HeaderREPLs

HeaderREPLs.jl is a Julia package that allows you to extend Julia's amazing REPL to support "mini-applications" that need to communicate more information to the user. They allow you to define a custom "header" type that you can use to print extra information above the prompt.

To demonstrate, here we build a CountingHeader type and show how the header is printed:

mutable struct CountingHeader <: AbstractHeader    # note must be mutable and contain nlines field
    n::Int            # internal data needed by the header
    nlines::Int       # the number of lines needed for display---update this in print_header
end

function HeaderREPLs.print_header(io::IO, header::CountingHeader)
    if header.nlines == 0
        if header.n > 0
            printstyled(io, "Header:\n"; color=:light_magenta)
            for i = 1:header.n
                printstyled(io, "  ", i, '\n'; color=:light_blue)
            end
        end
        header.nlines = nlines(header.n)
    end
    return nothing
end

In test/runtests.jl there is a complete example showing how you initialize this mode and specify the user interaction: briefly, a pipe character ('|') switches into "count" mode, and once in "count" mode '+' and '-' increment and decrement n.

Here, let's check out the outcome with a series of images.

This is what you see when you first press '|'. n starts at 0, and our print_header method above displays nothing in that case.

This is what you get after pressing '+' three times.

Now after pressing '-'. Note the line moved up to stay aligned.

Note we can enter Julia commands as usual. In this demo, "count" mode is non-sticky, so it reverts back to the julia> prompt.

In theory at least, "count" mode works as you'd expect when you traverse the history: when you get to a "count" line it shows the (current) header.

The package exports a few utilities that may make it easier to define custom REPL modes. Aside from key-binding initialization utilities (see the source for details), perhaps the two most useful are find_prompt and count_display_lines. Use ? for more information.

Currently this package overwrites a few methods in Julia's "REPL" standard library. This generates some warnings. Hopefully we can eliminate them in a future version of this package.