3.14 Sequences
Sequence Constructors in Guide: PLT Scheme introduces sequences.
A sequence encapsulates an ordered stream of values. The elements of a sequence can be extracted with one of the for syntactic forms or with the procedures returned by sequence-generate.
The sequence datatype overlaps with many other datatypes. Among built-in datatypes, the sequence datatype includes the following:
lists (see Pairs and Lists)
vectors (see Vectors)
hash tables (see Hash Tables)
strings (see Strings)
byte strings (see Byte Strings)
input ports (see Ports)
In addition, make-do-sequence creates a sequence given a thunk that returns procedures to implement a generator, and the prop:sequence property can be associated with a structure type.
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 cause the bytes to be read from the port.
Inidvidual 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 Sequence Predicate and Constructors
v : any/c |
Return #t if v can be used as a sequence, #f otherwise.
end : number? |
start : number? |
end : number? |
step : number? = 1 |
Returns a sequence whose elements are numbers. The single-argument case (in-range end) is equivalent to (in-range 0 end 1). The first number in the sequence is start, and each successive element is generated by adding step to the previous element. The sequence stops before an element that would be greater or equal to end if step is non-negative, or less or equal to end if step is negative.
An in-range application can provide better performance for number iteration when it appears directly in a for clause.
(in-naturals [start]) → sequence? |
start : exact-nonnegative-integer? = 0 |
Returns an infinite sequence of exact integers starting with start, where each element is one more than the preceeding element.
An in-naturals application can provide better performance for integer iteration when it appears directly in a for clause.
lst : list? |
Returns a sequence equivalent to lst.
An in-list application can provide better performance for list iteration when it appears directly in a for clause.
vec : vector? |
start : exact-nonnegative-integer? = 0 |
stop : (or/c exact-nonnegative-integer? false/c) = #f |
step : (and/c exact-integer? (not/c zero?)) = 1 |
Returns a sequence equivalent to vec when no optional arguments are supplied.
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 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. The start and stop values are not checked against the size of vec, so access can fail when an element is demanded from the sequence.
An in-vector application can provide better performance for vector iteration when it appears directly in a for clause.
str : string? |
start : exact-nonnegative-integer? = 0 |
stop : (or/c exact-nonnegative-integer? false/c) = #f |
step : (and/c exact-integer? (not/c zero?)) = 1 |
Returns a sequence equivalent to str when no optional arguments are supplied.
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.
bstr : bytes? |
start : exact-nonnegative-integer? = 0 |
stop : (or/c exact-nonnegative-integer? false/c) = #f |
step : (and/c exact-integer? (not/c zero?)) = 1 |
Returns a sequence equivalent to bstr when no optional arguments are supplied.
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.
(in-input-port-bytes in) → sequence? |
in : input-port? |
Returns a sequence equivalent to in.
(in-input-port-chars in) → sequence? |
in : input-port? |
Returns a sequence whose elements are read as characters form in (as opposed to using in directly as a sequence to get bytes).
in : input-port? = (current-input-port) | ||||||||||||
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Returns a sequence whose elements are the result of (read-line in mode) until an end-of-line is encountered. Note that the default mode is 'any, whereas the default mode of read-line is 'linefeed.
hash : hash? |
Returns a sequence equivalent to hash.
(in-hash-keys hash) → sequence? |
hash : hash? |
Returns a sequence whose elements are the keys of hash.
(in-hash-values hash) → sequence? |
hash : hash? |
Returns a sequence whose elements are the values of hash.
(in-hash-pairs hash) → sequence? |
hash : hash? |
Returns a sequence whose elements are pairs, each containing a key and its value from hash (as opposed to using hash directly as a sequence to get the key and value as separate values for each element).
(in-value v) → sequence |
v : any/c |
Returns a sequence that produces a single value: v. This form is mostly useful for let-like bindings in forms such as for*/list.
(in-indexed seq) → sequence? |
seq : sequence? |
Returns a sequence where each element has two values: the value produced by seq, and a non-negative exact integer starting with 0. The elements of seq must be single-valued.
(in-parallel seq ) → sequence? |
seq : sequence? |
Returns a sequence where each element has as many values as the number of supplied seqs; the values, in order, are the values of each seq. The elements of each seq must be single-valued.
(stop-before seq pred) → sequence? |
seq : sequence? |
Returns a sequence that contains the elements of seq (which must be single-valued), but only until the last element for which applying pred to the element produces #t, after which the sequence ends.
(stop-after seq pred) → sequence? |
seq : sequence? |
Returns a sequence that contains the elements of seq (which must be single-valued), but only until the element (inclusive) for which applying pred to the element produces #t, after which the sequence ends.
(make-do-sequence thunk) → sequence? | ||||||||||||
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Returns a sequence whose elements are generated by the procedures and initial value returned by the thunk. The generator is defined in terms of a position, which is initialized to the third result of the thunk, and the element, which may consist of multiple values.
The thunk results define the generated elements as follows:
The first result is a pos->element procedure that takes the current position and returns the value(s) for the current element.
The second result is a next-pos procedure that takes the current position and returns the next position.
The third result is the initial position.
The fourth result takes the current position and returns a true result if the sequence includes the value(s) for the current position, and false if the sequence should end instead of including the value(s).
The fifth result is like the fourth result, but it takes the current element value(s) instead of the current position.
The sixth result is like the fourth result, but it takes both the current position and the current element values(s) and determines a sequence end after the current element is already included in the 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 the other two functions always return #t.
Associates a procedure to a structure type that takes an instance of the structure and returns a sequence. If v is an instance of a structure type with this property, then (sequence? v) produces #t.
Examples: | ||||||||||
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(engine boxcar caboose) |
3.14.2 Sequence Generators
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seq : sequence? |
Returns two thunks to extract elements from the sequence. The first returns #t if more values are available for the sequence. The second returns the next element (which may be multiple values) from the sequence; if no more elements are available, the exn:fail:contract exception is raised.