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doc ///
Key
Iterator
Headline
class for iterators
Description
Text
This is a class designed to simplify writing @TO iterator@ methods.
Each instance is a nullary @TO FunctionClosure@ that serves as the
@TO next@ method for the iterator.
Example
iter = iterator {1, 2, 3}
code iter
Text
Each call of @TT "next iter"@ is equivalent to @TT "iter()"@.
Example
next iter
iter()
next iter
iter()
Text
Every @TT "Iterator"@ object is an iterator for itself.
Example
primes = Iterator (
p := 2;
() -> (
r := p;
p = nextPrime(p + 1);
r));
iterator primes === primes
Text
However, we cannot "rewind" an @TT "Iterator"@ object. Every time that
it is iterated upon using @TO "for"@, @TO scan@, etc., iteration will
resume where it left off previously.
Example
for p in primes list if p > 20 then break else p
for p in primes list if p > 20 then break else p
Text
Contrast this with most other classes with the @TO iterator@ method
installed, like, strings, for which a new @TT "Iterator"@ object is
created every time it is iterated upon, and so iteration starts over
from the beginning
Example
s = "Hello, world!"
for c in s list c
for c in s list c
SeeAlso
iterator
next
StopIteration
///
doc ///
Key
iterator
(iterator, Iterator)
(iterator, String)
(iterator, VisibleList)
Headline
get an iterator
Usage
iterator x
Inputs
x:Thing
Outputs
:Thing -- likely an @TO Iterator@
Description
Text
An iterator is an object that is used to traverse through @TT "x"@.
Usually, but not necessarily, this will be an instance of the
@TO Iterator@ class.
Example
iter = iterator {1, 2, 3}
Text
The class of an iterator should have a @TO next@ method installed that
gets the next element of @TT "x"@.
Example
next iter
next iter
next iter
Text
If @TT "x"@ contains only a finite number of elements, then @TO next@
should return the symbol @TO StopIteration@ after exhausting them all.
Example
next iter
Text
Instances of classes with this method installed can be used like lists
in @TO "for"@ loops and @TO scan@.
Example
lookup(iterator, String)
for i in "foo" list i
scan("foo", print)
Text
They can also be passed to @TO apply@ and @TO select@. In each case, an
@TO Iterator@ object will be returned that is an iterator for itself.
Example
apply("foo", toUpper)
for i in oo list i
select("foo", i -> i == "o")
for i in oo list i
SeeAlso
Iterator
next
StopIteration
///
doc ///
Key
next
(next, Iterator)
Headline
get the next object from an iterator
Usage
next x
Inputs
x:Thing -- an iterator
Outputs
:Thing -- the next object from @TT "x"@
Description
Text
This gets the next object from an iterator, i.e., something returned by
@TO iterator@.
Example
iter = iterator {1, 2, 3}
next iter
next iter
next iter
Text
If the iterator is exhausted, then the symbol @TO StopIteration@ is
returned.
Example
next iter
SeeAlso
Iterator
iterator
StopIteration
///
doc ///
Key
StopIteration
Headline
stop iteration
Description
Text
This symbol is returned by @TO next@ to signal that iteration is complete.
Example
iter = iterator {1, 2, 3}
next iter
next iter
next iter
next iter
SeeAlso
Iterator
iterator
next
///
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