Interpolate variable value inside macro call

General Usage
1

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”

unit_str = "yr"

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

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

hl_unit = @u_str("yr")
2

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?

3

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

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

Unfortunately, it still does not work…

4

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 is useful to automatically import for you some “Units, dimensions, and fundamental constants” but you can manually import them and then call convert?

1 Like
5

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…

6

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.

1 Like