# Scoping issues with Macro in Module

**URL:** <https://discourse.julialang.org/t/scoping-issues-with-macro-in-module/54383>\
**Category:** General Usage\
**Tags:** macros, module, quicktypes\
**Created:** [February 1, 2021, 2:58pm UTC](https://discourse.julialang.org/t/scoping-issues-with-macro-in-module/54383 "2021-02-01T14:58:22Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![mrVeng](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mrveng/32/8836_2.png) [@mrVeng](https://discourse.julialang.org/u/mrVeng)\
**Post date:** [February 1, 2021, 2:58pm UTC](https://discourse.julialang.org/t/scoping-issues-with-macro-in-module/54383/1 "2021-02-01T14:58:22Z")

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Hi everyone,

I need to create a mutable struct where I do not know the number of fields a priori. I could alternatively just use a NamedTuple for this, but in this case the struct is preferable to me. I can do this wonderfully already with the nice `QuickTypes` module:

```julia

using QuickTypes
param_names = (:μ, :σ, :p)
param_values = [1., 2., [.2, .3, .5] ]
@eval @qstruct_fp MyModel($(param_names...) )
MyModel(param_values...)

```

However, when I pack this into a module, I run into scoping issues, and no matter how much I try to solve this I do not manage to succeed. I already searched this forum and Google without success. An example code is provided below:

```julia
module TestModule
## Load and reexport QuickTypes
    using Reexport
    @reexport using QuickTypes
## Define Macro
    abstract type Supertype1 end
    macro make_new_struct(modelname, parametername)
        esc(:( @eval @qstruct_fp $modelname($(parametername...) ) <: Supertype1 ) )
        #do some other stuff
    end
## Export everything
    export @make_new_struct
    export Supertype1 
end

using Main.TestModule
TestModule.@make_new_struct(MyNewName, param_names) #LoadError: MethodError: no method matching iterate(::Symbol

```

I would be really relieved if someone could help me solve this issue!

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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 1, 2021, 4:12pm UTC](https://discourse.julialang.org/t/scoping-issues-with-macro-in-module/54383/2 "2021-02-01T16:12:28Z")

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I am not sure you are hitting scoping issues. In fact, I am not sure if your macro makes sense. The problem seems to happen because of the `parametername...` inside your macro. The `iterate` fails because you are trying to splat a `Symbol` (splat uses iterate to expand `parametername`, but `parametername` is a `Symbol` not a list). It seems to me that you misunderstand how macros work, they receive the code you wrote as arguments, not the values inside any variables (macros **cannot** receive values). Therefore `parametername` is just a symbol `:param_names` inside your macro, and your macro has not access to any information of what would be inside a variable with the same name in runtime.

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<div class="post-metadata">

**Author:** ![mrVeng](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mrveng/32/8836_2.png) [@mrVeng](https://discourse.julialang.org/u/mrVeng)\
**Post date:** [February 1, 2021, 4:43pm UTC](https://discourse.julialang.org/t/scoping-issues-with-macro-in-module/54383/3 "2021-02-01T16:43:20Z")

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Thank you for your reply! I am indeed not knowledgeable in this area. However, what `@qstruct_fp` does is just creating a parametric struct, i.e.

```julia
a #UndefVarError: a not defined
b #UndefVarError: a not defined
@qstruct_fp test123(a,b)
test123(1., 2) #test123{Float64,Int64} 

```

so when I call this Macro I actually dont need the values, but just the fieldnames - which is a case for Macros I assume?

If there are more elegant methods for creating fully parametric structs from a given vector/tuple of fieldnames, then I would be open to learn from that.

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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 1, 2021, 5:43pm UTC](https://discourse.julialang.org/t/scoping-issues-with-macro-in-module/54383/4 "2021-02-01T17:43:55Z")

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I think your code may work (did not test it), but then you need to pass the field names directly to the macro, instead of _hiding_ them as values inside a variable (in this your macro will only see the variable name). Did you consider taking a variable number of arguments in the macro? (i.e., appending `...` on the parameter `parametername`)

I think your example with `@eval @qstruct_fp MyModel($(param_names...) )` worked because you splatted and interpolated the `param_names` in the scope it is defined (and not inside the macro).