euler2.factor

: fib ( a b -- x y ) [ + ] keep swap ;

! My goal was to implement this with produce.
1 1
[ dup 4000000 < ] [ fib dup ] produce
2nip ! Remove the workspace
dup length 1 - swap remove-nth ! Remove the extra value
[ dup odd? [ drop 0 ] when ]
map-sum

Looking good! Sticking with your usage of produce, there are a few built-in words that can clean things up a bit:

dup length 1 - swap remove-nth

...can be replaced with:

1 head*

And:

[ dup odd? [ drop 0 ] when ] map-sum

...can be replaced with:

[ even? ] filter sum

Also, head-slice* would be slightly more efficient compared to head* in this case, since it's modifying the virtual sequence rather than outputting a new sequence, but it won't realistically make a difference for this problem.

Thanks! I like how simple it is to replace logical sections of code with different words. After doing these two Euler problems, and you suggesting fixes that would be obvious to me in, say, Clojure, suggests that I am thinking too deeply about the stack. Needless to say, Factor is really messing with my brain, so that's good.