[Haskell-cafe] Functors and the Visitor Pattern

[Tom.Amundsen]

So, last night, I was having this problem with my Java code where I couldn't
figure out for the life of me how to write a piece of code without a big if
{} else if {} else if {} ... else {} structure. I was Googling "Java
Reflection" to try to determine how to "cast to the most concerete subclass
at runtime." Then it dawned on me that what I was trying to do has already
been solved by using the Visitor design pattern.

Then, after reading the Visitor design pattern page on Wiki, it said that
the visitor pattern is essentially an implementation of a functor. Aha! It
totally clicked. The Visitor pattern allows you to collect code for similar
operations, while spreading apart code for similar objects. Now that really
sounds like a functor!

Although, now I'm second guessing myself, because I can't figure out how we
could create some design pattern that simulates an applicative functor. I'm
pretty sure the Visitor pattern doesn't take you this far (but I am willing
to be corrected). So, is there a way to create applicative functors in
non-functional languages? What would that pattern look like?

- Tom

[Andres Loeh]

Post by Tom.Amundsen
Although, now I'm second guessing myself, because I can't figure out how we
could create some design pattern that simulates an applicative functor. I'm
pretty sure the Visitor pattern doesn't take you this far (but I am willing
to be corrected). So, is there a way to create applicative functors in
non-functional languages? What would that pattern look like?

Perhaps this paper can answer your question:

Jeremy Gibbons, Bruno C.d.S. Oliveira
The essence of the Iterator pattern
http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/iterator.pdf

HTH,
Andres

[Edward Kmett]

The concepts are fairly closely related but each entails something the other
does not. Functor entails parametric polymorphism with respect to the
contents of the container. And a visitor can extract a result from the
traversal.

As a result you may want to think in terms of a Traversable or Foldable
functor rather than just a Functor. These will let you extract a monadic or
applicative result from your container.

Now, as for your actual question, Applicative functors are really hard to
model in other languages. With C++ templates you can probably get a weak
approximation with something like the encoding used here, possibly mixed
with some boost magic for a usable function type:

http://www.reddit.com/r/programming/comments/8bx33/a_comparison_of_c_concepts_and_haskell_type/

But I can't think of anyone that would try to use it in production code,
and there are many common idioms that that style of translation can't
account for (i.e. polymorphic recursion).

-Edward Kmett

Post by Tom.Amundsen
So, last night, I was having this problem with my Java code where I couldn't
figure out for the life of me how to write a piece of code without a big if
{} else if {} else if {} ... else {} structure. I was Googling "Java
Reflection" to try to determine how to "cast to the most concerete subclass
at runtime." Then it dawned on me that what I was trying to do has already
been solved by using the Visitor design pattern.
Then, after reading the Visitor design pattern page on Wiki, it said that
the visitor pattern is essentially an implementation of a functor. Aha! It
totally clicked. The Visitor pattern allows you to collect code for similar
operations, while spreading apart code for similar objects. Now that really
sounds like a functor!
Although, now I'm second guessing myself, because I can't figure out how we
could create some design pattern that simulates an applicative functor. I'm
pretty sure the Visitor pattern doesn't take you this far (but I am willing
to be corrected). So, is there a way to create applicative functors in
non-functional languages? What would that pattern look like?
- Tom
--
http://www.nabble.com/Functors-and-the-Visitor-Pattern-tp23851113p23851113.html
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
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[Daryoush Mehrtash]

There was a google talk on Visitor pattern in Java and Common Lisp that you
might find interesting

Daryoush

Post by Tom.Amundsen
So, last night, I was having this problem with my Java code where I couldn't
figure out for the life of me how to write a piece of code without a big if
{} else if {} else if {} ... else {} structure. I was Googling "Java
Reflection" to try to determine how to "cast to the most concerete subclass
at runtime." Then it dawned on me that what I was trying to do has already
been solved by using the Visitor design pattern.
Then, after reading the Visitor design pattern page on Wiki, it said that
the visitor pattern is essentially an implementation of a functor. Aha! It
totally clicked. The Visitor pattern allows you to collect code for similar
operations, while spreading apart code for similar objects. Now that really
sounds like a functor!
Although, now I'm second guessing myself, because I can't figure out how we
could create some design pattern that simulates an applicative functor. I'm
pretty sure the Visitor pattern doesn't take you this far (but I am willing
to be corrected). So, is there a way to create applicative functors in
non-functional languages? What would that pattern look like?
- Tom
--
http://www.nabble.com/Functors-and-the-Visitor-Pattern-tp23851113p23851113.html
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
_______________________________________________
Haskell-Cafe mailing list
http://www.haskell.org/mailman/listinfo/haskell-cafe

[wren ng thornton]

Post by Tom.Amundsen
So, last night, I was having this problem with my Java code where I couldn't
figure out for the life of me how to write a piece of code without a big if
{} else if {} else if {} ... else {} structure. I was Googling "Java
Reflection" to try to determine how to "cast to the most concerete subclass
at runtime." Then it dawned on me that what I was trying to do has already
been solved by using the Visitor design pattern.
Then, after reading the Visitor design pattern page on Wiki, it said that
the visitor pattern is essentially an implementation of a functor. Aha! It
totally clicked. The Visitor pattern allows you to collect code for similar
operations, while spreading apart code for similar objects. Now that really
sounds like a functor!
Although, now I'm second guessing myself, because I can't figure out how we
could create some design pattern that simulates an applicative functor. I'm
pretty sure the Visitor pattern doesn't take you this far (but I am willing
to be corrected). So, is there a way to create applicative functors in
non-functional languages? What would that pattern look like?

The Visitor pattern isn't a functor, it's a collection of things. The
type being visited is the functor[1], the set of methods on that type
for accepting a visitor is a catamorphism[2], and the visitor itself is
an algebra for the functor[3].


[1] Or rather, the coproduct of all related classes that can chain a
visitor forms a type, and that type is a functor.


[2] For the recursive Visitor pattern I use most often, that is. For the
non-recursive version it's usually fmap. This is the part where the
pattern gets a bit shaky because there are actually many different
patterns all called "Visitor". The main points of interest are whether
it's recursive or not, and whether it applies the visitor to itself, to
its children, or both.

non-recursive + itself == ($)

non-recursive + children == fmap (under open-recursion interpretation of
the type, aka all nodes are elements)

recursive + children == fmap (under closed-recursion interpretation, aka
only fringe nodes are elements)

recursive + both == cata (usually, though it depends how you aggregate)

recursive + itself == This is actually a variant of the Iterator pattern


[3] Though again there's some variation in different "Visitor" patterns.
Some variants of the pattern include some of the recursion pattern in
the visitor itself rather than in the methods on the visited type. It
can be harder to maintain since it's less modular, though it allows the
visit methods to serve as many different functions which can be helpful
if you need more than one of ($), fmap, cata, traverse,...

[Johan Tibell]

The Visitor pattern isn't a functor, it's a collection of things. The type
being visited is the functor[1], the set of methods on that type for
accepting a visitor is a catamorphism[2], and the visitor itself is an
algebra for the functor[3].
[1] Or rather, the coproduct of all related classes that can chain a
visitor forms a type, and that type is a functor.
[2] For the recursive Visitor pattern I use most often, that is. For the
non-recursive version it's usually fmap. This is the part where the pattern
gets a bit shaky because there are actually many different patterns all
called "Visitor". The main points of interest are whether it's recursive or
not, and whether it applies the visitor to itself, to its children, or both.
non-recursive + itself == ($)
non-recursive + children == fmap (under open-recursion interpretation of
the type, aka all nodes are elements)
recursive + children == fmap (under closed-recursion interpretation, aka
only fringe nodes are elements)
recursive + both == cata (usually, though it depends how you aggregate)
recursive + itself == This is actually a variant of the Iterator pattern

Could you be so kind to give an example for each?

Cheers,

Johan

[wren ng thornton]

Post by Johan Tibell

Post by wren ng thornton
[2] For the recursive Visitor pattern I use most often, that is. For the
non-recursive version it's usually fmap. This is the part where the pattern
gets a bit shaky because there are actually many different patterns all
called "Visitor". The main points of interest are whether it's recursive or
not, and whether it applies the visitor to itself, to its children, or both.
non-recursive + itself == ($)
non-recursive + children == fmap (under open-recursion interpretation of
the type, aka all nodes are elements)
recursive + children == fmap (under closed-recursion interpretation, aka
only fringe nodes are elements)
recursive + both == cata (usually, though it depends how you aggregate)
recursive + itself == This is actually a variant of the Iterator pattern

Could you be so kind to give an example for each?

In OOP you mean?

/* nonrecursive + self == application */
class A { T app(Visitor v) { return v.visit(this); } }
class B { T app(Visitor v) { return v.visit(this); } }
...
// An allomorphic function :: (A | B | ...) -> T
class Visitor {
T visit(A a) { ... }
T visit(B b) { ... }
...
}

This particular version often isn't too helpful because it's just
reflecting the method call, we could've just called visit directly
instead of calling app. But there are times where it is useful,
particularly when you want to have some visitors which are recursive and
some which are not. In which case it doesn't matter which method you
start with, but you do need both in order to reflect back on recursion.


/* nonrecursive + children == fmap (with real parametricity) */
class F<A> {
Children<A> as;
F(Children<A> as) { this.as = as; }

F<B> fmap(Visitor<A,B> v) {
Children<B> bs = new Children<B>();
for (A a : this.as)
bs.add( v.visit(a) );
return new F<B>(bs);
}
}
...
interface Visitor<A,B> { B visit(A a); }

This is a rather Haskellish take on this version. In practice people
often don't bother supporting parametricity (needed for making F a real
functor). That is, usually they'll do destructive updates to F, only
have endofunction visitors (so there's no change of types), or use
side-effect only visitors (see below).


/* recursive + children == fmap (side-effect only) */
abstract class Tree { abstract void rmap(Visitor v); }
class Branch extends Tree {
Children<Tree> subtrees;

void rmap(Visitor v) {
for (Tree t : this.subtrees)
v.visit(t);
}
}
class Leaf extends Tree {
void rmap(Visitor v) {
// Just in case we're the root node.
v.visit(this);
// Or we could do nothing instead,
// depending on desired semantics
}
}
class Visitor {
void visit(Branch t) { t.rmap(this); } // reflect to recurse
void visit(Leaf t) { ... } // don't reflect or you'll hit _|_
}

This highlights an additional axis of variation in the many different
visitor patterns, whether the "result" is returned directly (as in the
previous example), whether it is accumulated in the Visitor itself
(requiring explicit lookup later), or whether it's done via side-effects
on global state. The accumulator and side-effect versions are a bit more
general since their "return type" isn't restricted by the classes being
visited.


/* recursive + self/both == a kind of Iterator/catamorphism */
abstract class Tree { abstract void observe(Visitor v); }
class Branch extends Tree {
Children<Tree> subtrees;

void observe(Visitor v) {
v.visit(this);

for (Tree t : this.subtrees)
t.observe(v);
}
}
class Leaf extends Tree {
void observe(Visitor v) {
v.visit(this);
}
}
class Visitor {
void visit(Branch t) { ... }
void visit(Leaf t) { ... }
}

This is different from the recursive+children version because this
version keeps all of the recursion code on the side of the visited
classes, and it also meaningfully visits interior nodes. In the
recursive+children version the visitor ignored branches (though it
doesn't need to) and reflected back to initiate recursion, whereas this
version will recurse no matter what the visitor does (barring
exceptions, etc).

This version is a push iterator which forces you to visit all nodes,
rather than the more usual pull iterator where you need to call next()
to get the next node. We can convert between the two varieties by using
co-routines or threads or other control-flow tricks.

If we reverse the order of the recursive observe and the visit(this)
then we get something like a catamorphism. Whether it's actually a
catamorphism depends on what the visitor does, or rather what knowledge
about the shape of Branch and Leaf it makes use of. "Real" catamorphisms
are fairly rare in OOP, though you often find things like using a
visitor to add decoration to a tree (which is much like passing the
initial algebra to cata) or maintaining some aggregation in the visitor.

[Johan Tibell]

Post by wren ng thornton

Post by Johan Tibell
Could you be so kind to give an example for each?

In OOP you mean?

This cleared things up for me. Thanks!

-- Johan