= Replacements example

In this example, we want to design one call to assign replacement strings to variables.
Calls to these new variables are replaced by their assigned string.

== Litua input syntax document

Consider a simple document which invokes the ``let`` call. Its arguments define substitution pairs.

----
{let[name=세종대왕][occasion=52][achievement=Korean alphabet][who=tajpulo]}
----

Here, we want to replace e.g. ``name`` by ``세종대왕`` and ``who`` by ``tajpulo``.
Consecutively, we want to use calls to insert these substitution sequences:

----
{let[name=세종대왕][occasion=52][achievement=Korean alphabet][who=tajpulo]}

Dear {name},

I want to congratulate on the occasion of your {occasion}th birthday.
I am a big fan of your development of the {achievement}!

All the best,
{who}
----

So, ``let`` is supposed to define call hooks for each of its arguments.

== litua hooks file

We only need to consider the ``convert_node_to_string`` hook called for ``let``. In this case, we iterate over its arguments and for each ``arg``, we define a new ``convert_node_to_string`` hook which just returns the substitution value. Remember that ``val`` is a table since it might be a concatenation of several strings and calls. ``let`` calls themselves shall disappear and thus be represented as empty string (``return ""``).

[source,lua]
----
Litua.convert_node_to_string("let", function (node)
    for arg, val in pairs(node.args) do
        local content = ""
        for i=1,#val do
            content = content .. tostring(val[i])
        end

        Litua.convert_node_to_string(arg, function (_) return content end)
    end
    return ""
end)
----

== Output file

----
Dear 세종대왕,

I want to congratulate on the occasion of your 52th birthday.
I am a big fan of your development of the Korean alphabet!

All the best,
tajpulo
----