Precompilation "gotcha"s

Running code during module definition

Suppose you're working on an astronomy package and your source code has a line

const planets = map(makeplanet, ["Mercury", ...])

Julia will dutifully create planets and store it in the package's precompile cache file. This also runs makeplanet, and if this is the first time it gets run, it will compile makeplanet. Assuming that makeplanet is a method defined in the package, the compiled code for makeplanet will be stored in the cache file.

However, two circumstances can lead to puzzling omissions from the cache files:

• if makeplanet is a method defined in a dependency of your package, it will not be cached in your package. You'd want to add precompilation of makeplanet to the package that creates that method.
• if makeplanet is poorly-infered and uses runtime dispatch, any such callees that are not owned by your package will not be cached. For example, suppose makeplanet ends up calling methods in Base Julia or its standard libraries that are not precompiled into Julia itself: the compiled code for those methods will not be added to the cache file.

One option to ensure this dependent code gets cached is to create planets inside PrecompileTools.@compile_workload:

@compile_workload begin
    global planets
    const planet = map(makeplanet, ["Mercury", ...])
end

Note that your package definition can have multiple @compile_workload blocks.