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This HTML version of Think Perl 6 is provided for convenience, but it is not the best format of the book. You might prefer to read the PDF version.

Chapter 15  Some Final Advice

Everyone knows that debugging is twice as hard as writing
a program in the first place. So if you’re as clever as you
can be when you write it, how will you ever debug it?

— Brian Kernighan, "The Elements of Programming Style".

15.1  Make it Clear, Keep it Simple

Writing a real-life program is not the same thing as learning the art of programming or learning a new language.

Because the goal of this book is to lead you to learn more advanced concepts or new syntax, I have often been pushing new ways of doing things. But this does not mean that you should try to pack your most advanced knowledge into each of your programs. Quite the contrary.

The rule of thumb is “KISS”: keep it simple, stupid. The KISS engineering principle (originated in the US Navy around 1960) states that most systems work best if they are kept simple rather than made complicated; therefore simplicity should be a key goal in design and unnecessary complexity should be avoided. This does not mean, however, that you should write simplistic code.

As an example, if you’re looking for a literal substring within a string, use the simple index built-in function, rather than firing the regex engine for that. Similarly, if you know the position and length of the substring, then use the substr function. But if you need a more “fuzzy” match with perhaps some alternatives or a character class, then a regex is very likely the right tool.

Another related tenet is “YAGNI”: you aren’t gonna need it. This acronym comes from a school of programming known as “extreme programming” (XP). Even if you don’t adhere to all the XP principles, this idea is quite sound and well-founded: don’t add any functionality until it is really needed.

Try to make your programs as clear as possible, and as simple as you can. Use more advanced concepts if you have to, but don’t do it for the sake of showing how masterful you are. Don’t try to be clever or, at least, don’t be too clever.

Remember that code is not only used by the compiler, but is also by humans. Think about them.

Think about the person who will have maintain your code. As some people like to put it: “Always code as if the person who ends up maintaining your code is a violent psychopath who knows where you live.” And, if nothing else will convince you, remember that the person maintaining your code might be you a year from now. You may not remember then how that neat trick you used really works.

A final quote from Edsger Dijkstra on this subject: “Simplicity is prerequisite for reliability.”

15.2  Dos and Don’ts

Don’t repeat yourself (DRY):
Avoid code duplication. If you have the same code in different places of your program, then something is most likely wrong. Maybe the repeated code should go into a loop or a separate subroutine, or perhaps even in a module or a library. Remember that copy and paste is a source of evil.
Don’t reinvent the wheel:
Use existing libraries and modules when you can; it is likely that they have been thoroughly tested and will work better than the quick fix you’re about to write. The Perl 6 ecosystem has a large and growing collection of software modules (see that you can use in your programs.
Use meaningful identifiers
If your variables, subroutines, methods, classes, grammars, modules, and programs have sensible names that convey clearly what they are or what they do, then your code will be clearer and might need fewer comments. Very short names like $i or $n are usually fine for loop variables, but pretty much anything else needs a name that clearly explains what the content or the purpose is. Names like $array or %hash may have sometimes been used in some examples of this book to indicate more clearly the nature of the data structure, but they are not recommended in real-life programs. If your hash contains a collection of words, call it %words or %word-list, not %hash. The % sigil indicates that it is a hash anyway.
Make useful comments and avoid useless ones
A comment like this:
my $count = 1;     # assigns 1 to $count
is completely useless. In general, your comments should explain neither what your code is doing nor even how it is doing it (this should be obvious if your code is clear), but rather why you are doing that: perhaps you should refer to a math theorem, a law of physics, a design decision, or a business rule.
Remove dead code and code scaffolding
Even when writing new code, you may at some point create variables that you don’t use in the final version of your code. If so, remove them; don’t let them distract the attention of your reader. If you modify an existing program, clean up the place after you’ve changed it. Remember the boy scout’s rule: leave the place better and cleaner than you found it.
Test aggressively
Nobody can write any piece of significant software without having a number of initial bugs. Edsger Dijkstra is quoted as saying: “If debugging is the process of removing software bugs, then programming must be the process of putting them in.” It is unfortunately very true. Even though Dijkstra also said that “Testing shows the presence, not the absence of bugs,” testing is an essential part of software development. Write extensive test plans, use them often, and update them as the functionality evolves. See Section ?? (p. ??) for some automated testing tools.
Avoid premature optimization
In the words of Donald Knuth: “Premature optimization is the source of all evil (or at least most of it) in programming.”
Don’t use magical numbers:
Consider this:
my $time-left = 31536000;

What is this 31,536,000 number coming out of nowhere? There’s no way to know just by looking at this line of code. Compare with this:

my $secondsInAYear = 365 * 24 * 60 * 60;
# ...
my $time-left = $secondsInAYear;

Isn’t the second version clearer? Well, to tell the truth, it would be even better to use a constant in such a case:

constant SECONDS-PER-YEAR = 365 * 24 * 60 * 60;

Avoid hardcoded values:
Hard-coded values are bad. If you have to use some, define them as variables or constants at the beginning of your program, and use those variables or constants instead. Hard-coded file paths are especially bad. If you have to use some, use some variables with relative paths:
my $base-dir = '/path/to/application/data';
my $input-dir = "$base-dir/INPUT";
my $result-dir = "$base-dir/RESULT";
my $temp-dir = "$base-dir/TEMP";
my $log-dir = "$base-dir/LOG";
At least, if the path must change, you have to change only the top code line.
Don’t ignore errors returned by subroutines or built-in functions:
Not all return values are useful; for example, we usually don’t check the return value of a print statement, but that’s usually fine because we are interested in the side effect, the fact of printing something out to the screen or to a file, rather than in the return value. In most other cases, you need to know if something went wrong in order to take steps to either recover from the error condition, if possible, or to abort the program gracefully (e.g., with an informative error message) if the error is too serious for the program to continue.
Format your code clearly and consistently
The compiler might not care about code indentation, but human readers do. Your code formatting should help clarify the structure and control flow of your programs.
Be nice and have fun.

15.3  Use Idioms

Any language has its own “best practice” methods of use. These are the idioms that experienced programmers use, ways of doing things that have become preferred over time. These idioms are important. They protect you from reinventing the wheel. They are also what experienced users expect to read; they are familiar and enable you to focus on the overall code design rather than get bogged down in detailed code concerns. They often formalize patterns that avoid common mistakes or bugs.

Even though Perl 6 is a relatively new language, a number of such idioms have become honed over time. Here are a few of these idiomatic constructs 1.

Creating a hash from a list of keys and a list of values
Using slices:
my %hash; %hash{@keys} = @values;

Using the zip operator and a metaoperator with the pair constructor:

my %hash = @keys Z=> @values;

For existence tests, the hash values only need to be true. Here is a good way to create a hash from a list of keys:

my %exists = @keys X=> True;

Or, better yet, use a set:

my $exists = @keys.Set;
say "exists" if $exists{$object};
Making mandatory attributes (or subroutine parameters):
This is a nice way of making a mandatory attribute in a class:
has $.attr = die "The 'attr' attribute is mandatory";
This code uses the default value mechanism: if a value is supplied, then the code for the default value does not run. If no value is supplied, then the code dies with the appropriate error message. The same mechanism can be used for subroutine parameters.

Or you could use the is required trait:

> class A { has $.a is required }; 
The attribute '$!a' is required, 
but you did not provide a value for it.

You can even pass a explanatory message:

> class A { has $.a is required("We need it") }; 
The attribute '$!a' is required because We need it,
but you did not provide a value for it.
Iterating over the subscripts of an array
The first solution that comes to mind might be:
for 0 .. @array.end -> $i {...}

That’s fine, but this is probably even better:

for @array.keys -> $i {...}
Iterating over the subscripts and values of an array
The .kv method, in combination with a pointy block taking two parameters, allows you to easily iterate over an array:
for @array.kv -> $i, $value {...}
Printing the number of items in an array
Two possible solutions:
say +@array; 
# or:
say @array.elems; 
Do something every third time
Use the %% divisibility operator on the loop variable:
if $i %% 3 {...}  
Do something n times:
Use the right-open range operator:
for 0 ..^ $n {...}
# or, simpler:
for ^$n {...} 
Split a string into words (splitting on space):
A method call without an explicit invocant always uses the $_ topical variable as an implicit invocant. Thus, assuming the string has been topicalized into $_:
@words = .split(/\s+/);
# or, simpler:
@words = .words;
An infinite loop
A loop statement with no parentheses and no argument loops forever:
while True {...}
# or, more idiomatic:
loop {...}   

Of course, the body of the loop statement must have some kind of flow control statement to exit the loop at some point.

Returning the unique elements of a list
The unique method removes duplicates from the input list:
return @array.unique;

Or, if you know that your list is sorted, you can use the squish function (which removes adjacent duplicates).

Adding up the items of a list
Use the reduce function or the reduction metaoperator:
my $sum = @a.reduce(* + *);
# or, simpler:
my $sum = [+] @a;
# or yet simpler, using the sum built-in:
my $sum = @a.sum;  
Swapping two variables
Use the .= mutating method call with the reverse function:
( $x, $y ) =   $y, $x;
# or:
( $x, $y ) .= reverse; # equivalent to: ($x, $y) = ($x, $y).reverse
Generating random integers between 2 and 6
Use the .. range operator and the pick method:
$z = 2 + Int(5.rand);
# or, better:
$z = (2..6).pick;
Count by steps of 3 in an infinite loop
Use the ... sequence operator with the “*” whatever star operator and a pointy block:
for 3, * + 3 ... * -> $n {...}
# or:
for 3, 6, 9 ... * -> $n {...}    
Loop on a range of values, discounting the range limits:
Use the open range operator:
for ($start+1) .. ($end-1) -> $i {...}
# or, better:
for $start ^..^ $end -> $i {...}

15.4  What’s Next?

A book like this one can’t tell you everything about programming, nor about Perl 6. At this point, you should know how to write a program to solve an average-difficulty problem, but a lot of work has been done in the last decades to solve harder problems. So where should you go from here?

Read books about algorithmics, the science of algorithms. Many good books exist on the subject, but I especially recommend the following two (you should be aware, though, that they are not easy):

  • Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein, Introduction to Algorithms, The MIT Press
  • Donald Knuth, The Art of Computer Programming, Addison Wesley (many volumes, many editions).

Read other books about programming, even if they target other programming languages or no specific programming language. They’re likely to have a different approach on various points; this will offer you a different perspective and perhaps better comprehension, and will complement what you have read here. Read tutorials, articles, blogs, and forums about programming. Participate when you can. Read the Introduction to Perl 6 which exists in eight different languages as of this writing ( Read the official Perl 6 documentation (

This book has more than a thousand code examples, which is quite a lot, but may not be sufficient if you really want to learn more. You should also read code samples written by others. Look for open source libraries or modules and try to understand what they do and how they do it. Try to use them.

Having said that, I should stress that you can read as many books as you want about the theory of swimming, but you’ll never know swimming until you really get around to doing it. The same is true about learning to program and learning a programming language. Write new code. Modify existing examples, and see what happens. Try new things. Go ahead, be bold, dive into the pool and swim. The bottom line is: you will really learn by doing.

Learning the art of programming is great fun. Enjoy it!

When two solutions are suggested, the second one is usually the more idiomatic one.

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