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First Closure demonstration
Functions. example from a Well House Consultants training course
More on Functions. [link]

This example is described in the following article(s):
   • What are closures in Lua? - [link]
   • New Lua Examples - for last weeks delegates - [link]

Source code: cl2d Module: U106
--[[ Closure demonstration

Strictly speaking, a function is a 'closure' ... and that means
that local variables used within functions defined inside an outer
function can be referenced within an inner function too.

There are a number of benefits of this, especially when you consider
that Lua's functions are first class variables, meaning that they
can be redefined as variables - but let's start with a simple example
which shows the use of a closure for a sort routine.

The significance here is that we do NOT have to make thistable global,
nor find some other mechanism through which we should pass it to the
function - because psord is defined within postman, its local variables
are available to postman too.


function postman(thistable)
        function psord(a,b)
                if thistable[a] < thistable[b] then
                        return true
                return false
        local keys = {}
        for k,_ in pairs(thistable) do
                keys[#keys+1] = k
        rs = ""
        for k,v in ipairs(keys) do
                rs = rs .. k .. " " .. v .. " at no. ".. thistable[v].."\n"
        return rs

deliveries = {John = 4, Jim = 10, Jean = 7, Joanna = 3, Jo = 9}
hegoes = postman(deliveries)
io.write (hegoes)

--[[ Sample output

[trainee@easterton toweb]$ lua cl2d
1 Joanna at no. 3
2 John at no. 4
3 Jean at no. 7
4 Jo at no. 9
5 Jim at no. 10
[trainee@easterton toweb]$


Learn about this subject
This module and example are covered on the following public courses:
 * Learning to program in Lua
 * Lua Programming
 * Lua Programming
Also available on on site courses for larger groups

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Other Examples
This example comes from our "Functions." training module. You'll find a description of the topic and some other closely related examples on the "Functions." module index page.

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