Brian Wisti

Continuing the Perl 6 adventures with Allen B. Downey's Think Python book.

Chapter 2

Maybe a brief bit about objects in Chapter 1 would be appropriate, because Perl 6 lets you query an object directly about the type associated with a value.

> 'Hello, World!'.WHAT
Str()
> 17.WHAT
Int()
> 3.2.WHAT
Rat()

Strings that have numbers are still strings by default.

> '17'.WHAT
Str()
> '3.2'.WHAT
Str()

Commas make lists.

> 1,000,000
1 0 0

Variables need to be declared - my is a favorite.

> $message = 'And now for something completely different'
Variable $message is not declared
> my $message = 'And now for something completely different'
And now for something completely different

Simple string and number variables are prefixed with $. Perl 6 looks at "type" in at least two ways: the type of a value (Str, Int, Rat, and so on) and the type of the variable that holds the value. For now, just remind folks to declare variables with my and use $ sigil for the variables they're working with at this point.

> my $n = 17;
17
> my $pi = 3.1415926535897932
3.1415926535897932

You can ask a variable what type of value it is holding right now using WHAT.

> $message.WHAT
Str()
> $n.WHAT
Int()
> $pi.WHAT
Rat()

Exercise 1

Perl 6 doesn't actually care about leading zero in a value ordinarily. Caveats about literals in other languages don't apply here, except to remind folks about the difference between Int and Str.

> my $zipcode = 02492;
2492
> $zipcode = '02492';
02492

Rules for variable names are similar to Python and other languages, though that sigil means reserved names aren't such a big deal.

> my $76trombones = 'big parade';
Cannot declare a numeric variable
> my $more@ = 1000000
Two terms in a row
> my $class = 'Advanced Theoretical Zymurgy'
Advanced Theoretical Zymurgy

Note that Perl 6 errors are more descriptive than the infamous "SyntaxError: invalid syntax" of Python.

Oh, and Perl 6 allows - in variable names.

> my $class-name = 'Advanced Theoretical Zymurgy';
Advanced Theoretical Zymurgy

Perl 6 does not do floor division by default. It will convert Int to Rat as needed for an operation.

> my $minute = 59
59
> $minute / 60
0.983333

Use say - remember say? - to get output when running a Perl 6 program as opposed to the interactive shell.

my $miles = 26.2
say $miles * 1.61

Run it.

$ perl6 sample-01.p6
===SORRY!===
Two terms in a row
at sample-01.p6:2
------> <BOL>⏏say $miles * 1.61
    expecting any of:
        postfix
        infix or meta-infix
        infix stopper
        statement end
        statement modifier
        statement modifier loop

Yeah. Better start using semicolons now. Probably should've been using semicolons from the beginning, but I was being lazy. Speaking of lazy, I just noticed that perl6 doesn't seem to require the use v6 pragma. Still, good form is important.

use v6;
my $miles = 26.2;
say $miles * 1.61;

Run it.

$ perl6 sample-01.p6
42.182

That's better.

Assignment lines don't produce output when running outside the REPL.

use v6;
say 1;
my $x = 2;
say $x;

See?

$ perl6 sample-02.p6
1
2

Exercise 2

Type the following statements in the Perl 6 interpreter to see what they do:

5;
my $x = 5;
$x + 1;

Now put the same statements into a script and run it. What is the output? Modify the script by transforming each expression into a say statement and then run it again.

say 5;
my $x = 5;
say $x + 1;

Order of Operations

No huge difference here for simple operations. PEMDAS. When in doubt, use parens.

String Operations

Remember that Perl 6 is still Perl.

> '2' - 1'
1
> 'eggs' / 'easy'
Cannot convert string to number: base-10 number must begin with valid digits or '.' in '⏏eggs' (indicated by ⏏)

Well, almost.

Python concatenates strings with +. Perl 6 concatenates strings with ~.

my $first = 'throat';
my $second = 'warbler';
say $first ~ $second; # says 'throatwarbler'

Python does string repetition with *. Perl 6 does string repetition with x.

> 'spam' x 3
spamspamspam

Comments

# This is a line comment, same as in loads of other languages.

Debugging

Perl 6 error messages are fairly helpful.

> my $bad name = 5;
Two terms in a row

Typos are a common source of errors when caffeine passes the critical threshold. Here is one of the more common typo errors: misspelling a variable name.

> my $principal = 327.68;
327.68
> my $interest = $principle * $rate;
Variable $principle is not declared

Exercises

Exercise 3

Assume that we execute the following assignment statements:

my $width = 17;
my $height = 12.0;
my $delimiter = '.';

For each of the following expressions, write the value of the expression and the type (of the value of the expression).

1. $width / 2 = 8.5 = Rat()
2. $width / 2.0 = 8.5 = Rat() Perl 6 just don't care about floor division
3. $height / 3 = 4 = Rat() Looks like an Int but it's a Rat
4. 1 + 2 * 5 = 11
5. $delimiter x 5 = .....

Use the Perl 6 interpreter to check your answers.

Exercise 4

Practice using the Perl 6 interpreter as a calculator:

Volume of a sphere w/radius r is 4/3πr³. What is the volume of a sphere with radius 5? Python hint doesn't apply.

One solution

 use v6;
 my $radius = 5;
 my $PI = 3.1415926;
 my $area = 4/3 * $pi * $radius**3;
 say $area;

It works.

book cover prices is $24.95, bookstores get 40% discount. Shipping is $3 for first copy and 75 cents per additional copy. What's the total wholesale cost for 60 copies?

Just your basic calculator solution:

 > 24.95 * 0.6 * 60 + 0.75 * (60 - 1) + 3
 945.45

I had to break the next one down carefully, as I am a bear of very small brain.

If I leave my house at 6:52 am and run 1 mile at an easy pace (8:15 per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at easy pace again, what time do I get home for breakfast?

I am willing to bet good money that my solution is more verbose than it needs to be.

use v6;

my $minute = 60;
my $hour = $minute * 60;

# Turn start time to seconds after midnight
my $start-time = (6 * 60 + 52) * 60;
my $easy-pace  = 8 * 60 + 15;
my $tempo-pace = 7 * 60 + 12;
my $running-time = $easy-pace + 3 * $tempo-pace + $easy-pace;
# Find finish time as seconds after midnight
my $finish-time = $start-time + $running-time;

# Convert finish time to a clock time, first in hours:
my $finish-hour = ($finish-time / $hour).Int;
# ... then the seconds left over, expressed as minutes.
my $finish-minute = ($finish-time % $hour / 60).Int;

say "Finish time was $finish-hour:$finish-minute";

That gave me:

$ perl6 jogging.p6
Finish time was 7:30