namespace eval ::TclReadLine::prompts {
    variable shortenPath {
        original {}
        short {}
    }
}

proc ::TclReadLine::prompts::user-at-host {} {
    return $::tcl_platform(user)@[info hostname]
}

proc ::TclReadLine::prompts::shorten-path path {
    # Cache the short path, since this runs on every key press.
    variable shortenPath
    if {[dict get $shortenPath original] eq $path} {
        return [dict get $shortenPath short]
    }

    set splitPath [file split $path]
    set result {}
    foreach fragment [lrange $splitPath 0 end-1] {
        regexp {^\.{0,2}.} $fragment fragment
        lappend result $fragment
    }

    set short [file join {*}$result [lindex $splitPath end]]
    dict set shortenPath original $path
    dict set shortenPath short $short

    return $short
}

# Looks like this:
# tclsh-8.6.3 [Downloads] %
set ::TclReadLine::prompts::path {tclsh[info patchlevel] \[[pwd]\]% }

# Looks like the above but with color.
set ::TclReadLine::prompts::pathColor {\033\[36mtclsh-[info patchlevel]\033\[0m \[\033\[34m[file tail [pwd]\033\[0m]\]\033\[31m % \033\[0m}

# A fish shell-style prompt with a clock.
# Looks like this:
# 14:49:52 user@host /h/d/Downloads>
set  ::TclReadLine::prompts::fish {[clock format [clock seconds] -format {%H:%M:%S}] [::TclReadLine::prompts::user-at-host] [::TclReadLine::prompts::shorten-path [pwd]]> }

set ::TclReadLine::PROMPT $::TclReadLine::prompts::fish