Lp-section-4
Syntax of FOR Loop$s
LP4: Syntax of FOR Loop$s
To describe the syntax of FOR loop$s we first describe the most
elaborate FOR loop$. Then we note which elements can be omitted.
The most elaborate FOR loop$ is of the form
(LOOP$ FOR v1 OF-TYPE spec1
target1
AS v2 OF-TYPE spec2
target2
...
AS vn
OF-TYPE specn targetn
UNTIL :GUARD guard1
until-expr
WHEN :GUARD guard2
when-expr
op :GUARD guard3
body-expr)
where
- each vi is a legal variable symbol, they are all distinct,
and are collectively called the iteration variables,
- each speci is a type-spec for the corresponding
variable vi,
- each targeti is of one of the forms below and specifies the range
of values to be taken by the corresponding variable vi. Abstractly
you may think of the “range” here simply as a list of the values as
described below, where list-expr is a term (which is expected to
evaluate to a true list), lo-expr and hi-expr are terms (which
are expected to evaluate to integers), and step-expr is a term (which
is expected to evaluate to a positive integer). In the examples given below,
let rv be the list reverse function, so (rv '(a b c)) is (C B A).
- IN list-expr — range is the value of list-expr.
For example, the range of “IN (rv '(a b c))” is (C B
A).
- ON list-expr — range is the list of successive
non-nil tails of the value of list-expr. For example, the range
of “ON (rv '(a b c))” is ((C B A) (B A) (A)). The range
value is produced by (TAILS list-expr).
- FROM lo-expr TO hi-expr BY step-expr
— If “BY step-expr” is omitted, “BY 1” is used.
The range contains all the integers from the value of lo-expr to the
greatest integer less than or equal to the value of hi-expr such that
each is separated from its higher neighbor by the value of step-expr.
For example, the range of “FROM 1 TO 10 BY 2” is (1 3 5 7
9). The range value is produced by (FROM-TO-BY lo-expr
hi-expr step-expr).
- each guardi is a term,
- each until-expr, when-expr, and body-expr
is a tame term, and
- op is one of the operators
SUM, COLLECT, APPEND, ALWAYS, or THEREIS.
The symbols FOR, IN, ON, FROM, TO, BY,
OF-TYPE, WHEN, UNTIL, SUM, COLLECT, APPEND,
ALWAYS, and THEREIS when used in loop$ statements may be in
any package.
Common Lisp prohibits loops with both a WHEN clause and either an
ALWAYS or a THEREIS operator. For example, if you are tempted to
write “WHEN p ALWAYS q” you can instead write
“ALWAYS (if p q t).”
The following elements may be omitted from the most elaborate form and
still produce legal loop$ statements:
- any line beginning with AS, UNTIL or WHEN,
- any OF-TYPE speci, and
- any :GUARD guardi.
A FOR loop$ expression with just one iteration variable and in
which the iterative expressions mention no free variable other than the
iteration variable is called a simple loop$ (or, sometimes, a
simple loop). An example of a simple loop is
(loop$ for x in lst when (evenp x) collect (+ 1 (sq x))).
A FOR loop$ expression is called a fancy loop$ if it
is not simple. Both of the following loop$s are fancy.
(loop$ for x in xlst as y on ylst collect (expr x y))
(loop$ for x in xlst collect (expr x z))
The first is fancy because it has two iteration variables, x and
y. The second is fancy because the body freely uses the variable z
which is not the iteration variable.
Now go to lp-section-5 (or return to the Table of Contents).
