A short tour of the type system in Perl6. Using type annotations to make your code more robust.

1. The Perl6 type system

2. my $scalar;
our @array;
my %hash;
$scalar = “hi there”;
@array = 1, 2, “a string”;
%hash = one => 1, two => “2”;

3. my Any $scalar;
our Any @array;
my Any %hash;
$scalar = “hi there”;
@array = 1, 2, “a string”;
%hash = one => 1, two => “2”;

4. my Int $scalar;
our Int @array;
my Int %hash;
$scalar = “hi there”;
@array = 1, 2, “a string”;
%hash = one => 1, two => “2”;

5. my Int $scalar;
our Int @array;
my Int %hash;
$scalar = “hi there”;
@array = 1, 2, “a string”;
%hash = one => 1, two => “2”;
Type check failed in assignment to
'$scalar'; expected 'Int' but got 'Str'
in block <unit> at types.pl:4

6. AST
Any
Array
Associative
Backtrace
Backtrace::Frame
Bag
BagHash
Baggy
Blob
Block
Bool
Buf
Callable
Capture
Channel
Code
Complex
Cool
CurrentThreadScheduler
Cursor
Date
DateTime
Dateish
Duration
Enum
EnumMap
Exception
Failure
FatRat
Grammar
Hash
IO
IO::Handle
IO::Pipe
IO::Socket
Instant
Int
Iterable
Iterator
Junction
Label
List
Lock
Macro
Match
Metamodel::C3MRO
Metamodel::ClassHOW
Metamodel::Finalization
Metamodel::Naming
Metamodel::Primitives
Method
Mix
MixHash
Mixy
Mu
Nil
Num
Numeric
ObjAt
Pair
PairMap
Parameter
Parcel
Pod::Block
Pod::Item
Positional
Proc
Proc::Async
Proc::Status
Promise
Proxy
QuantHash
Range
Rat
Rational
Real
Regex
Routine
Scheduler
Signature
Str
Thread
Version
Whatever

7. sub sum_0_to_n($n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);

8. sub sum_0_to_n($n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3); 6

9. sub sum_0_to_n($n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(“3”);
6

10. sub sum_0_to_n($n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(“3”);
6
6

11. sub sum_0_to_n($n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(“3”);
say sum_0_to_n(“foo”);
6
6

12. sub sum_0_to_n($n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(“3”);
say sum_0_to_n(“foo”);
6
6
Cannot convert string to number:
base-10 number must begin with valid
digits or '.' in '⏏foo' (indicated by ⏏)
In method Numeric at m-CORE:1576
In sub infix:<-> at m-CORE:5277
In sub infix:<-> at m-CORE:5277
In sub sub_0_to_n at sum.pl:3
In block <unit> at sum.pl:7

13. sub sum_0_to_n(Numeric $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(“foo”);
6

14. sub sum_0_to_n(Numeric $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(“foo”);
6
===SORRY!=== Error while compiling
sum.pl
Calling 'sum_0_to_n' will never work
with argument types (str)
Expected: :(Numeric $n)
at summer.pl:10
------> say ⏏sum_0_to_n("foo");

15. sub sum_0_to_n(Numeric $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
my $arg = “foo”;
say sum_0_to_n($arg);
6

16. sub sum_0_to_n(Numeric $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
my $arg = “foo”;
say sum_0_to_n($arg);
6
Type check failed in binding $n;
expected 'Numeric' but got 'Str'
in sub sum_0_to_n at sum.pl:1
in block <unit> at sum.pl:8

17. sub sum_0_to_n(Numeric $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(3.5);
6

18. sub sum_0_to_n(Numeric $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(3.5);
6
Hangs… infinite loop

19. sub sum_0_to_n(Int $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(3.5);
6

20. sub sum_0_to_n(Int $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(3.5);
6
===SORRY!=== Error while compiling
sum.pl
Calling 'sum_0_to_n' will never work
with argument types (Rat)
Expected: :(Int $n)
at sum.pl:9

21. sub sum_0_to_n(Int $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6

22. sub sum_0_to_n(Int $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6
Hangs… Infinite loop

23. sub sum_0_to_n(Int $n where * >= 0) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6

24. sub sum_0_to_n(Int $n where * >= 0) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6
Constraint type check failed for
parameter '$n'
in sub sum_0_to_n at sum.pl:4
in block <unit> at sum.pl:9

25. subtype NonNegInt of Int where * >= 0;
sub sum_0_to_n(NonNegInt $n) {
return 0 unless $n;
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6
Constraint type check failed for
parameter '$n'
in sub sum_0_to_n at sum.pl:4
in block <unit> at sum.pl:9

26. subtype NonNegInt of Int where * >= 0;
multi sub sum_0_to_n(0) { 0 }
multi sub sum_0_to_n(NonNegInt $n) {
return $n + sum_0_to_n($n – 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6
Constraint type check failed for
parameter '$n'
in sub sum_0_to_n at sum.pl:4
in block <unit> at sum.pl:9

27. subtype NonNegInt of Int where * >= 0;
multi sub sum_0_to_n(0) { 0 }
multi sub sum_0_to_n(NonNegInt $n) {
return $n + sum_0_to_n($n – 1);
}
multi sub sum_0_to_n(Int $n where * < 0) {
return $n + sum_0_to_n($n + 1);
}
say sum_0_to_n(3);
say sum_0_to_n(-3);
6
-6

28. subtype NonNegInt of Int where * >= 0;
multi sub sum_0_to_n(0) { 0 }
multi sub sum_0_to_n(NonNegInt $n) {
return $n + sum_0_to_n($n – 1);
}
multi sub sum_0_to_n(Int $n where * < 0) {
return $n + sum_0_to_n($n + 1);
}
sub MAIN(Int :$value!) {
say sum_0_to_n($value);
}
> perl6 sum.pl
Usage:
sum.pl –value=<Int>

29. subtype NonNegInt of Int where * >= 0;
multi sub sum_0_to_n(0) { 0 }
multi sub sum_0_to_n(NonNegInt $n) {
return $n + sum_0_to_n($n – 1);
}
multi sub sum_0_to_n(Int $n where * < 0) {
return $n + sum_0_to_n($n + 1);
}
sub MAIN(Int :$value!) {
say sum_0_to_n($value);
}
> perl6 sum.pl
Usage:
sum.pl –value=<Int>
> perl6 sum.pl –value=foo
Usage:
sum.pl --value=<Int>

30. subtype NonNegInt of Int where * >= 0;
multi sub sum_0_to_n(0) { 0 }
multi sub sum_0_to_n(NonNegInt $n) {
return $n + sum_0_to_n($n – 1);
}
multi sub sum_0_to_n(Int $n where * < 0) {
return $n + sum_0_to_n($n + 1);
}
sub MAIN(Int :$value!) {
say sum_0_to_n($value);
}
> perl6 sum.pl
Usage:
sum.pl –value=<Int>
> perl6 sum.pl –value=foo
Usage:
sum.pl –value=<Int>
> perl6 sum.pl – value=4
10

31. subtype NonNegInt of Int where * >= 0;
multi sub sum_0_to_n(0) returns Int { 0 }
multi sub sum_0_to_n(NonNegInt $n) returns Int {
return $n + sum_0_to_n($n – 1);
}
multi sub sum_0_to_n(Int $n where * < 0) returns Int {
return $n + sum_0_to_n($n + 1);
}
sub MAIN(Int :$value!) {
say sum_0_to_n($value);
}

32. class Dog {
has Str $.name;
has Int $.age is rw;
}

33. class Dog {
has Str $.name;
has Int $.age is rw;
}
class Owner {
has Str $.name;
has Dog @.dogs;
}

34. class Dog {
has Str $.name;
has Int $.age is rw;
}
class Owner {
has Str $.name;
has Dog @.dogs;
}
my $dog = Dog.new(name => 'Fido',
age => 'foo');
Type check failed in assignment to '$!age'; expected 'Int' but got 'Str'
in block at src/gen/m-CORE.setting:1009
in method BUILDALL at src/gen/m-CORE.setting:992
in method bless at src/gen/m-CORE.setting:981
in method new at src/gen/m-CORE.setting:967
in block <unit> at dogs.pl:10

35. class Dog {
has Str $.name;
has Int $.age is rw;
}
class Owner {
has Str $.name;
has Dog @.dogs;
}
my $dog = Dog.new(name => 'Fido',
age => '2');
$dog.age = “foo”;
Type check failed in assignment to '$!age'; expected 'Int' but got 'Str'
In block <unit> at dogs.pl:11

36. class Dog {
has Str $.name;
has Int $.age is rw;
}
class Owner {
has Str $.name;
has Dog @.dogs;
}
my $dog = Dog.new(name => 'Fido',
age =>2);
my $owner = Owner.new(name => 'Bob',
dogs => ($dog, 'foo'));
Type check failed in assignment to '@!dogs'; expected 'Dog' but got 'Str'
in method REIFY at src/gen/m-CORE.setting:10861
in method reify at src/gen/m-CORE.setting:9471
in method gimme at src/gen/m-CORE.setting:9954
in method sink at src/gen/m-CORE.setting:10438
in method BUILDALL at src/gen/m-CORE.setting:1083
in method bless at src/gen/m-CORE.setting:981
in method new at src/gen/m-CORE.setting:967
in block <unit> at dogs.pl:11

37. class Dog {
has Str $.name;
has Int $.age;
}
class Owner {
has Str $.name;
has Dog @.dogs;
}
my $dog = Dog.new(name => 'Fido',
age =>2);
my $owner = Owner.new(name => 'Bob',
dogs => ($dog));
If ($owner ~~ Dog) {
say “owner is-a Dog”;
} elsif ($owner ~~ Owner) {
say “owner is-a Owner”;
} else {
say “owner is-a something else”;
}

38. given $thing {
when Dog { say '$thing is-a Dog' }
when 3 { say '$thing is 3' }
when "blah" { say '$thing is blah' }
when /pattern/ { say '$thing matches /pattern/' }
when (10 .. 20) { say '$thing is between 10 and 20' }
when any(1, 4, 6) { say '$thing is 1, 4 or 6' }
}

39. class Dog {
has Str $.name;
has Int $.age;
}
class Owner {
has Str $.name;
has Dog @.dogs;
method ACCEPTS(Dog $d) {
return $d ~~ any(@.dogs);
}
}

40. class Dog {
has Str $.name;
has Int $.age;
}
class Owner {
has Str $.name;
has Dog @.dogs;
method ACCEPTS(Dog $d) {
return $d ~~ any(@.dogs);
}
}
my $fido = Dog.new(name => 'Fido', age => 3)
my $bob = Owner.new(name => 'Bob', dogs => ($fido));
my $fred = Owner.new(name => 'Fred');
given $fido {
When $fred { say “owned by Fred” }
When $bob { say “owned by Bob” }
}