How to Pass Arguments to elisp Functions with Optional Parameters

Learn how to selectively pass arguments to `elisp` functions while leaving optional parameters empty, with practical examples and clear explanations.
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Understanding Optional Parameters in elisp Functions

In the world of Emacs Lisp (elisp), one common scenario you may encounter is the need to call a function with multiple optional parameters. This can lead to confusion, especially when you want to pass a specific argument while leaving others empty. Let’s break down how you can accomplish this effectively with a practical example.

The Problem Explained

Consider the following function definition in elisp:

[[See Video to Reveal this Text or Code Snippet]]

This function takes three parameters, two of which (basedir and targets) are optional. If you want to call this function using only the default-input, you might wonder about how to handle the empty optional parameters. For instance, if you attempt to call:

[[See Video to Reveal this Text or Code Snippet]]

You might question whether this is the correct approach. Let’s dive deeper into the solution.

The Solution: Using Nil Values for Optional Arguments

Understanding nil as Default

In elisp, optional parameters default to nil if not specified. Therefore, passing nil explicitly for the optional parameters is perfectly valid. Here’s a clearer breakdown:

Function definition: The parameters basedir and targets are marked as optional.

Default value: If you do not provide them when calling the function, they will automatically assume the value of nil.

Explicit nil: Specifying nil as an argument is redundant but still accepted by the function call.

Correct Function Call

Given the explanation above, calling the function like this:

[[See Video to Reveal this Text or Code Snippet]]

is indeed correct. Here’s why:

The first nil corresponds to basedir (left empty).

The second nil corresponds to targets (also left empty).

The string "something" is passed as default-input.

Practical Example

Let's say you need to search within a directory using the default-input of "my-search-term". You can execute the following:

[[See Video to Reveal this Text or Code Snippet]]

In this instance, basedir and targets are set to nil, and the function will effectively use "my-search-term" as the input parameter.

Conclusion

Handling optional parameters in elisp can be a bit tricky, especially if you're unsure whether omitting them will affect your function's execution. The context here helps clarify that you can either leave them out completely or specify nil without any issues.

So next time you need to call a function with optional parameters, remember that nil is your ally! Happy coding!