How to give a default value to a notebook "parameter"

[marckruzik]

Is your feature request related to a problem? Please describe.
I would like to execute a notebook with different values (such as different values for a path), but also with a default value. The current problem is due to a lack of an official way to have parameters when importing a notebook, as we rely on variables already defined to transfer information.

I'm giving a full example to explain.

---

Here is the content of a notebook with a defined value:

string path = "C:";
Console.WriteLine(path);

When I call it, the path always has the same value.

#!import notebook.dib

C:

---

So I edit this notebook to remove the variable definition.

Console.WriteLine(path);

When I call it, the path has the value defined before importing the notebook.

string path = "C:";
#!import notebook.dib

C:

But now, the imported notebook is no longer working by itself. Trying to execute it directly outputs an error.
Error: (1,1): error CS0103: The name 'path' does not exist in the current context

Describe the solution you'd like
The easiest way to use would be to allow something like #!import notebook.dib arg0 arg1, and access args[] in the imported notebook. It's a common solution.

Describe alternatives you've considered
Writing a file on disk before importing the notebook, and the imported notebook reads the value in the file or gives the variable a default value.
I tried various ways, such as accessing variables from #!who, looking into #!share, etc. dotnet-repl allows to fill @input.
Some languages other than C# allow to check if a variable is defined, but I don't think it's the direction to follow.

[jonsequitur]

Would you want to define the variable in the imported notebook using an @input, e.g.

#!set --name variableX --value @input

Following the dotnet-repl implementation, the importing notebook could then provide them much like your proposed example, though I would probably opt for specifying the values by name, something like:

#!import notebook.dib {"variableX":"c:\\"}

[marckruzik]

You are right about specifying the values by name, it is better.

About @input, it adds a layer of complexity. Especially as saving a value (using PowerShell secrets) makes it hard to edit the value. It is overengineered, and it misses the point of having a simple way to set a default value.

In my case, what is really missing is an easy way to check in the Kernel if a variable is already defined.

I made a working example (thanks to Copilot and heavy use of displaying values).

// main_notebook.dib
string variableX = "value defined in main notebook";
#!import sub_notebook.dib

// sub_notebook.dib
using Microsoft.DotNet.Interactive;
using Microsoft.DotNet.Interactive.Commands;
using Microsoft.DotNet.Interactive.Events;

public static string VariableExists(string variableName)
{
    var kernel = Kernel.Root.FindKernelByName("csharp");
    var command = new RequestValueInfos("csharp");
    var result = kernel.SendAsync(command).Result;

    var variables = result.Events
        .OfType<ValueInfosProduced>()
        .SelectMany(v => v.ValueInfos)
        .ToList();

    var exists = variables.Any(v => v.Name == variableName);
    if (exists)
    {
        return variables.First(v => v.Name == variableName).FormattedValue.Value;
    }
    else
    {
        return null;
    }
}

string variableX = VariableExists("variableX") ?? "value defined in sub notebook";

Console.WriteLine(variableX);

When executing main_notebook.dib:

value defined in main notebook

When executing sub_notebook.dib:

value defined in sub notebook

So both notebooks can be executed.
In this example, it is possible to have a variable defined in a main notebook (acting as a parameter), while still being able to check in a sub notebook if the variable exists, in order to set a default value (using VariableExists("variableX") ?? "default value").

I think @input is made to ask user for input, it is not made to provide a default value.
I suggest we separate the two concerns, to first "have a proper way to check if a value exists and set a default value if not", and then "import notebooks with parameters to fill inputs". I apologize for the confusion.