package require stream

proc naturals {{start 0}} {
  set nextNum [expr {$start + 1}]
  stream create $start [list naturals $nextNum]
}

namespace import ::tcl::mathop::+

proc addThousand {num} {
  expr {$num + 1000}
}

proc isEven {num} {
  expr {$num % 2 == 0}
}

set thousands [stream map addThousand [naturals]]
set evenThousands [stream select isEven $thousands]
set tenThousands [stream take 10 $evenThousands]
set sum [stream foldl + 0 $tenThousands]
puts "sum: $sum"