Doesn't this traverse the list twice?

Presented with hard evidence I can't argue about efficiency.

I think my second point is more important-- The function doesn't implement "double-integer", it implements "multiply-all-numbers-together-and-then-by-2".. :)

The tests should be updated to check for functions that, when given more than one arg, do more than double the first, or just check that the function's arity is 1.

Actually, he's right. There is a big differente: https://gist.github.com/jmhdez/46de62a0520bd0c8081b

Could you please expand a bit more on the "inefficient" part?

Your second point is purely pedantic, I will ignore it.

Fair enough.

This is inefficient since partial creates a function that can take any number of args, plus it is incorrect, because it can take more than one arg (the function name being double-integer). I.e., your function could be passed 3 and 4, (double-integer 3 4) ; 24