This is really where my problem arose. I understood everything up
to here, but I sort of had this idea, "hey zip returns a tuple so
that somehow the compiler
was going to figure out for me that function(e) has two values"
and is thus
a binary function so this should work (the 0.0 on the end was my
start value):

reduce!(function(e) { return (e[1]-e[0])*(e[1]-e[0]); })(0.0)

However, of course the compiler see's the tuple as just one value
- which is where I made my mistake.