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Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

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Perl - calls to methods that use => - what do they mean?

Have you seen Perl method calls that look like this?
  $present = new box(-smell => "chocolate", noise => "silent");
and wondered "what is all this => stuff" and "what's the - sign for"?

That's a call that could be fairly typical of the sort you'll see in the documentation of any number of standard modules on the CPAN (Comprehensive Perl Archive Network), and it's an API (Application / Programmer Interface) that's designed to be robust, easy to follow, and doesn't add a heavy footprint of variables and names into your program.

Internally, the code:
  $present = new box(-smell => "chocolate", noise => "silent");
is the same as:
  $present = box::new("box", "-smell", "chocolate", "noise", "silent");
- in other words, run the new sub in the box package, passing in five parameters. The first parameter appears to be redundant (and indeed it is until we start using inheritance), and the other parameters are pairs of attribute names and values.

=> is a synonym for ,
• We can leave the quotes off the attribute names if they are single words before the =>
• The extra - sign ensures that the property name is never going to be a Perl keyword, and the class needs to strip it off if present.

There's a full example, showing this call (and the class / package that implements it) [here] from last week's course. The example is a generic base class with a common setter that handles the various standard ways of passing options (i.e. as above, and also as a reference to a hash).

There's a further example - showing both syntaxes of the constructor and also accessor methods - [here], and the simple class that implements it [here].

Topic covered on our Perl for Larger Projects course and also as appropriate / requested on tailored private courses.
(written 2012-01-16, updated 2012-01-28)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P218 - Perl - More Objects
  [227] Bellringing and Programming and Objects and Perl - (2005-02-25)
  [246] When to bless a Perl variable - (2005-03-15)
  [531] Packages in packages in Perl - (2005-12-16)
  [588] Changing @INC - where Perl loads its modules - (2006-02-02)
  [592] NOT Gone phishing - (2006-02-05)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [930] -> , >= and => in Perl - (2006-11-18)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1320] Perl for Larger Projects - Object Oriented Perl - (2007-08-25)
  [1435] Object Oriented Programming in Perl - Course - (2007-11-18)
  [1664] Example of OO in Perl - (2008-06-03)
  [1665] Factory method example - Perl - (2008-06-04)
  [1819] Calling base class constructors - (2008-10-03)
  [1949] Nuclear Physics comes to our web site - (2008-12-17)
  [2427] Operator overloading - redefining addition and other Perl tricks - (2009-09-27)
  [2651] Calculation within objects - early, last minute, or cached? - (2010-02-26)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2811] Igloos melt in the summer, but houses do not - (2010-06-15)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [3097] Making Perl class definitions more conventional and shorter - (2010-12-20)
  [3098] Learning Object Orientation in Perl through bananas and perhaps Moose - (2010-12-21)
  [3377] What do I mean when I add things in Perl? - (2011-08-02)
  [3941] Building an object based on another object in Perl - (2012-12-03)
  [4096] Perl design patterns example - (2013-05-20)
  [4098] Using object orientation for non-physical objects - (2013-05-22)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)


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Expanding a list of parameters in Tcl - {*} and eval
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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