5. Custom pre- and post-processing

Since all packages are different, and may have different demands on how to create a nice example for the documentation it is important that the package maintainer does not feel limited by the by default provided syntax that this package offers. While you can generally come a long way by utilizing line filtering there might be situations where you need to manually hook into the generation and change things. In Literate this is done by letting the user supply custom pre- and post-processing functions that may do transformation of the content.

All of the generators (Literate.markdown, Literate.notebook and Literate.script) accept preprocess and postprocess keyword arguments. The default "transformation" is the identity function. The input to the transformation functions is a String, and the output should be the transformed String.

preprocess is sent the raw input that is read from the source file (modulo the default line ending transformation). postprocess is given different things depending on the output: For markdown and script output postprocess is given the content String just before writing it to the output file, but for notebook output postprocess is given the dictionary representing the notebook, since, in general, this is more useful.

Example: Adding current date

As an example, lets say we want to splice the date of generation into the output. We could of course update our source file before generating the docs, but we could instead use a preprocess function that splices the date into the source for us. Consider the following source file:

# # Example
# This example was generated DATEOFTODAY

x = 1 // 3

where DATEOFTODAY is a placeholder, to make it easier for our preprocess function to find the location. Now, lets define the preprocess function, for example

function update_date(content)
    content = replace(content, "DATEOFTODAY" => Date(now()))
    return content

which would replace every occurrence of "DATEOFTODAY" with the current date. We would now simply give this function to the generator, for example:

Literate.markdown("input.jl", "outputdir"; preprocess = update_date)

Example: Replacing include calls with included code

Let's say that we have some individual example files file1, file2, ... etc. that are runnable and also following the style of Literate. These files could be for example used in the test suite of your package.

We want to group them all into a single page in our documentation, but we do not want to copy paste the content of file1, ... for robustness: the files are included in the test suite and some changes may occur to them. We want these changes to also be reflected in the documentation.

A very easy way to do this is using preprocess to interchange include statements with file content. First, create a runnable .jl following the format of Literate

# # Replace includes
# This is an example to replace `include` calls with the actual file content.


# Cool, we just saw the result of the above code snippet. Here is one more:


Let's say we have saved this file as examples.jl. Then, you want to properly define a pre-processing function:

function replace_includes(str)

    included = ["file1.jl", "file2.jl"]

    # Here the path loads the files from their proper directory,
    # which may not be the directory of the `examples.jl` file!
    path = "directory/to/example/files/"

    for ex in included
        content = read(path*ex, String)
        str = replace(str, "include(\"$(ex)\")" => content)
    return str

(of course replace included with your respective files)

Finally, you simply pass this function to e.g. Literate.markdown as

Literate.markdown("examples.jl", "path/to/save/markdown";
                  name = "markdown_file_name", preprocess = replace_includes)

and you will see that in the final output file (here markdown_file_name.md) the include statements are replaced with the actual code to be included!

This approach is used for generating the examples in the documentation of the TimeseriesPrediction.jl package. The example files, included together in the stexamples.jl file, are processed by literate via this make.jl file to generate the markdown and code cells of the documentation.