The document discusses Perl 6 types and type checking. It shows examples of declaring scalar, array and hash variables with implicit and explicit types like Any, Int and Str. It then demonstrates type checking failures when assigning values of the wrong type. It also covers defining custom types through subtypes and multi dispatch to handle different types.

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” }
}