if not modules then modules = { } end modules ['trac-set'] = {
    version   = 1.001,
    comment   = "companion to luat-lib.mkiv",
    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files"
}

local type, next, tostring = type, next, tostring
local concat = table.concat
local format, find, lower, gsub = string.format, string.find, string.lower, string.gsub
local is_boolean = string.is_boolean

setters = { }

local data = { } -- maybe just local

-- We can initialize from the cnf file. This is sort of tricky as
-- laster defined setters also need to be initialized then. If set
-- this way, we need to ensure that they are not reset later on.

local trace_initialize = false

local function report(what,filename,name,key,value)
    texio.write_nl(format("%s setter, filename: %s, name: %s, key: %s, value: %s",what,filename,name,key,value))
end

function setters.initialize(filename,name,values) -- filename only for diagnostics
    local data = data[name]
    if data then
        data = data.data
        if data then
            for key, value in next, values do
                key = gsub(key,"_",".")
                value = is_boolean(value,value)
                local functions = data[key]
                if functions then
                    if #functions > 0 and not functions.value then
                        if trace_initialize then
                            report("doing",filename,name,key,value)
                        end
                        for i=1,#functions do
                            functions[i](value)
                        end
                        functions.value = value
                    else
                        if trace_initialize then
                            report("skipping",filename,name,key,value)
                        end
                    end
                else
                    -- we do a simple preregistration i.e. not in the
                    -- list as it might be an obsolete entry
                    functions = { default = value }
                    data[key] = functions
                    if trace_initialize then
                        report("storing",filename,name,key,value)
                    end
                end
            end
        end
    end
end

-- user interface code

local function set(t,what,newvalue)
    local data, done = t.data, t.done
    if type(what) == "string" then
        what = aux.settings_to_hash(what) -- inefficient but ok
    end
    for w, value in next, what do
        if value == "" then
            value = newvalue
        elseif not value then
            value = false -- catch nil
        else
            value = is_boolean(value,value)
        end
        for name, functions in next, data do
            if done[name] then
                -- prevent recursion due to wildcards
            elseif find(name,w) then
                done[name] = true
                for i=1,#functions do
                    functions[i](value)
                end
                functions.value = value
            end
        end
    end
end

local function reset(t)
    for name, functions in next, t.data do
        for i=1,#functions do
            functions[i](false)
        end
        functions.value = false
    end
end

local function enable(t,what)
    set(t,what,true)
end

local function disable(t,what)
    local data = t.data
    if not what or what == "" then
        t.done = { }
        reset(t)
    else
        set(t,what,false)
    end
end

function setters.register(t,what,...)
    local data = t.data
    what = lower(what)
    local functions = data[what]
    if not functions then
        functions = { }
        data[what] = functions
    end
    local default = functions.default -- can be set from cnf file
    for _, fnc in next, { ... } do
        local typ = type(fnc)
        if typ == "string" then
            local s = fnc -- else wrong reference
            fnc = function(value) set(t,s,value) end
        elseif typ ~= "function" then
            fnc = nil
        end
        if fnc then
            functions[#functions+1] = fnc
            if default then
                fnc(default)
                functions.value = default
            end
        end
    end
end

function setters.enable(t,what)
    local e = t.enable
    t.enable, t.done = enable, { }
    enable(t,string.simpleesc(tostring(what)))
    t.enable, t.done = e, { }
end

function setters.disable(t,what)
    local e = t.disable
    t.disable, t.done = disable, { }
    disable(t,string.simpleesc(tostring(what)))
    t.disable, t.done = e, { }
end

function setters.reset(t)
    t.done = { }
    reset(t)
end

function setters.list(t) -- pattern
    local list = table.sortedkeys(t.data)
    local user, system = { }, { }
    for l=1,#list do
        local what = list[l]
        if find(what,"^%*") then
            system[#system+1] = what
        else
            user[#user+1] = what
        end
    end
    return user, system
end

function setters.show(t)
    commands.writestatus("","")
    local list = setters.list(t)
    local category = t.name
    for k=1,#list do
        local name = list[k]
        local functions = t.data[name]
        if functions then
            local value, default, modules = functions.value, functions.default, #functions
            value   = value   == nil and "unset" or tostring(value)
            default = default == nil and "unset" or tostring(default)
            commands.writestatus(category,format("%-25s   modules: %2i   default: %5s   value: %5s",name,modules,default,value))
        end
    end
    commands.writestatus("","")
end

-- we could have used a bit of oo and the trackers:enable syntax but
-- there is already a lot of code around using the singular tracker

-- we could make this into a module

function setters.new(name)
    local t
    t = {
        data     = { }, -- indexed, but also default and value fields
        name     = name,
        enable   = function(...) setters.enable  (t,...) end,
        disable  = function(...) setters.disable (t,...) end,
        register = function(...) setters.register(t,...) end,
        list     = function(...) setters.list    (t,...) end,
        show     = function(...) setters.show    (t,...) end,
    }
    data[name] = t
    return t
end

trackers    = setters.new("trackers")
directives  = setters.new("directives")
experiments = setters.new("experiments")

-- nice trick: we overload two of the directives related functions with variants that
-- do tracing (itself using a tracker) .. proof of concept

local trace_directives  = false local trace_directives  = false  trackers.register("system.directives",  function(v) trace_directives  = v end)
local trace_experiments = false local trace_experiments = false  trackers.register("system.experiments", function(v) trace_experiments = v end)

local e = directives.enable
local d = directives.disable

function directives.enable(...)
    (commands.writestatus or logs.report)("directives","enabling: %s",concat({...}," "))
    e(...)
end

function directives.disable(...)
    (commands.writestatus or logs.report)("directives","disabling: %s",concat({...}," "))
    d(...)
end

local e = experiments.enable
local d = experiments.disable

function experiments.enable(...)
    (commands.writestatus or logs.report)("experiments","enabling: %s",concat({...}," "))
    e(...)
end

function experiments.disable(...)
    (commands.writestatus or logs.report)("experiments","disabling: %s",concat({...}," "))
    d(...)
end

-- a useful example

directives.register("system.nostatistics", function(v)
    statistics.enable = not v
end)