fib.fs

( in this example I use "TOS" to mean "Top of Stack" )

( Here is a one-line word, `f`, that computes the nth fibonacci number. )
: f 0 1 rot 1 ?DO swap over + LOOP swap . ; 

10 f . ( compute the 10th fibonacci number. )
       ( `.` pops the computed fibonacci number off the stack and prints it. )
       ( the stack is empty once again. )


( Here is a heavily annotated version of the same functionality. )
( Note the addition of the `next2fibs` word to increase readability in the `fib` loop. )

: next2fibs 
  ( copy the bottom fib to TOS ) 
  swap over 
  ( add top two values. now top, bottom are fib on TOS )
  + 
;

: fib             ( define the word `fib` )
  0 1             ( push the first two fibs on TOS. 0 is the bottom fib number. 1 is the top fib number.)
  rot 1           ( use `rot` [rotate] to grab the val below the 2 fibs. This value is used in initiating the loop. )

                  ( recall: prior to calling `fib`, the user pushed an int onto the stack. )
                  ( that int denotes which fibonacci number it should produce. )

  ?DO             ( loop _from_ 1 _to_ the value pushed on the stack prior to calling `f` )
                  ( `10 fib`: this will loop from 1 to to 10 )
    next2fibs
  LOOP 

  swap .          ( drop the bottom fib, leaving the top fib on TOS)
; 

( this produces the same result as `10 f .` on line 6 )
10 fib .