# A question about defining a function inside another using metaprogramming

**URL:** <https://discourse.julialang.org/t/a-question-about-defining-a-function-inside-another-using-metaprogramming/20905>\
**Category:** General Usage\
**Tags:** first-steps\
**Created:** [February 17, 2019, 4:00pm UTC](https://discourse.julialang.org/t/a-question-about-defining-a-function-inside-another-using-metaprogramming/20905 "2019-02-17T16:00:40Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![pdenapo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pdenapo/32/5686_2.png) [@pdenapo](https://discourse.julialang.org/u/pdenapo)\
**Post date:** [February 17, 2019, 4:00pm UTC](https://discourse.julialang.org/t/a-question-about-defining-a-function-inside-another-using-metaprogramming/20905/1 "2019-02-17T16:00:40Z")

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Consider the following code for defining a function from a symbolic expression, using the metaprogramming facilities of Julia:

```julia
expression_G = :(x^3)
G = eval(:(x->$expression_G))
println(G(3)) 

```

it works as expected and prints the value 27. However, if you do exactly the same inside a function.

```julia
function f()
 expression_G = :(x^3)
 G = eval(:(x->$expression_G))
 println(G(3)) 
end 

f()

```

you get an error message like this one:

```julia
include("example2.jl")
ERROR: LoadError: MethodError: no method matching (::getfield(Main, Symbol("##5#6")))(::Int64)
The applicable method may be too new: running in world age 25570, while current world is 25571.
Closest candidates are:
  #5(::Any) at /home/pablo/programitas/test/example2.jl:3 (method too new to be called from this world context.)
Stacktrace:
 [1] f() at /home/pablo/programitas/test/example2.jl:4
 [2] top-level scope at none:0
 [3] include at ./boot.jl:326 [inlined]
 [4] include_relative(::Module, ::String) at ./loading.jl:1038
 [5] include(::Module, ::String) at ./sysimg.jl:29
 [6] include(::String) at ./client.jl:403
 [7] top-level scope at none:0
in expression starting at /home/pablo/programitas/test/example2.jl:7

```

What is happening and can I do something about it? (I guess that  
this is related to when the code is actually gets compiled.)

---

<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [February 17, 2019, 4:22pm UTC](https://discourse.julialang.org/t/a-question-about-defining-a-function-inside-another-using-metaprogramming/20905/2 "2019-02-17T16:22:44Z")

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See [Methods · The Julia Language](https://docs.julialang.org/en/v1/manual/methods/#Redefining-Methods-1).

The function won’t be visible until execution returns to the top level, which is when the world-age-related updates happen. You could use `Base.invokelatest(G, 3)` if you want to call `G` inside `f`, but there’s a bit of a performance penalty for doing that.

Note that typically, there’s a better way than `eval`ing expressions in a function to solve the same problem.