3.14.1 Sequences

Sequence Constructors in The Racket Guide introduces sequences.

A sequence encapsulates an ordered collection of values. The elements of a sequence can be extracted with one of the for syntactic forms, with the procedures returned by sequence-generate, or by converting the sequence into a stream.

The sequence datatype overlaps with many other datatypes. Among built-in datatypes, the sequence datatype includes the following:

• exact nonnegative integers (see below)

• strings (see Strings)

• byte strings (see Byte Strings)

• lists (see Pairs and Lists)

• mutable lists (see Mutable Pairs and Lists)

• vectors (see Vectors)

• hash tables (see Hash Tables)

• dictionaries (see Dictionaries)

• sets (see Sets)

• input ports (see Ports)

• streams (see Streams)

An exact number k that is a non-negative integer acts as a sequence similar to (in-range k), except that k by itself is not a stream.

Custom sequences can be defined using structure type properties. The easiest method to define a custom sequence is to use the gen:stream generic interface. Streams are a suitable abstraction for data structures that are directly iterable. For example, a list is directly iterable with first and rest. On the other hand, vectors are not directly iterable: iteration has to go through an index. For data structures that are not directly iterable, the iterator for the data structure can be defined to be a stream (e.g., a structure containing the index of a vector).

For example, unrolled linked lists (represented as a list of vectors) themeselves do not fit the stream abstraction, but have index-based iterators that can be represented as streams:

Examples:

> (struct unrolled-list-iterator (idx lst)     #:methods gen:stream     [(define (stream-empty? iter)        (define lst (unrolled-list-iterator-lst iter))        (or (null? lst)            (and (>= (unrolled-list-iterator-idx iter)                     (vector-length (first lst)))                 (null? (rest lst)))))      (define (stream-first iter)        (vector-ref (first (unrolled-list-iterator-lst iter))                    (unrolled-list-iterator-idx iter)))      (define (stream-rest iter)        (define idx (unrolled-list-iterator-idx iter))        (define lst (unrolled-list-iterator-lst iter))        (if (>= idx (sub1 (vector-length (first lst))))            (unrolled-list-iterator 0 (rest lst))            (unrolled-list-iterator (add1 idx) lst)))]) > (define (make-unrolled-list-iterator ul)     (unrolled-list-iterator 0 (unrolled-list-lov ul))) > (struct unrolled-list (lov)     #:property prop:sequence     make-unrolled-list-iterator) > (define ul1 (unrolled-list '(#(cracker biscuit) #(cookie scone)))) > (for/list ([x ul1]) x) '(cracker biscuit cookie scone)

The prop:sequence property provides more flexibility in specifying iteration, such as when a pre-processing step is needed to prepare the data for iteration. The make-do-sequence function creates a sequence given a thunk that returns procedures to implement a sequence, and the prop:sequence property can be associated with a structure type to implement its implicit conversion to a sequence.

For most sequence types, extracting elements from a sequence has no side-effect on the original sequence value; for example, extracting the sequence of elements from a list does not change the list. For other sequence types, each extraction implies a side effect; for example, extracting the sequence of bytes from a port causes the bytes to be read from the port. A sequence’s state may either span all uses of the sequence, as for a port, or it may be confined to each distinct time that a sequence is initiated by a for form, sequence->stream, sequence-generate, or sequence-generate*. Concretely, the thunk passed to make-do-sequence is called to initiate the sequence each time the sequence is used.

Individual elements of a sequence typically correspond to single values, but an element may also correspond to multiple values. For example, a hash table generates two values—a key and its value—for each element in the sequence.

3.14.1.1 Sequence Predicate and Constructors

See Vectors for information on using vectors as sequences.

The optional arguments start, stop, and step are analogous to in-range, except that a #f value for stop is equivalent to (vector-length vec). That is, the first element in the sequence is (vector-ref vec start), and each successive element is generated by adding step to index of the previous element. The sequence stops before an index that would be greater or equal to end if step is non-negative, or less or equal to end if step is negative.

If start is not a valid index, or stop is not in [-1, (vector-length vec)] then the exn:fail:contract exception is raised. If start is less than stop and step is negative, then the exn:fail:contract:mismatch exception is raised. Similarly, if start is more than stop and step is positive, then the exn:fail:contract:mismatch exception is raised.

An in-vector application can provide better performance for vector iteration when it appears directly in a for clause.

See Strings for information on using strings as sequences.

The optional arguments start, stop, and step are as in in-vector.

An in-string application can provide better performance for string iteration when it appears directly in a for clause.

See Byte Strings for information on using byte strings as sequences.

The optional arguments start, stop, and step are as in in-vector.

An in-bytes application can provide better performance for byte string iteration when it appears directly in a for clause.

Examples:

> (define table (hash 'a 1 'b 2)) > (for ([(key value) (in-hash table)])     (printf "key: ~a value: ~a\n" key value)) key: b value: 2 key: a value: 1

See Hash Tables for information on using hash tables as sequences.

Examples:

> (define table (hash 'a 1 'b 2)) > (for ([key (in-hash-keys table)])     (printf "key: ~a\n" key)) key: b key: a

Examples:

> (define table (hash 'a 1 'b 2)) > (for ([value (in-hash-values table)])     (printf "value: ~a\n" value)) value: 2 value: 1

Examples:

> (define table (hash 'a 1 'b 2)) > (for ([key+value (in-hash-pairs table)])     (printf "key and value: ~a\n" key+value)) key and value: (b . 2) key and value: (a . 1)

An in-directory sequence traverses nested subdirectories recursively. To generate a sequence that includes only the immediate content of a directory, use the result of directory-list as a sequence.

Each of the procedures listed above is called only once per position. Among the last three procedures, as soon as one of the procedures returns #f, the sequence ends, and none are called again. Typically, one of the functions determines the end condition, and #f is used in place of the other two functions.

Using a pre-existing sequence:

Examples:

> (struct my-set (table)     #:property prop:sequence     (lambda (s)       (in-hash-keys (my-set-table s)))) > (define (make-set . xs)     (my-set (for/hash ([x (in-list xs)])               (values x #t)))) > (for/list ([c (make-set 'celeriac 'carrot 'potato)])     c) '(celeriac potato carrot)

Using make-do-sequence:

Examples:

> (define-struct train (car next)     #:property prop:sequence (lambda (t)                                (make-do-sequence                                 (lambda ()                                   (values train-car                                           train-next                                           t                                           (lambda (t) t)                                           (lambda (v) #t)                                           (lambda (t v) #t)))))) > (for/list ([c (make-train 'engine                             (make-train 'boxcar                                         (make-train 'caboose                                                     #f)))])     c) '(engine boxcar caboose)

3.14.1.2 Sequence Conversion

If extracting an element from seq involves a side-effect, then the effect is performed each time that either stream-first or stream-rest is first used to access or skip an element.

3.14.1.3 Sequence Combinations

In case initiating s involves a side effect, the sequence s is not initiated until the resulting sequence is initiated, at which point the first i elements are extracted from the sequence.

If all given ss are streams, the result is also a stream.

If s is a stream, then the result is also a stream.

If s is a stream, then the result is also a stream.

If s is a stream, then the result is also a stream.