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class:: Pkey
summary:: access a key in an event stream
related:: Classes/Penvir
categories:: Streams-Patterns-Events>Patterns>Data Sharing
description::
Pkey simplifies backward access to values in an event being processed by link::Classes/Pbind:: or another event pattern.
ClassMethods::
method::new
argument::key
The name of the event variable to read from.
argument::repeats
The number of items returned before finishing. Using code::nil::, the default value, makes the stream infinite.
Examples::
code::
// \b should thus take twice the value of \a in each event:
p = Pbind(\a, Pwhite(1, 10, inf), \b, Pkey(\a) * 2).asStream;
p.next(()) // for Pbind, must pass in a default event even if empty
( 'a': 10, 'b': 20 )
( 'a': 2, 'b': 4 )
( 'a': 5, 'b': 10 )
( 'a': 4, 'b': 8 )
( 'a': 2, 'b': 4 )
// note: to check if a Pkey's result /equals/ another value,
// be sure to use |==|
p = Pbind(
\a, Pseries(1, 1, inf),
\b, Pif(
Pkey(\a) == 2,
"equals 2",
"nope"
)
).asStream;
p.next(()); // 'b' is always "nope"
p = Pbind(
\a, Pseries(1, 1, inf),
\b, Pif(
Pkey(\a) |==| 2,
"equals 2",
"nope"
)
).asStream;
p.next(()); // 'b' is as expected
::
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