Perl 5 to Perl 6 - Scalars

Perl 5 to Perl 6 - Scalars

Tags: Perl, Perl 5, Perl 6, newsletter, scalars

Abstract

The first entry of the Perl 6 Tricks and Treats newsletter in our quest to switch from Perl 5 to Perl 6. Looking at the scalars.

Welcome back to the Perl 6 Tricks and Treats

I have to apologize as it has been more than 2 months since I sent out the last newsletter. After I came back from Oslo, where I gave a Perl 6 training class a client contacted with an urgent project that had a tight schedule. That meant that up till yesterday I have been working quite intensively at the client. That did not leave time for writing either the Test Automation Tips or the Perl 6 Tricks and Treats.

Anyway, the dead-line is now over and they are moving into a calmer period in which I only visit them once or twice a week.

Perl 5 to Perl 6 : scalars

So today I wanted to start writing about Perl 6 again but I was not really sure what to write about next. Hence I decided to go over the Perl 6 training material and clean up parts that were incorrect or that were lacking during the training in Oslo.

Actually there was a part that was quite neglected since I wrote the first version a few years ago and it might be an easier approach for me to clean that up first.

So I am looking at the Perl 5 to Perl 6 chapter and will go over its sections and send them out. That might be less clear to people who don't know Perl 5. I apologize in advance. It also means I am going to send a number of short postings in the next few days.

I wonder if I should also write similar chapters for people coming from other languages as well and which languages would be the most important?

I start with scalars including the functions that seems to be the most important to me. If you think differently, please tell me so I can include those examples too.

print Hello World

In Perl 5 you normally use print() or starting from 5.10 you can use say() to print to the screen. The latter will append a newline to whatever you had to say.

In Perl 6 they work the same.

    use v6;
    
    print "Hello World\n";
    say "Hello World";

Variable declaration using "my"

In Perl 6 you (almost) always need to declare your variables "my". Think as if you had always added "use strict". There are exceptions such as one-liners and certain constructs that auto-declare the scoped variables for you.

    use v6;

    my $name = "Moose";

Scalar variable interpolation

In Perl 6 scalar variables still interpolate in double quoted strings:

    use v6;
    
    my $name = "Foobar";
    say "Hello $name";     # Hello Foobar

In order to make the Object Oriented people happy Perl 6 also allows this:

    use v6;
    
    my $name = "Foobar";
    "Hello $name".say;     # Hello Foobar

Reading from standard input

In Perl 5 you would use <STDIN>.

As reading a single line from standard input is usually preceded by a print statement the keyword to do that in Perl 6 is called prompt().

    use v6;
    
    my $line = prompt('Please type in your name: ');
    say "Hi $line, how are you?";        # Hi Foobar, how are you?

If you try this you will see that it chomps the newline off automatically. Repeated reading without a prompt will be explained later.

Getting part of a string (substr)

The substr() function of Perl 5 is the same except that there is no 4 parameter version. As all the other functions it can also work in the object oriented way.

    use v6;

    my $line = "The brown cat climbed the green tree";
    say substr $line, 4, 3;   # bro
    $line.substr(4, 3).say;   # bro

Length of a string

In Perl 5 we used the length() function.

In order to avoid ambiguity in Perl 6 there is no length() function. Instead there are several functions with more precise meaning of the type of unit you are asking for. So bytes() will return the number of bytes in the string while, chars() will return the number of characters. Of course in my example you won't see any difference as the string is written in ascii where each character is one byte long.

    use v6;

    my $a = "This is a string";
    say chars $a;  # 16
    say bytes $a;  # 16

There are also the graphs() and codes() functions but they are not yet implemented in Rakudo. See S32-setting-library/Str.pod for more details ( http://perlcabal.org/syn/S32/Str.html )

chomp

In Perl 5 there is a chomp() function to remove a trailing newline after reading a line from the standard input or from a file.

In Perl 6 this function will be almost never used as reading from the standard input or from files will automatically chomp off the newlines. In any case, in Perl 6 chomp behaves differently as it returns the chomped string and does NOT change the original string.

    use v6;
    my $a = "abcd\n";
    my $b = chomp $a;  # $b is now "abcd"

defined

The defined() function is the same in Perl 6 as in Perl 5 checking if a scalar value has any value different from undef.

String concatenation using dot (.)

In Perl 5 dot (.) is used for string concatenation. I think it is a lot less used than one would think as in many cases we use string interpolation. Maybe one of the most frequent use-cases is actually the short-cut string concatenation.

In Perl 6 the string concatenation is done using the ~ operator. I think it will be even less used than in Perl 5 as the variable and code interpolation in Perl 6 is much stronger. Anyway here is the example:

    use v6;
    my $str = "Foo" ~ "Bar";
    $str.say;      # FooBar

I think the only place where it will be used a lot is the short-cut version that looks like this ~=

    $str ~= " and Moo";
    say $str;      # FooBar and Moo

This can of course lead to some confusion but =~ is not the regular expression operator in Perl 6 and the user will be told so if he gets it wrong.

String repetition (x)

String repetition that was the x operator stayed the same:

    use v6;
    say "abc" x 3;   # abcabcabc

index, rindex

The index() and rindex() functions of Perl 5 that can locate a substring stayed the same in Perl 6 as well.

    use v6;
    my $s = "The brown cat climbed the green tree";
    say index $s, "b";    # 4

lc, uc

lc() and uc() turning strings to all lower case and all upper case are the same in Perl 6 as in Perl 5.

    use v6;
    say lc "Hello World";    # hello world
    say uc "Hello World";    # HELLO WORLD

Conclusion

I know these are no big tricks here just plain code but I hope this will help some people in learning Perl 6.

Stay tuned for arrays, hashes, files, control structure and the more advanced stuff.