Perl 6 from Haskell - Nutshell

Learning Perl 6 from Haskell, in a nutshell: What do I already know?

Haskell and Perl 6 are very different languages. This is obvious. However, that does not mean there are not similarities or shared ideas! This page attempts to get a Haskell user up and running with Perl 6. The Haskell user may find that they need not abandon all of their Haskelly thoughts while scripting in Perl 6.

Note that this should not be mistaken for a beginner tutorial or overview of Perl 6; it is intended as a technical reference for Perl 6 learners with a strong Haskell background.


Types vs Values

In Haskell, you have type level programming and then value level programming.

plusTwo :: Integer -> Integer   -- Types
plusTwo x = x + 2               -- Values

You do not mix types and values in Haskell like the below

plusTwo 2          -- This is valid
plusTwo Integer    -- This is not valid

In Perl 6, types (AKA type objects) live on the same level as values

sub plus-two(Int $x --> Int{ $x + 2 }
plus-two(2);    # This is valid 
plus-two(Int);  # This is valid 

I will illustrate this unique aspect of Perl 6 with one more example:

multi sub is-string(Str $ --> True{}
multi sub is-string(Any $ --> False{}
is-string('hello');    #True 
is-string(4);          #False 


In Haskell, you have a Maybe type that allows you to forgo the worry of null types. Let's say you have a hypothetical function that parses a String to an Integer:

parseInt :: String -> Maybe Integer
case parseInt myString of
  Just x  -> x
  Nothing -> 0

In Perl 6, since type objects coexist with regular objects, we have the concept of Defined and Undefined objects. Plain type objects are undefined while instantiated objects are defined.

sub parse-int(Str $s --> Int{ ... }
my $string = {...};
given parse-int($string{
  when Int:D { $_ }
  when Int:U { 0 }

So in Perl 6 we have type constraints that indicate the definedness of a type. These are

Int:D# This is a defined Int. 
Int:U# This is an undefined Int, AKA a type object 
Int:_# This is either defined or undefined. 

If we wanted to be explicit in the above example (probably a good idea), we could add the :_ constraint on the return type. This would let the user know that they should account for both defined and undefined return values. We could also use other methods and constructs that specifically test for definedness.

sub parse-int(Str $s --> Int:_{ ... }
# One way to do it 
my $string = {...};
given parse-int($string{
  when Int:D { $_ }
  when Int:U { 0 }
# Another way to do it 
my Int $number = parse-int($string);
if $number.defined { $number } else { 0 }
# A better way 
with parse-int($string{ $_ } else { 0 }
# With the defined-or operator 
parse-int($string// 0

The with operator that you see above is like if, except it explicitly tests for definedness and then passes the result to the following block. Similarly, without tests that the object is undefined and also passes the result to the following block.

For more natural control flow with undefined and defined types, Perl 6 introduces andthen and orelse.

sub parse-int(Str $s --> Int:_{ ... }
my $string = {...};
my $result = parse-int($stringorelse 0;
sub hello() { say 'hi' }
hello() andthen say 'bye';

TODO: include a better example for andthen that makes sense. Maybe using promise objects?

So in practice, Perl 6 does not have the concept of a null type, but rather of defined or undefined types.

Data Definitions

Perl 6 is fundamentally an Object Oriented language. However, it also gives you the freedom to write in virtually any paradigm you wish. If you only want to pure functions that take an object and return a new object, you can certainly do so.

Here is a Haskell code example:

data Point = Point x y
moveUp :: Point -> Point
moveUp (Point x y= Point x (y + 1)

And an equivalent Perl 6 example:

class Point { has $.xhas $.y}
sub move-up(Point $p --> Point{ => $p.x=> $p.y + 1)

The code I illustrated above is an example of a Product Type. If instead you'd like to write a Sum Type, there is not an exact equivalent in Perl 6. The closest thing would be an Enum.

data Animal = Dog | Cat | Bird | Horse
testAnimal :: Animal -> String
testAnimal Dog   = "Woof"
testAnimal Horse = "Neigh"

A Perl 6 Enum does not fit the same exact use cases, but it can be used in putting constraints on types.

enum Animal < Dog Cat Bird Horse >;
proto sub test-animalAnimal        ) {*}
multi sub test-animalDog           ) { 'Woof' }
multi sub test-animalAnimal::Horse ) { 'Neigh'  }   # more explicit 
say test-animal Animal::Dog# more explicit 
say test-animal Horse;

Type Aliases and Subsets

In Haskell, you can alias an existing type to simply increase clarity of intent and re-use existing types.

type Name = String
fullName :: Name -> Name -> Name
fullName first last = first ++ last

The equivalent in Perl 6 is the following.

my constant Name = Str;
sub full-name ( Name \firstName \last --> Name ) { first ~ last }

It should be noted that in Perl 6, one can also create a subset of an existing type.

subset Name of Str where *.chars < 20;
sub full-name(Name $firstName $last{
  $first ~ $last
full-name("12345678901234567890111""Smith"# This does not compile, as the first parameter 
                                              # doesn't fit the Name type 



explain how Perl 6 roles compare to Haskell typeclasses


Definitions and Signatures



Haskell makes heavy use of pattern matching in function definitions.

greeting :: String -> String
greeting  ""   = "Hello, World!"
greeting "bub" = "Hey bub."
greeting  name = "Hello, " ++ name ++ "!"

Perl 6 does this as well! You just use the multi keyword to signify that it is a multiple dispatch function.

proto greeting ( Str   --> Str ) {*}
multi greeting ( ""    --> "Hello, World!" ) {}
multi greeting ( "bub" --> "Hey bub." ) {}
multi greeting ( \name ) { "Hello, " ~ name ~ "!" }

The proto declarator is not necessary, but can sometimes aid in making sure that all multis follow your business rules. Using a variable name in the signature of the proto would provide more information in error messages, and for introspection.

proto greeting ( Str \name --> Str ) {*}
say &greeting.signature;                  # (Str \name --> Str) 

An interesting thing to note in the Perl 6 code above is that passing values like 'bub' as a function parameter is just syntax sugar for a where guard.


Using the example from the "Pattern Matching" section of this page, you can see the guards that are used behind the scenes to constrain our function arguments.

multi greeting ( ""    --> "Hello, World!" ) {}
multi greeting ( "bub" --> "Hey bub." ) {}
# The above is the same as the below 
multi greeting(Str \name where ''    ) {'Hello, World!'}
multi greeting(Str \name where 'bub' ) {'Hey bub.'}
# The above is the same as the below, again. 
multi greeting(Str \name where $_ ~~ ''   ) {'Hello, World!'}
multi greeting(Str \name where $_ ~~ 'bub'{'Hey bub.'}

$_ is known as the topic variable. It assumes the form of whatever is appropriate. The smart match operator ~~ figures out the best way to determine if the left matches the right, be it number ranges, strings, etc. Our three examples above go from most sugared (top), to least sugared (bottom).

The bottom examples above could be wrapped in curly braces, making it more obvious that it is a code block. Note that a where clause may also take an explicit Callable.

multi greeting(Str \name where { $_ ~~ '' } ) {'Hello, World!'}
multi greeting(Str \name where -> $thing { $thing ~~ '' } ) {'Hello, World!'}
multi greeting ( Str \name where { Bool.pick } --> 'True' ){}
multi greeting ( Str \name where &some-subroutine ){}

If you read the section in this page on subsets, you'll notice that "where" is used in the making of subsets as well as here. The usage of "where" in both areas is exactly the same.

When using where, note that the order of definition is important, just like in Haskell.

multi greeting ( Str \name where '' --> 'Hello, World!' ){}
multi greeting ( Str \name where { Bool.pick } --> 'True' ){}
multi greeting ( Str \name where 'bub' --> 'Hey, bub.' ){}
say greeting ''   ; # will never say True 
say greeting 'bub'# about 50% of the time it will say True 






.assuming vs currying

method chaining vs currying



show function composition operator. Maybe explain a more perl6ish way to do this though.

Case / Matching

Haskell makes heavy use of case matching like the below:

case number of
  2 -> "two"
  4 -> "four"
  8 -> "eight"
  _ -> "don't care"

In Perl 6 you can achieve this same thing with the given/when structure:

my $number = {...};
given $number {
  when 2  { "two" }
  when 4  { "four" }
  when 8  { "eight" }
  default { "don't care" }

Note that the order of the when's is also significant, just like with the where's in the guard section of this page.



explain difference between perl6 Array, Sequence, List. Explain data shapes in regards to the @ sigil. Explain how you can convert an Array to a flattened list of objects with |@

data shapes become quite intuitive, but it takes a bit of practice.

List Comprehensions

TODO compare haskell list comprehensions to Perl 6 gather/take


Fold in Haskell is called Reduce in Perl 6.

mySum = foldl `+0 numList
my @numbers = {...};
reduce { $^a + $^b }0|@numbers;
@numbers.reduce({$^a + $^b}with => 0)

However, in Perl 6, if you want to use an infix operator (+ - / % etc) there is a nice little helper called the Reduction Metaoperator.

my @numbers = {...};
[+@numbers    # This is the same 
[+0@numbers # as this 

It inserts the operator in between all values in the list and produces a result, just like Fold.

In Haskell you, you have foldl and foldr. In Perl 6, this difference is determined by the associativity attached to the operator/subroutine.

sub two-elem-list ( \a, \b ) { ( ab ) }
# you can use a subroutine as an infix operator 
say 'a' [&two-elem-list'b'# (a b) 
# as the reduction prefix meta operator takes an infix operator, it will work there too; 
[[&two-elem-list]] 1..5;           # ((((1 2) 3) 4) 5) 
say (1..5).reduce: &two-elem-list# ((((1 2) 3) 4) 5) 
# right associative 
sub right-two-elem-list( \a, \b ) is assoc<right> { ( ab ) }
say (1..5).reduce: &right-two-elem-list# (1 (2 (3 (4 5)))) 
# XXX there is possibly a bug here as this currently doesn't look at 
# XXX the associativity of &right-two-elem-list and just always does left assoc 
say [[&right-two-elem-list]] 1..5;
# chaining 
say [<1..5;            # True 
say (1..5).reduce: &[<]; # True 




Haskell and Perl 6 both allow you to specify ranges of values.

myRange1 = 10..100
myRange2 = 1..        -- Infinite
myRange3 = 'a'..'h'   -- Letters work too
my $range1 = 10..100;
my $range2 = 1..*;      # Infinite 
my $range3 = 'a'..'h';  # Letters work too 

Laziness vs Eagerness

In the examples above, you have the concept of laziness displayed very plainly. Perl 6 has laziness only where it makes the most sense. For example, in the range 10..100, this is eager because it has a definite end. If a list does not have a definite end, then the list should clearly be lazy.

(1 .. 100).is-lazy# False 
(1 .. Inf).is-lazy# True 

These are the "sane defaults" that Perl 6 takes pride in. But they are still defaults and can be changed into one or the other.

(1 .. 100);       # True 
(1 .. 100) False 

Contexts (let-in / where)


explain how given/when and with/without and for loops open lexical scopes with the argument as the context.

compare it to let/in and where constructs maybe?


Parser Combinators vs Grammars