Optimising switching algorithms based on value of struct field

General Usage
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4

Actually, there is one sidenote:

Say I have predefined getE1(g), getE2(g) .... I need to programatically change s.efunc when I change one of the props. Naively I would do something like this:

function setPropB(s)
  s.prop_b = true
  s.efunc(s) = getE2(s)
end

this immediately gives the error

ERROR: syntax: Global method definition around ... needs to be placed at the top level, or use "eval".

Instead trying

function setPropB(s)
  s.prop_b = true
  eval(:(s.efunc(s) = begin; getE2(s); end ))
end

works, but is MetaProgramming, if I’m not mistaken, which is often discouraged here. Are there any caveats to using eval in this way?