Burke_et_al-NIL_Reference_Manual_0286-1984

Iteration Constructs 34 NIL Manual
See "Iso the loop macro, page 144. loop is a "programmahle iteration facility". which allows
one to combine various sorts of iterations (such as those provided by dotimes and dovector) with
vilrious sorts of result accumuhttion (such as those provided by mapcar. mapcan. every. some.
and other things such as summing and counting). Because it is complicated. it is documented
fully in Ch4tptcr 17. page 144. An earlier version of that chapter appeared as [51. which is
expected to be re.vised· similarly.
5.6.3 Block and Tagbody
block Clnd tagbody together implement the flow-of-control functionality pf()\'idcd hy standard
prog. prog could have been implcmcnted as a milCro in terms uf these and let. and in filet is
descrihed in that filshion by COMMON LISP.
block nome {declaralion}- ljorm}*
block· evaluates the jiJrms. If a lcxicaJly apparent return-from is evaluated with a tag of
lIome (or l1ame is nil arid a return is evaluated). then the vaJue(s) of the form given to
return or return-from arc returned as the \"alue of the block form. Otherwise'. the block
filrm returns the valuc(s)uf the evalumion of the lClst fonll.
Note that the argument to return or return-from is evaluated in the environment in
which it occurs, 1101 the environment where the block was established.
return fonn
Evaluates jbnl1 , and returns the value(s) it returns from the nearest lexic.llly apparent
block with a name of nil. Many special forms implicitly establish blocks named nil. such
as prog. do. do list. dotimes, dovector, and (usually) loop.
Note that this differs subtly from LISP MACIIINFl.JSP. In LlSI' MA(,IU~F. I.JSP. return
returns from the innermost prog (Le.. block) which is 1101 named t. In NIL (and COMMON
LISP), a return to a block name of nil only matches a block name of nil, and the block
name t is not distinguished in any way.
return-from name form
Evaluates fonn, and returns the value(s) it returns from the nearest lexically apparent
block with a name of name. name is not evaluated.
tagbody {tag J form}-
The body of a tagbody is examined sequentially. If a fonn is atomic. then it is a tag and
is ignored, otherwise it is evaluated. If during the evaluation a lexica11y apparent call to
go is evaluated with an argument of one of the tags. then control is returned to that
point within the tagbody fonn. which resumes its interpretation. If the interpretation
reaches the end of the tagbody. the result is nit
MC:NII.MAN:PCONS81
23-DEC-83