Provided by: tcl8.6-doc_8.6.13+dfsg-2_all 
NAME
coroutine, yield, yieldto - Create and produce values from coroutines
SYNOPSIS
coroutine name command ?arg...?
yield ?value?
yieldto command ?arg...? │
name ?value...? │
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DESCRIPTION
The coroutine command creates a new coroutine context (with associated command) named name
and executes that context by calling command, passing in the other remaining arguments
without further interpretation. Once command returns normally or with an exception (e.g.,
an error) the coroutine context name is deleted.
Within the context, values may be generated as results by using the yield command; if no
value is supplied, the empty string is used. When that is called, the context will
suspend execution and the coroutine command will return the argument to yield. The
execution of the context can then be resumed by calling the context command, optionally
passing in the single value to use as the result of the yield call that caused the context
to be suspended. If the coroutine context never yields and instead returns conventionally,
the result of the coroutine command will be the result of the evaluation of the context.
The coroutine may also suspend its execution by use of the yieldto command, which instead │
of returning, cedes execution to some command called command (resolved in the context of │
the coroutine) and to which any number of arguments may be passed. Since every coroutine │
has a context command, yieldto can be used to transfer control directly from one coroutine │
to another (this is only advisable if the two coroutines are expecting this to happen) but │
any command may be the target. If a coroutine is suspended by this mechanism, the │
coroutine processing can be resumed by calling the context command optionally passing in │
an arbitrary number of arguments. The return value of the yieldto call will be the list of │
arguments passed to the context command; it is up to the caller to decide what to do with │
those values. │
The recommended way of writing a version of yield that allows resumption with multiple │
arguments is by using yieldto and the return command, like this: │
proc yieldm {value} { │
yieldto return -level 0 $value │
} │
The coroutine can also be deleted by destroying the command name, and the name of the
current coroutine can be retrieved by using info coroutine. If there are deletion traces
on variables in the coroutine's implementation, they will fire at the point when the
coroutine is explicitly deleted (or, naturally, if the command returns conventionally).
At the point when command is called, the current namespace will be the global namespace
and there will be no stack frames above it (in the sense of upvar and uplevel). However,
which command to call will be determined in the namespace that the coroutine command was
called from.
EXAMPLES
This example shows a coroutine that will produce an infinite sequence of even values, and
a loop that consumes the first ten of them.
proc allNumbers {} {
yield
set i 0
while 1 {
yield $i
incr i 2
}
}
coroutine nextNumber allNumbers
for {set i 0} {$i < 10} {incr i} {
puts "received [nextNumber]"
}
rename nextNumber {}
In this example, the coroutine acts to add up the arguments passed to it.
coroutine accumulator apply {{} {
set x 0
while 1 {
incr x [yield $x]
}
}}
for {set i 0} {$i < 10} {incr i} {
puts "$i -> [accumulator $i]"
}
This example demonstrates the use of coroutines to implement the classic Sieve of
Eratosthenes algorithm for finding prime numbers. Note the creation of coroutines inside a
coroutine.
proc filterByFactor {source n} {
yield [info coroutine]
while 1 {
set x [$source]
if {$x % $n} {
yield $x
}
}
}
coroutine allNumbers apply {{} {while 1 {yield [incr x]}}}
coroutine eratosthenes apply {c {
yield
while 1 {
set n [$c]
yield $n
set c [coroutine prime$n filterByFactor $c $n]
}
}} allNumbers
for {set i 1} {$i <= 20} {incr i} {
puts "prime#$i = [eratosthenes]"
}
This example shows how a value can be passed around a group of three coroutines that yield │
to each other: │
proc juggler {name target {value ""}} { │
if {$value eq ""} { │
set value [yield [info coroutine]] │
} │
while {$value ne ""} { │
puts "$name : $value" │
set value [string range $value 0 end-1] │
lassign [yieldto $target $value] value │
} │
} │
coroutine j1 juggler Larry [ │
coroutine j2 juggler Curly [ │
coroutine j3 juggler Moe j1]] "Nyuck!Nyuck!Nyuck!" │
DETAILED SEMANTICS
This example demonstrates that coroutines start from the global namespace, and that
command resolution happens before the coroutine stack is created.
proc report {where level} {
# Where was the caller called from?
set ns [uplevel 2 {namespace current}]
yield "made $where $level context=$ns name=[info coroutine]"
}
proc example {} {
report outer [info level]
}
namespace eval demo {
proc example {} {
report inner [info level]
}
proc makeExample {} {
puts "making from [info level]"
puts [coroutine coroEg example]
}
makeExample
}
Which produces the output below. In particular, we can see that stack manipulation has
occurred (comparing the levels from the first and second line) and that the parent level
in the coroutine is the global namespace. We can also see that coroutine names are local
to the current namespace if not qualified, and that coroutines may yield at depth (e.g.,
in called procedures).
making from 2
made inner 1 context=:: name=::demo::coroEg
SEE ALSO
apply(3tcl), info(3tcl), proc(3tcl), return(3tcl)
KEYWORDS
coroutine, generator