# Interpolate variable value inside macro call

**URL:** <https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740>\
**Category:** General Usage\
**Tags:** macros\
**Created:** [February 19, 2022, 1:00pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740 "2022-02-19T13:00:48Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![Gregstrq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gregstrq/32/20620_2.png) [@Gregstrq](https://discourse.julialang.org/u/Gregstrq)\
**Post date:** [February 19, 2022, 1:00pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740/1 "2022-02-19T13:00:48Z")

</div>

I am trying to parse a file containing half-lives of nuclear isotopes. One of the columns contains the units of the half-life time as a string, and I would like to convert it into Unitful units.

As an example, let’s say I parsed one of the lines and obtained that the unit of half-life is “year”

```julia
unit_str = "yr"

```

Now, I want to convert it to the Unitful units using something along the lines of

```julia
using Unitful
hl_unit = @u_str($unit_str)

```

I thought `$unit_str` would interpolate the actual value inside the macro call, however, apparently, it does not work like this.  
**How can I actually interpolate the value of `unit_str` inside the macro call?**  
What I want in the end is to have

```julia
hl_unit = @u_str("yr")

```

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [February 19, 2022, 1:18pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740/2 "2022-02-19T13:18:38Z")

</div>

Macros transform code, this is not possible. You sure you should be using `@u_str`? There is not a function that does the same thing and can be called over non-literals?

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<div class="post-metadata">

**Author:** ![Gregstrq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gregstrq/32/20620_2.png) [@Gregstrq](https://discourse.julialang.org/u/Gregstrq)\
**Post date:** [February 19, 2022, 1:27pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740/3 "2022-02-19T13:27:59Z")

</div>

As far as I am aware, there is only a macro version.

Could it be possible to wrap the call to `@u_str` inside another macro which does the interpolation?  
I have tried to define

```julia
macro interpolate_ustr(str)
    return :(@u_str($str))
end

```

Unfortunately, it still does not work…

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [February 19, 2022, 1:42pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740/4 "2022-02-19T13:42:10Z")

</div>

Macros cannot touch values inside variables in general. They can output code that passes the variable to a function, but they themselves directly only see what a programmer see looking at a code not in execution yet (this is, variable names).

I do not use this library, but it seems to me that [`@u_str`](https://painterqubits.github.io/Unitful.jl/stable/#Important-note-on-namespaces) is useful to automatically import for you some “Units, dimensions, and fundamental constants” but you can manually import them and then call convert?

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<div class="post-metadata">

**Author:** ![Gregstrq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gregstrq/32/20620_2.png) [@Gregstrq](https://discourse.julialang.org/u/Gregstrq)\
**Post date:** [February 19, 2022, 1:49pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740/5 "2022-02-19T13:49:05Z")

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I guess I can directly access the names of the units by constructing the corresponding symbols.  
Thank you, @Henrique_Becker.

It is hard to wrap ones head around macroprogramming…

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [February 19, 2022, 1:53pm UTC](https://discourse.julialang.org/t/interpolate-variable-value-inside-macro-call/76740/6 "2022-02-19T13:53:39Z")

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> [@Gregstrq](#):
>
> It is hard to wrap ones head around macroprogramming…

It sure is, My advice is: avoid `macro` solutions unless you are entirely sure why you are using a `macro`; and keep in mind that macros are “basically” a glorified Regex match and replace/rearrange over code. They just see lines and code and change them to other lines of code.