'\"
'\" Copyright (c) 2009 Donal K. Fellows.
'\"
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'\" RCS: @(#) $Id: coroutine.n,v 1.2 2009/03/26 10:43:47 dkf Exp $
'\" 
.so man.macros
.TH coroutine n 8.6 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
coroutine, yield \- Create and produce values from coroutines
.SH SYNOPSIS
.nf
\fBcoroutine \fIname command\fR ?\fIarg...\fR?
\fByield\fR ?\fIvalue\fR?
\fIname\fR ?\fIvalue\fR?
.fi
.BE
.SH DESCRIPTION
.PP
The \fBcoroutine\fR command creates a new coroutine context (with associated
command) named \fIname\fR and executes that context by calling \fIcommand\fR,
passing in the other remaining arguments without further interpretation. Once
\fIcommand\fR returns normally or with an exception (e.g., an error) the
coroutine context \fIname\fR is deleted.
.PP
Within the context, values may be generated as results by using the
\fByield\fR command; if no \fIvalue\fR is supplied, the empty string is used.
When that is called, the context will suspend execution and the
\fBcoroutine\fR command will return the argument to \fByield\fR. The execution
of the context can then be resumed by calling the context command, optionally
passing in the value to use as the result of the \fByield\fR call that caused
the context to be suspended. If the coroutine context never yields and instead
returns conventionally, the result of the \fBcoroutine\fR command will be the
result of the evaluation of the context.
.PP
The coroutine can also be deleted by destroying the command \fIname\fR, and
the name of the current coroutine can be retrieved by using \fBinfo
coroutine\fR. 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).
.SH EXAMPLES
.PP
This example shows a coroutine that will produce an infinite sequence of
even values, and a loop that consumes the first ten of them.
.PP
.CS
proc allNumbers {} {
    \fByield\fR
    set i 0
    while 1 {
        \fByield\fR $i
        incr i 2
    }
}
\fBcoroutine\fR nextNumber allNumbers
for {set i 0} {$i < 10} {incr i} {
    puts "received [\fInextNumber\fR]"
}
rename nextNumber {}
.CE
.PP
In this example, the coroutine acts to add up the arguments passed to it.
.PP
.CS
\fBcoroutine\fR accumulator apply {{} {
    set x 0
    while 1 {
        incr x [\fByield\fR $x]
    }
}}
for {set i 0} {$i < 10} {incr i} {
    puts "$i -> [\fIaccumulator\fR $i]"
}
.CE
.PP
This example demonstrates the use of coroutines to implement the classic Sieve
of Eratosthenes algorithm for finding prime numbers. Note the creation of
coroutines inside a coroutine.
.PP
.CS
proc filterByFactor {source n} {
    \fByield\fR [info coroutine]
    while 1 {
        set x [\fI$source\fR]
        if {$x % $n} {
            \fByield\fR $x
        }
    }
}
\fBcoroutine\fR allNumbers apply {{} {while 1 {\fByield\fR [incr x]}}}
\fBcoroutine\fR eratosthenes apply {c {
    \fByield\fR
    while 1 {
        set n [\fI$c\fR]
        \fByield\fR $n
        set c [\fBcoroutine\fR prime$n filterByFactor $c $n]
    }
}} allNumbers
for {set i 1} {$i <= 20} {incr i} {
    puts "prime#$i = [\fIeratosthenes\fR]"
}
.CE
.SH "SEE ALSO"
apply(n), info(n), proc(n), return(n)
.SH KEYWORDS
coroutine, generator
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