - a form.
results - the values of the forms if a normal return occurs,
or else, if an explicit return occurs, the values that were transferred.
block establishes a block named name
and then evaluates forms as an implicit progn.
The special operators block and return-from work together to
provide a structured, lexical, non-local exit facility. At any point lexically
contained within forms, return-from can be used with the
given name to return control and values from the block
form, except when an intervening block with the same name
has been established, in which case the outer block is
shadowed by the inner one.
The block named name has
lexical scope and dynamic extent.
Once established, a block may only be exited once,
whether by normal return or explicit return.
(block empty) NIL
(block whocares (values 1 2) (values 3 4)) 3, 4
(let ((x 1))
(block stop (setq x 2) (return-from stop) (setq x 3))
(block early (return-from early (values 1 2)) (values 3 4)) 1, 2
(block outer (block inner (return-from outer 1)) 2) 1
(block twin (block twin (return-from twin 1)) 2) 2
;; Contrast behavior of this example with corresponding example of CATCH.
(flet ((b1 () (return-from b 1)))
(block b (b1) (print 'unreachable))
- See Also:
return, return-from, Section 3.1 Evaluation
- Allegro CL Implementation Details: