Why doesn't set -x work within eval within a function?

I ran into an issue trying to dynamically set some variables based on output of a program, in a fish function.

I narrowed my issues down to a MWE:

function example
    eval (echo 'set -x FOO 1;')
end

calling:

>example
>echo $FOO

results in no output -- ie the FOO environment variable has not been set. How should I have done this?