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error checking through pcall
Trapping and handling errors. example from a Well House Consultants training course
More on Trapping and handling errors. [link]

This example is described in the following article(s):
   • Error trapping in Lua - no exceptions. - [link]

Source code: wonky Module: U111

--[[
In this program, the first block is the "control" - it works
well as the "number" variable is a function. In the second
block, we call a numeric value. "pcall" traps the error and
lets us handle it in our code, but if we call it direct (the
very last example), it's going to crash out with a Lua error
]]


-- ======== Calling a function

function number()
        return 54
        end

-- trap any error from a call to number
-- protected function call

lots,more = pcall(number)

-- so it's under programmer's control
print (lots, more)

lots = number()
print ("again")

-- =============== Trying to call a numeric value

number = 42

-- trap any error from a call to number
-- protected funcion call

lots,more = pcall(number)

-- so it's under programmer's control
print (lots, more)

lots = number()
print ("again")

--[[

[trainee@holt lm10]$ lua wonky
true 54
again
false attempt to call a number value
lua: wonky:33: attempt to call global 'number' (a number value)
stack traceback:
        wonky:33: in main chunk
        [C]: ?
[trainee@holt lm10]$

]]

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

Books covering this topic
Yes. We have over 700 books in our library. Books covering Lua are listed here and when you've selected a relevant book we'll link you on to Amazon to order.

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

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
• Our Solutions centre provides a number of longer technical articles.
• Our Opentalk forum archive provides a question and answer centre.
The Horse's mouth provides a daily tip or thought.
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• All of these resources can be searched through through our search engine
• And there's a global index here.

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This web site is written and maintained by Well House Consultants.

Purpose of this website
This is a sample program, class demonstration or answer from a training course. It's main purpose is to provide an after-course service to customers who have attended our public private or on site courses, but the examples are made generally available under conditions described below.

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