2.18 Iterations and Comprehensions: for, for/list, ...
Iterations and Comprehensions in The Racket Guide introduces iterations and comprehensions.
The for iteration forms are based on SRFI-42
[SRFI-42].
2.18.1 Iteration and Comprehension Forms
| (for (for-clause ...) body ...+) |
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| for-clause | | = | | [id seq-expr] | | | | | | | [(id ...) seq-expr] | | | | | | | #:when guard-expr | | | | | | | #:unless guard-expr |
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Iteratively evaluates body. The for-clauses
introduce bindings whose scope includes body and that
determine the number of times that body is evaluated.
In the simple case, each for-clause has one of its first two
forms, where [id seq-expr] is a shorthand for [(id) seq-expr]. In this simple case, the seq-exprs are evaluated
left-to-right, and each must produce a sequence value (see
Sequences).
The for form iterates by drawing an element from each
sequence; if any sequence is empty, then the iteration stops, and
#<void> is the result of the for expression. Otherwise
a location is created for each id to hold the values of each
element; the sequence produced by a seq-expr must return as
many values for each iteration as corresponding ids.
The ids are then bound in the body, which is
evaluated, and whose results are ignored. Iteration continues with the
next element in each sequence and with fresh locations for each
id.
A for form with zero for-clauses is equivalent to a
single for-clause that binds an unreferenced id to
a sequence containing a single element. All of the ids must
be distinct according to bound-identifier=?.
If any for-clause has the form #:when guard-expr,
then only the preceding clauses (containing no #:when or #:unless)
determine iteration as above, and the body is effectively
wrapped as
| (when guard-expr |
| (for (for-clause ...) body ...+)) |
using the remaining for-clauses. A for-clause of
the form #:unless guard-expr corresponds to the same transformation
with unless in place of when.
Examples: |
| (1 a #t)(1 a #f)(3 c #t)(3 c #f) | | (a 1)(b 20) | | here | | > (for ([i '()]) | | (error "doesn't get here")) |
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Iterates like
for, but that the last expression in the
bodys must
produce a single value, and the result of the
for/list
expression is a list of the results in order.
When evaluation of a
body is skipped due to a
#:when
or
#:unless clause, the result list includes no corresponding
element.
Examples: |
| '((1 #\a #t) (1 #\a #f) (3 #\c #t) (3 #\c #f)) | | > (for/list () 'any) | '(any) | | '() |
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Iterates like
for/list, but the result are accumulated into
a vector instead of a list. If the optional
#:length
form is used, then
length-expr must evaluate to an
exact-nonnegative-integer?, and the result vector is
constructed with this length. In this case, the iteration can be
performed more efficiently, and terminates when the vector is full or
the requested number of iterations have been performed, whichever
comes first. If the provided
length-expr evaluates to a
length longer than the number of iterations then the remaining slots
of the vector are intialized to the default argument of
make-vector.
Like
for/list, but the result is an immutable
hash
table;
for/hash creates a table using
equal? to
distinguish keys,
for/hasheq produces a table using
eq?, and
for/hasheqv produces a table using
eqv?. The last expression in the
bodys must return
two values: a key and a value to extend the hash table accumulated by
the iteration.
Example: |
| '#hash((1 . "1") (2 . "2") (3 . "3")) |
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| (for/and (for-clause ...) body ...+) |
Iterates like
for, but when last expression of
body produces
#f, then iteration terminates, and the result of the
for/and expression is
#f. If the
body
is never evaluated, then the result of the
for/and
expression is
#t. Otherwise, the result is the (single)
result from the last evaluation of
body.
Examples: |
| > (for/and ([i '(1 2 3 "x")]) | | (i . < . 3)) |
| #f | | 4 | | #t |
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| (for/or (for-clause ...) body ...+) |
Iterates like
for, but when last expression of
body produces
a value other than
#f, then iteration terminates, and
the result of the
for/or expression is the same
(single) value. If the
body is never evaluated, then the
result of the
for/or expression is
#f. Otherwise, the result is
#f.
Examples: |
| > (for/or ([i '(1 2 3 "x")]) | | (i . < . 3)) |
| #t | | 1 | | #f |
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| (for/sum (for-clause ...) body ...+) |
|
Iterates like
for, but each result of the last
body
is accumulated into a result with
+.
Iterates like
for, but each result of the last
body
is accumulated into a result with
*.
| (for/lists (id ...) (for-clause ...) body ...+) |
Similar to
for/list, but the last
body expression
should produce as many values as given
ids, and the result is
as many lists as supplied
ids. The
ids are bound to
the lists accumulated so far in the
for-clauses and
bodys.
Iterates like
for, but after
body is evaluated the first
time, then the iteration terminates, and the
for/first
result is the (single) result of
body. If the
body is never evaluated, then the result of the
for/first expression is
#f.
Iterates like
for, but the
for/last result is the (single)
result of the last evaluation of
body. If the
body is never evaluated, then the result of the
for/last expression is
#f.
| (for/fold ([accum-id init-expr] ...) (for-clause ...) . body) |
Iterates like
for. Before iteration starts, the
init-exprs are evaluated to produce initial accumulator
values. At the start of each iteration, a location is generated
for each
accum-id, and the corresponding current accumulator
value is placed into the location. The last expression in
body must produce as many values as
accum-ids, and
those values become the current accumulator values. When iteration
terminates, the results of the
fold/for expression are the
accumulator values.
Example: |
| 10 | '(2 1.7320508075688772 1.4142135623730951 1) |
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| (for* (for-clause ...) body ...+) |
Like
for, but with an implicit
#:when #t between
each pair of
for-clauses, so that all sequence iterations are
nested.
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| (for*/or (for-clause ...) body ...+) |
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| (for*/fold ([accum-id init-expr] ...) (for-clause ...) body ...+) |
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Example: |
| '((1 #\a) (1 #\b) (2 #\a) (2 #\b)) |
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2.18.2 Deriving New Iteration Forms
| (for/fold/derived orig-datum | | ([accum-id init-expr] ...) (for-clause ...) body ...+) |
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Like
for/fold, but the extra
orig-datum is used as the source for all syntax errors.
Examples: |
| | eval:487:0: for/digits: bad sequence binding clause at: a | in: (for/digits (a (in-list (quote (1 2 3)))) (b (in-list | (quote (4 5 6)))) (+ a b)) | | 963 |
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Like
for*/fold, but the extra
orig-datum is used as the source for all syntax errors.
Examples: |
| | eval:490:0: for*/digits: bad sequence binding clause at: ds | in: (for*/digits (ds (in-list (quote ((8 3) (1 1))))) (d | (in-list ds)) d) | | 1138 |
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Defines
id as syntax. An
(id . rest) form is
treated specially when used to generate a sequence in a
clause of
for (or one of its variants). In that
case, the procedure result of
clause-transform-expr is called
to transform the clause.
When id is used in any other expression position, the result
of expr-transform-expr is used. If it is a procedure of zero
arguments, then the result must be an identifier other-id,
and any use of id is converted to a use of
other-id. Otherwise,expr-transform-expr must
produce a procedure (of one argument) that is used as a macro
transformer.
When the clause-transform-expr transformer is used, it is
given a clause as an argument, where the clause’s form is
normalized so that the left-hand side is a parenthesized sequence of
identifiers. The right-hand side is of the form (id . rest).
The result can be either #f, to indicate that the forms
should not be treated specially (perhaps because the number of bound
identifiers is inconsistent with the (id . rest) form), or a
new clause to replace the given one. The new clause might
use :do-in. To protect identifiers in the result of
clause-transform-expr, use for-clause-syntax-protect
instead of syntax-protect.
Examples: |
| | | > (for/list ([d (in-digits 1138)]) d) | '(8 3 1 1) | | > (map in-digits (list 137 216)) | '((7 3 1) (6 1 2)) |
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| (:do-in ([(outer-id ...) outer-expr] ...) | | outer-check | | ([loop-id loop-expr] ...) | | pos-guard | | ([(inner-id ...) inner-expr] ...) | | pre-guard | | post-guard | | (loop-arg ...)) |
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A form that can only be used as a
seq-expr in a
clause of
for (or one of its variants).
Within a for, the pieces of the :do-in form are
spliced into the iteration essentially as follows:
| (let-values ([(outer-id ...) outer-expr] ...) |
| outer-check |
| (let loop ([loop-id loop-expr] ...) |
| (if pos-guard |
| (let-values ([(inner-id ...) inner-expr] ...) |
| (if pre-guard |
| (let body-bindings |
| (if post-guard |
| (loop loop-arg ...) |
| done-expr)) |
| done-expr)) |
| done-expr))) |
where body-bindings and done-expr are from the
context of the :do-in use. The identifiers bound by the
for clause are typically part of the ([(inner-id ...) inner-expr] ...) section.
The actual loop binding and call has additional loop
arguments to support iterations in parallel with the :do-in
form, and the other pieces are similarly accompanied by pieces from
parallel iterations.
For an example of :do-in, see define-sequence-syntax.
Provided
for-syntax: Like
syntax-protect, but allows
the
for expander to
disarm the result syntax object,
and arms the pieces of a clause instead of the entire syntax object.
Use this function to protect the result of a
clause-transform-expr that is bound by
define-sequence-syntax.
2.18.3 Do Loops
| (do ([id init-expr step-expr-maybe] ...) | | (stop?-expr finish-expr ...) | | expr ...) |
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| step-expr-maybe | | = | | | | | | | | | step-expr |
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Iteratively evaluates the exprs for as long as
stop?-expr returns #f.
To initialize the loop, the init-exprs are evaluated in order
and bound to the corresponding ids. The ids are
bound in all expressions within the form other than the
init-exprs.
After the ids have been bound, the stop?-expr is
evaluated. If it produces #f, each expr is evaluated
for its side-effect. The ids are then effectively updated
with the values of the step-exprs, where the default
step-expr for id is just id; more
precisely, iteration continues with fresh locations for the
ids that are initialized with the values of the corresponding
step-exprs.
When stop?-expr produces a true value, then the
finish-exprs are evaluated in order, and the last one is
evaluated in tail position to produce the overall value for the
do form. If no finish-expr is provided, the value of
the do form is #<void>.