Bob Nystrom
85a889e49f
Refactor Sequence.map and Sequence.where to return lazy MapSequence and WhereSequence wrappers instead of eagerly building a List. Add new MapSequence and WhereSequence classes that delegate iteration and apply the transformation or predicate on-the-fly. Update documentation to reflect lazy semantics and the need to call .list to materialize results. Adjust all existing tests to call .list on map/where results. Add new tests in test/core/sequence/ verifying lazy behavior with infinite Fibonacci iterators.
3.5 KiB
^title Sequence Class ^category core
An abstract base class for any iterable object. Any class that implements the core iterator protocol can extend this to get a number of helpful methods.
Methods
all(predicate)
Tests whether all the elements in the sequence pass the predicate.
Iterates over the sequence, passing each element to the function predicate.
If it returns something false, stops iterating
and returns the value. Otherwise, returns true.
:::dart
[1, 2, 3].all {|n| n > 2} // False.
[1, 2, 3].all {|n| n < 4} // True.
any(predicate)
Tests whether any element in the sequence passes the predicate.
Iterates over the sequence, passing each element to the function predicate.
If it returns something true, stops iterating and
returns that value. Otherwise, returns false.
:::dart
[1, 2, 3].any {|n| n < 1} // False.
[1, 2, 3].any {|n| n > 2} // True.
contains(element)
Returns whether the sequence contains any element equal to the given element.
count
The number of elements in the sequence.
Unless a more efficient override is available, this will iterate over the sequence in order to determine how many elements it contains.
count(predicate)
Returns the number of elements in the sequence that pass the predicate.
Iterates over the sequence, passing each element to the function predicate
and counting the number of times the returned value evaluates to true.
:::dart
[1, 2, 3].count {|n| n > 2} // 1.
[1, 2, 3].count {|n| n < 4} // 3.
each(function)
Iterates over the sequence, passing each element to the given function.
:::dart
["one", "two", "three"].each {|word| IO.print(word) }
join(sep)
Returns a string representation of the list. The string representations of the
elements in the list is concatenated with intervening occurrences of sep.
It is a runtime error if sep is not a string.
join
Calls join with the empty string as the separator.
list
Creates a list containing all the elements in the sequence.
:::dart
(1..3).list // [1, 2, 3]
map(transformation)
Creates a new sequence that applies the transformation to each element in the
original sequence while it is iterated.
The list method can be used to turn the resulting sequence into a list.
:::dart
[1, 2, 3].map {|n| n * 2}.list // [2, 4, 6].
reduce(function)
Reduces the sequence down to a single value. function is a function that takes two arguments, the accumulator and sequence item and returns the new accumulator value. The accumulator is initialized from the first item in the sequence. Then, the function is invoked on each remaining item in the sequence, iteratively updating the accumulator.
It is a runtime error to call this on an empty sequence.
reduce(seed, function)
Similar to above, but uses seed for the initial value of the accumulator. If the sequence is empty, returns seed.
where(predicate)
Creates a new sequence containing only the elements from the original sequence
that pass the predicate.
During iteration, each element in the original sequence is passed to the
function predicate. If it returns false, the element is skipped.
The list method can be used to turn the resulting sequence into a list.
:::dart
(1..10).where {|n| n % 2 == 1}.list // [1, 3, 5, 7, 9].