proc f {} { yield [set a 0] yield [set b 1] while 1 { yield [incr a $b] lassign [list $b $a] a b } } coroutine fib fThe call to coroutine is considered to be fib(0) and returns 0. Add a nullary yield (or the customary yield [info coroutine]) before the first yield to make the first call to fib the zeroth call.It's about six times slower than ::math::fibonacci, but it's hard to compare timing when the coroutine needs to be destroyed and reinstated during every iteration, and the call to fib wrapped in a for statement to advance the sequence.Time by:

time {for {set i 0} {$i <= 46} {incr i} fib ; rename fib "" ; coroutine fib f} 100000 ;# relative 5.5 time {::math::fibonacci 46} 100000 ;# relative 1.0

This command allows the Fibonacci series to be restarted if an argument is passed to fib:

proc f {} { yield while 1 { yield [set a 0] yield [set b 1] while {[yield [incr a $b]] eq {}} { lassign [list $b $a] a b } } } coroutine fib fIt is only slightly faster than the first command. Using this code and calling fib 46 times instead of using a loop makes this implementation about three times slower than ::math::fibonacci.Time by:

time {for {set i 0} {$i <= 46} {incr i} fib ; fib stop} 100000 ;# relative 4.9 time { fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib ; fib fib ; fib ; fib ; fib ; fib ; fib fib reset } 100000 ;# relative 3.1