# MethodError for Composite Types

**URL:** <https://discourse.julialang.org/t/methoderror-for-composite-types/93246>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [January 20, 2023, 3:51am UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246 "2023-01-20T03:51:55Z")\
**Posts on this page:** 10\
**Page:** 1

<div class="post-metadata">

**Author:** ![jonlym](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonlym/32/20007_2.png) [@jonlym](https://discourse.julialang.org/u/jonlym)\
**Post date:** [January 20, 2023, 3:51am UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/1 "2023-01-20T03:51:55Z")

</div>

I would like to create a composite type, initialize some constant objects of that type, and then use the objects with methods I’ve coded. For example, below I have created a folder with 3 files:

1. `RectangleMod.jl` - Defines the `Rectangle` type and a corresponding method to calculate its area:

```julia
module RectangleMod

export Rectangle, get_area

struct Rectangle
    width::Number
    height::Number
end

function get_area(rectangle::Rectangle)
    return rectangle.width * rectangle.height;
end

end

```

1. `savedRectangles.jl` - Stores an object of `Rectangle` type

```julia
module savedRectangles
 
include("./RectangleMod.jl")
using .RectangleMod

export MY_RECTANGLE

const MY_RECTANGLE = Rectangle(1.0, 2.0);

end

```

1. `test_rectangle.jl` - Imports `MY_RECTANGLE` and tries to calculate its area:

```julia
include("./savedRectangles.jl")
include("./RectangleMod.jl")

using .savedRectangles
using .RectangleMod

get_area(MY_RECTANGLE)

```

However, I get the following error when I run `test_rectangle.jl`:

```julia
ERROR: LoadError: MethodError: no method matching get_area(::Main.savedRectangles.RectangleMod.Rectangle)
Closest candidates are:
  get_area(::Rectangle) at c:\Users\jlym\Downloads\test_rectangle\RectangleMod.jl:10
Stacktrace:
 [1] top-level scope
   @ c:\Users\jlym\Downloads\test_rectangle\test_rectangle.jl:7
in expression starting at c:\Users\jlym\Downloads\test_rectangle\test_rectangle.jl:7

```

The stacktrace suggests `MY_RECTANGLE` is type `::Main.savedRectangles.RectangleMod.Rectangle`, which is not recognized as a `::Rectangle` type. What can I do to resolve this?

EDIT: Updated code snippers to enable syntax highlighting

---

<div class="post-metadata">

**Author:** ![mkitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mkitti/32/12459_2.png) [@mkitti](https://discourse.julialang.org/u/mkitti)\
**Post date:** [January 20, 2023, 7:02am UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/2 "2023-01-20T07:02:07Z")

</div>

You should use `include` for a file at most once if at all.

There are larger file organization issues here, but I will try to address this minimally.

Change file 3, test\_rectangle.jl, to be this:

```julia
include("./savedRectangles.jl")

using .savedRectangles
using .savedRectangles.RectangleMod

get_area(MY_RECTANGLE)

```

---

<div class="post-metadata">

**Author:** ![Rex\_Wang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rex_wang/32/35108_2.png) [@Rex\_Wang](https://discourse.julialang.org/u/Rex_Wang)\
**Post date:** [January 20, 2023, 7:31am UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/3 "2023-01-20T07:31:42Z")

</div>

> [@jonlym](#):
>
> ```julia
> module savedRectangles
>  
> include("./RectangleMod.jl")
> using .RectangleMod
> end
> 
> ```

In file 2, you treat `RectangleMod` as a submodule of `savedRectangles`, making `MY_RECTANGLE` a type of `savedRectangles.RectangleMod`.

It might be better to treat them as separated packages. Try

```julia
include("./RectangleMod.jl")

module savedRectangles
using Main.RectangleMod

export MY_RECTANGLE

const MY_RECTANGLE = Rectangle(1.0, 2.0);

end

```

in file 2.

EDIT: Add prefix `Main.` to `RectangleMod`

---

<div class="post-metadata">

**Author:** ![jonlym](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonlym/32/20007_2.png) [@jonlym](https://discourse.julialang.org/u/jonlym)\
**Post date:** [January 20, 2023, 6:28pm UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/4 "2023-01-20T18:28:26Z")

</div>

Thank you! The code ran successfully.

Is this how developers solve the issue when they develop their own packages? It seems cumbersome to have to know how your imported modules import their modules. I’m open to suggestions on restructuring whole code.

---

<div class="post-metadata">

**Author:** ![jonlym](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonlym/32/20007_2.png) [@jonlym](https://discourse.julialang.org/u/jonlym)\
**Post date:** [January 20, 2023, 6:31pm UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/5 "2023-01-20T18:31:20Z")

</div>

Thank you for your suggestion! I moved the `include` line outside the module but received this error:

```julia
ERROR: LoadError: UndefVarError: RectangleMod not defined
Stacktrace:
 [1] include(fname::String)
   @ Base.MainInclude .\client.jl:476
 [2] top-level scope
   @ c:\Users\jlym\Downloads\test_rectangle\test_rectangle.jl:2
in expression starting at c:\Users\jlym\Downloads\test_rectangle\savedRectangles.jl:3

```

Was there any other changes I should have made?

* * *

**RectangleMod.jl**

```julia
module RectangleMod

export Rectangle, get_area

struct Rectangle
    width::Number
    height::Number
end

function get_area(rectangle::Rectangle)
    return rectangle.width * rectangle.height;
end

end

```

**savedRectangles.jl**

```julia
include("./RectangleMod.jl")

module savedRectangles
using .RectangleMod

export MY_RECTANGLE

const MY_RECTANGLE = Rectangle(1.0, 2.0);

end

```

**test\_rectangle.jl**

```julia
include("./RectangleMod.jl")

include("./savedRectangles.jl")

using .RectangleMod

using .savedRectangles

get_area(MY_RECTANGLE)

```

---

<div class="post-metadata">

**Author:** ![Raf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raf/32/3383_2.png) [@Raf](https://discourse.julialang.org/u/Raf)\
**Post date:** [January 20, 2023, 8:50pm UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/6 "2023-01-20T20:50:56Z")

</div>

> Is this how developers solve the issue when they develop their own packages? It seems cumbersome to have to know how your imported modules import their modules. I’m open to suggestions on restructuring whole code.

Mostly we don’t use modules so much - the point for breaking a project into modules is often also the point for breaking them into packages.

The way multiple dispatch works you also have less namespace issues, so you can have more code in a single namespace.

---

<div class="post-metadata">

**Author:** ![Rex\_Wang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rex_wang/32/35108_2.png) [@Rex\_Wang](https://discourse.julialang.org/u/Rex_Wang)\
**Post date:** [January 21, 2023, 8:07am UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/7 "2023-01-21T08:07:24Z")

</div>

Sorry for the careless. I just edit the post which now get passed.

The module `RectangleMod` defined in `Main` should be explicitly called with `Main.`.

This works fine for temporary use, but for stability, I recommend writing as a package (try [PkgTemplates](https://juliaci.github.io/PkgTemplates.jl/stable/user/#A-More-Complicated-Example-1)), which can be directly called by `using xxx` anywhere.

---

<div class="post-metadata">

**Author:** ![jonlym](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonlym/32/20007_2.png) [@jonlym](https://discourse.julialang.org/u/jonlym)\
**Post date:** [January 22, 2023, 9:58pm UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/8 "2023-01-22T21:58:38Z")

</div>

Thank you all for the help! I agree that creating a package is the best solution. I used `PkgTemplates` and followed along [this tutorial](https://julialang.org/contribute/developing_package/) to create the following `Rectangles` package.

**Package File Structure**

```julia
Rectangles
│ .gitignore
│ LICENSE
│ Manifest.toml
│ Project.toml
│ README.md
│
├───.github
│ └───workflows
│ CI.yml
│ CompatHelper.yml
│ TagBot.yml
│
├───src
│ RectangleMod.jl
│ Rectangles.jl
│
└───test
        runtests.jl

```

**Rectangles/src/RectangleMod.jl**

```julia
module RectangleMod

export Rectangle, get_area

struct Rectangle
    width::Real
    height::Real
end

function get_area(rectangle::Rectangle)::Real
    return rectangle.width * rectangle.height;
end

end

```

**Rectangles/src/Rectangles.jl**

```julia
module Rectangles

export Rectangle, get_area

include("./RectangleMod.jl")
using .RectangleMod

end

```

**Rectangles/test/run\_tests.jl**

```julia
using Rectangles
using Test

@testset "Rectangles" begin
    rectangle = Rectangle(1.0, 2.0)

    @test get_area(rectangle) ≈ 1.0*2.0
end

```

The test passes:

```julia
julia> cd("C:\\Users\\jonat\\.julia\\dev\\Rectangles")

julia>

(@v1.8) pkg> activate .
  Activating project at `C:\Users\jonat\.julia\dev\Rectangles`

(Rectangles) pkg> test
     Testing Rectangles
      Status `C:\Users\jonat\AppData\Local\Temp\jl_La3LcN\Project.toml`
  [7f953efd] Rectangles v0.1.0 `C:\Users\jonat\.julia\dev\Rectangles`
  [8dfed614] Test `@stdlib/Test`
      Status `C:\Users\jonat\AppData\Local\Temp\jl_La3LcN\Manifest.toml`
  [7f953efd] Rectangles v0.1.0 `C:\Users\jonat\.julia\dev\Rectangles`
  [2a0f44e3] Base64 `@stdlib/Base64`
  [b77e0a4c] InteractiveUtils `@stdlib/InteractiveUtils`
  [56ddb016] Logging `@stdlib/Logging`
  [d6f4376e] Markdown `@stdlib/Markdown`
  [9a3f8284] Random `@stdlib/Random`
  [ea8e919c] SHA v0.7.0 `@stdlib/SHA`
  [9e88b42a] Serialization `@stdlib/Serialization`
  [8dfed614] Test `@stdlib/Test`
     Testing Running tests...
Test Summary: | Pass Total Time
Rectangles | 1 1 0.1s
     Testing Rectangles tests passed

```

I can install the package:

```julia
(@v1.8) pkg> add "C:\Users\jonat\.julia\dev\Rectangles"
    Updating git-repo `C:\Users\jonat\.julia\dev\Rectangles`
    Updating registry at `C:\Users\jonat\.julia\registries\General.toml`
   Resolving package versions...
    Updating `C:\Users\jonat\.julia\environments\v1.8\Project.toml`
  [7f953efd] + Rectangles v0.1.0 `C:\Users\jonat\.julia\dev\Rectangles#main`
    Updating `C:\Users\jonat\.julia\environments\v1.8\Manifest.toml`
  [7f953efd] + Rectangles v0.1.0 `C:\Users\jonat\.julia\dev\Rectangles#main`

```

I can import the package:

```julia
julia> using Rectangles

```

But I can’t access anything from it.

```julia
julia> Rectangle(1.0, 2.0)
ERROR: UndefVarError: Rectangle not defined
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

```

Are there any errors standing out? Or would you propose a different way to organize my files?

EDIT: Corrected filename as suggested below.

---

<div class="post-metadata">

**Author:** ![mkitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mkitti/32/12459_2.png) [@mkitti](https://discourse.julialang.org/u/mkitti)\
**Post date:** [January 22, 2023, 10:40pm UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/9 "2023-01-22T22:40:58Z")

</div>

You named the file “RectanglesMod.jl” but you included “RectangleMod.jl” without the ‘s’.

Otherwise, this works fine for me.

```julia
julia> using Rectangles

julia> Rectangle(1.0, 2.0)
Rectangle(1.0, 2.0)

```

Rather than adding the package, you might try using `dev`:

```julia
]dev Rectangles

```

---

<div class="post-metadata">

**Author:** ![jonlym](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonlym/32/20007_2.png) [@jonlym](https://discourse.julialang.org/u/jonlym)\
**Post date:** [January 23, 2023, 12:59am UTC](https://discourse.julialang.org/t/methoderror-for-composite-types/93246/10 "2023-01-23T00:59:46Z")

</div>

Oh, good catch! It works now! 🥳  
Thank you everyone again for all the help!