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Quiz 00.1 on Modules, packages and environments

Q1: What is printed ?

Given a file foo.jl with the following code:

println("A - I am in foo.jl")

module Foo

println("B - I am in module Foo in foo.jl")
export x

const x=1
const y=2
z() = println("C - I am in a function of module Foo")
end

module Foo2

println("D - I am in module Foo2 in foo.jl")
export a

const a=1
const b=2
c() = println("E - I am in a function of module Foo2")
end

Which print statements will appear after running the command include("foo.jl") ?

RESOLUTION

Including the file would result in evaluating the code in the file and hence in the statements A, B and D to be printed. Statements C and E are within function definition and would occur only when functions z or c would have been called.

The correct answer is:

  • "Statements A, B and D only"

Q2: Inclusion of a module

Given a file foo.jl with the following code:

module Foo

export x

const x=1
const y=2
z() = println("Hello world!")
end

and the following sequence of commands:

include("foo.jl")         # Command 1
x                         # Command 2
Foo.x                     # Command 3
using Foo                 # Command 4
using .Foo                # Command 5
x                         # Command 6
Foo.z()                   # Command 7

Which statements are correct ?

RESOLUTION

The include("foo.jl") statement evaluates the included content, but it doesn't yet bring it into scope. You can't yet refer directly to the objects of the Foo module, you need to use the qualified name as in command 3. Foo is a module, not a package, so command 4 will complain that it doesn't find the "package" Foo. After the module has been brought to scope we can refer to x directly as in command 6. Command 7, as we are using the qualified name, is independent of whether z was exported by Foo or not, and hence it works, and would have worked even without the using .Foo of command 5.

The correct answers are:

  • "Command 3 returns the value 1"
  • "Command 4 returns an ArgumentError: Package Foo not found in current path:"
  • "Command 6 returns the value 1"

Q3: Submodules

Given a file Foo.jl with the following code:

module Foo
export x, plusOne
x = 1
plusOne(x) = x + 1
module Foo2
  export plusTwo
  plusTwo(x) = plusOne(x)+1
end
end

After including the file we try to run the command Foo.Foo2.plusTwo(10). Which of the following statements is correct ?

RESOLUTION

The given command results in a UndefVarError: plusOne not defined. Indeed even if Foo2 is a submodule of Foo, it doesn't inherit the scope of parent modules. So its code can't find the 'plusOne' function. When in the REPL we run the command we are in the Main module. Adding using .Foo doesn't change anything, as the problem is in the scope of the Foo2 module, not in those of the REPL (Main - and , of course, typing using Foo looks-up for the package Foo, not the module Foo, and would end in a Package Foo not found error. So what can we do? One solution is using in the plusTwo function the full path of the plusOne function: plusTwo(x) = Main.Foo.plusOne(x)+1. While this works, it may be a less portable solution, as it then requires module Foo to be a child of Main. Perhaps a better solution is to use a relative path and use the statement using ..Foo in module Foo2 before the definition of plusTwo (trying to use a relative path directly in the function definition as in plusTwo(x) = ..Foo.plusOne(x)+1 would result in a parsing error)

The correct answers are:

  • The result is an error that we can avoid if the function plusTwo in module Foo2 is defined as plusTwo(x) = Main.Foo.plusOne(x)+1
  • The result is an error that we can avoid if in module Foo2 the function plusTwo is preceded by the statement using ..Foo

Q4: Submodules2

Given a module Foo with the following code:

module Foo
export x
x = 1
module Foo2
  export plusTwo
  plusTwo(x) = x+2
end
module Foo3
  export plusThree
  [XXXX]
  plusThree(x) = plusTwo(x)+1
  end                  
end

Which of the following statements are correct in order for the function plusThree to work ?

RESOLUTION

The function plusThree needs to access a function on a sibling module. So the module Foo2 must be retrieved by going up to one level with the two dots and then naming the module, i.e. using ..Foo2 or using the full module path using Main.Foo.Foo2. import statements alone will not work as the plusThree function calls the plusTwo function using the unqualified name, without prefixing the module, so the plusThree function name needs to be exported.

The correct answers are:

  • "[XXXX] should be using Main.Foo.Foo2"
  • "[XXXX] should be using ..Foo2"

Q5: Reproducibility

Which elements do you have to provide to others to guarantee reproducibility of your results obtained with a Julia project?

RESOLUTION

To provide replicable results, assuming a deterministic algorithm or one where the random seed generator has been fixed, we need to provide the input data, the source code and the 'Manifest.toml' file that describes the exact version of all packages. The Project.toml file instead, when present, is used to describe in which conditions our scripts could be used (i.e. the list and eventually range of dependent packages), but not a unique environment state. The information of the Manifest.toml (and, for Julia versions before 1.7, the Julia version itself, as this info was not encoded in the Manifest.toml file) is enough, we don't need to provide the whole content of the user Julia folder.

The correct answers are:

  • "The input data of your analysis"
  • "The full source code of the scripts you have used"
  • "The file Manifest.toml of the environment where your code ran to produce the results"