### Posted to tcl by kbk at Fri Jun 12 21:26:28 GMT 2009view pretty

# In this perverse implementation of the Fibonacci sequence, every function # call is either a tailcall or an invocation of [list] to construct a closure. # There is no direct recursion, instead, control is handled in continuation # passing style. Note the strange use of [lindexc] to implement a conditional. namespace path ::tcl::mathop proc TRACE {} { puts [format %*s%s [info level] {} [info level [- [info level] 1]]] } proc TRACE args {} proc fib {n {c {{x} {set x}}}} { # apply c to nth element of fibonacci seq TRACE tailcall <c $n 2 [list [list nlt2 [list n $n] [list c $c]] { fib0 $nlt2 $n $c }] } proc fib0 {nlt2 n c} { # apply c to nth element of fibonacci seq, nlt2 is (n < 2) TRACE tailcall lindexc {fib1 apply} $nlt2 [list [list f [list n $n] [list c $c]] { tailcall $f $c $n }] } proc fib1 {c n} { # apply c to nth element of fibonacci sequence (n >= 2) TRACE tailcall -c $n 1 [list [list nm1 [list n $n] [list c $c]] { tailcall fib2 $nm1 $n $c }] } proc fib2 {nm1 n c} { # apply c to nth element of fibonacci sequence # n-1 has been completed and placed in $nm1 TRACE tailcall -c $n 2 [list [list nm2 [list nm1 $nm1] [list c $c]] { tailcall fib3 $nm1 $nm2 $c }] } proc fib3 {nm1 nm2 c} { # apply c to nth element of Fibonacci sequence. # n-1 and n-2 have been computed and placed in nm1 # and nm2 TRACE tailcall fib $nm1 [list [list fibnm1 [list nm2 $nm2] [list c $c]] { tailcall fib4 $fibnm1 $nm2 $c }] } proc fib4 {fibnm1 nm2 c} { # apply c to nth element of Fibonacci sequence. # (n-1)st element is known. TRACE tailcall fib $nm2 [list [list fibnm2 [list fibnm1 $fibnm1] [list c $c]] { tailcall +c $fibnm1 $fibnm2 $c }] } proc lindexc {lst idx c} { # apply c to [lindex $lst $idx] TRACE tailcall apply $c [lindex $lst $idx] } proc <c {x y c} { # apply c to the result of x<y TRACE tailcall apply $c [< $x $y] } proc +c {x y c} { # apply c to the sum of x and y TRACE tailcall apply $c [+ $x $y] } proc -c {x y c} { # apply c to the difference of x and y TRACE tailcall apply $c [- $x $y] } for {set i 0} {$i <= 10} {incr i} { puts [fib $i] }