# How to build functions from symbolic expressions with symbolics.jl

**URL:** <https://discourse.julialang.org/t/how-to-build-functions-from-symbolic-expressions-with-symbolics-jl/78879>\
**Category:** General Usage\
**Tags:** package, symbolic\
**Created:** [April 1, 2022, 3:25pm UTC](https://discourse.julialang.org/t/how-to-build-functions-from-symbolic-expressions-with-symbolics-jl/78879 "2022-04-01T15:25:57Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![F-YF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/f-yf/32/17363_2.png) [@F-YF](https://discourse.julialang.org/u/F-YF)\
**Post date:** [April 1, 2022, 3:25pm UTC](https://discourse.julialang.org/t/how-to-build-functions-from-symbolic-expressions-with-symbolics-jl/78879/1 "2022-04-01T15:25:57Z")

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I want to build functions from symbolic expressions with [symbolics.jl](https://symbolics.juliasymbolics.org/stable/tutorials/symbolic_functions/). I tried the following according to the documentation instructions.

```julia
using Symbolics
@variables x
p=(1+x)^2
f_expr=build_function(p,x)
f=eval(f_expr)
f(1) # get 4

```

It can run in REPL, but not in the function block. How do I implement it in a function block?

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<div class="post-metadata">

**Author:** ![goerch](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goerch/32/29122_2.png) [@goerch](https://discourse.julialang.org/u/goerch)\
**Post date:** [April 1, 2022, 3:42pm UTC](https://discourse.julialang.org/t/how-to-build-functions-from-symbolic-expressions-with-symbolics-jl/78879/2 "2022-04-01T15:42:16Z")

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I believe you see something like

```julia
The applicable method may be too new: running in world age 32545, while current world is 32546.

```

You could try something like

```julia
using Symbolics
function test()
    @variables x
    p=(1+x)^2
    f_expr=build_function(p,x)
    f=eval(f_expr)
    Base.@invokelatest f(1) # get 4
end
test()

```

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [April 1, 2022, 9:28pm UTC](https://discourse.julialang.org/t/how-to-build-functions-from-symbolic-expressions-with-symbolics-jl/78879/3 "2022-04-01T21:28:46Z")

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Just do `f=build_function(p,x, expression = Val{false})` like the docs say and you won’t have world age issues.

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<div class="post-metadata">

**Author:** ![F-YF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/f-yf/32/17363_2.png) [@F-YF](https://discourse.julialang.org/u/F-YF)\
**Post date:** [April 2, 2022, 1:38am UTC](https://discourse.julialang.org/t/how-to-build-functions-from-symbolic-expressions-with-symbolics-jl/78879/4 "2022-04-02T01:38:14Z")

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> [@ChrisRackauckas](#):
>
> f=build\_function(p,x, expression = Val{false})

It does work. Thanks a lot