Settembre 2015
@filippovitale
Polimorfismo parametrico,
polimorfismo su misura e
polimorfismo cosa?
3 semplici
Data Structure
Seq Set Map
package scala.collection
Seq Set Map
package scala.collection
v v v v
Seq Set Map
package scala.collection
valueA
valueB
valueC
Seq Set Map
package scala.collection
key value
key value
key value
Seq Set Map
package scala.collection
base trait per implementazioni
mutable e immutable
package scala.collection
Seq Set Map
package scala.collection.mutable
ArrayBuffer HashSet HashMap
package scala.collection.mutable
ArrayBuffer HashSet HashMap
“Mutability is an optimisation – perhaps premature”
package scala.collection
Seq Set
package scala.collection.immutable
Map
List HashSet HashMap
package scala.collection.mutable
ArrayBuffer HashSet HashMap
http://docs.scala-lang.org/tutorials/FAQ/collections.html
package scala.collection.immutable
List HashSet HashMap
Immutabilità implica:
- equational reasoning
- sharing with referential integrity
- thread safety
- …
“When you get used to immutable data,
ya kinda forget how to use mutable data
in a sensible way.” – Jessica Kerr
package scala.collection
Traversable
Seq Set Map
Quali metodi offre
Traversable?
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Metodi offerti da Traversable
def map[B](f: A => B)
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Metodi offerti da Traversable
def foreach(f: (A) => Unit): Unit
def map[B](f: A => B)
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Metodi offerti da Traversable
def foldLeft[B](z: B)(f: (B, A) => B): B
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Metodi offerti da Traversable
def foldLeft[B](z: B)(f: (B, A) => B): B
def /:[B](z: B)(op: (B, A) => B): B = foldLeft(z)(op)
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Metodi offerti da Traversable
def foldLeft[B](z: B)(f: (B, A) => B): B
def /:[B](z: B)(op: (B, A) => B): B = foldLeft(z)(op)
Theorems for free! – http://ttic.uchicago.edu/~dreyer/course/papers/wadler.pdf
Parametricity – http://yowconference.com.au/slides/yowlambdajam2014/Morris-ParametricityTypesAreDocumentation.pdf
Unire Strutture Dati
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
def ++[B](that: Traversable[B]): Traversable[B]
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
def ++[B](that: Traversable[B]): Traversable[B]
Traversable Seq List
List(1, 2, 3) ++ List(4, 5, 6) == ???
List(1, 2, 3) ++ List(4, 5, 6) == List(1, 2, 3, 4, 5, 6)
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
def ++[B](that: Traversable[B]): Traversable[B]
Traversable Set HashSet
Set(1, 2, 3) ++ Set(4, 5, 6) == ???
Set(1, 2, 3) ++ Set(4, 5, 6) == Set(5, 1, 6, 2, 3, 4)
Set(1, 2, 3) ++ Set(4, 5, 6) == Set(5, 1, 6, 2, 3, 4)
Set(1, 2) ++ Set(2, 3) == Set(1, 2, 3)
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
def ++[B](that: Traversable[B]): Traversable[B]
Traversable Map HashMap
Map("a" -> 1) ++ Map("b" -> 2) == Map("a" -> 1, "b" -> 2)
E in casi più complessi?
Map[String, Set[Int]]
Map[String, Set[Int]]
“a” Set(1, 2)
“key b” Set(4, 7, 5)
“key c” Set(9, 4)
“a” Set(2, 3)
“key c” Set(3, 4)
“key d” Set(5, 6)
Map("a" -> Set(1, 2)) ++ Map("a" -> Set(2, 3)) == ???
Map("a" -> Set(1, 2)) ++ Map("a" -> Set(2, 3)) == ???
1: Map("a" -> Set(1, 2))
2: Map("a" -> Set(1, 2, 3))
3: Map("a" -> Set(2, 3))
4: RuntimeException
5: Compiler Error
Map("a" -> Set(1, 2)) ++ Map("a" -> Set(2, 3)) == ???
1:
2:
3: Map("a" -> Set(2, 3))
4:
5:
Map("a" -> Set(1, 2)) ??? Map("a" -> Set(2, 3))
Map("a" -> Set(1, 2, 3))
Metodi offerti da Traversable
isEmpty
size
hasDefiniteSize
++
map
flatMap
filter
remove
partition
groupBy
foreach
reduceRightOpt
head
headOption
tail
last
lastOption
init
take
drop
slice
takeWhile
forall
exists
count
find
foldLeft
/:
foldRight
:
reduceLeft
reduceLeftOpt
reduceRight
dropWhile
span
splitAt
toArray
toList
toIterable
toSeq
toStream
sortWith
mkString
toString
Seq Set Map
✔ ✔ ✘
Quando vuoi
un lavoro fatto bene…
“a” Set(1, 2) “a” Set(2, 3)
“a” Set(1, 2) ++ Set(2, 3)
“a” Set(1, 2) “a” Set(2, 3)
“a” Set(1, 2, 3)
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]]
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]] = {
mb foreach { case (k, v) =>
???
}
}
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]] = {
val result = mutable.Map() ++ ma
mb foreach { case (k, v) =>
???
}
result.toMap
}
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]] = {
val result = mutable.Map() ++ ma
mb foreach { case (k, v) =>
if (result.contains(k))
result += k -> (result(k) ++ v)
else
result += k -> v
}
result.toMap
}
Implementazione
con Map immutable
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]] = {
}
(ma /: mb) { case (result,(k, v)) =>
if (result.contains(k))
result + (k -> (result(k) ++ v))
else
result + (k -> v)
}
val result = mutable.Map() ++ ma
mb foreach { case (k, v) =>
if (result.contains(k))
result += k -> (result(k) ++ v)
else
result += k -> v
}
result.toMap
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]] = {
(ma /: mb) { case (result,(k, v)) =>
if (result.contains(k))
result + (k -> (result(k) ++ v))
else
result + (k -> v)
}
}
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
}
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
// .map(_ ++ v).getOrElse(v)
}}
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
// .map(_ ++ v).getOrElse(v)
// .some(_ ++ v).none(v)
}}
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
// .map(_ ++ v).getOrElse(v)
// .some(_ ++ v).none(v)
// .fold(v)(_ ++ v)
}}
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
// .map(_ ++ v).getOrElse(v)
// .some(_ ++ v).none(v)
// .fold(v)(_ ++ v)
// .cata(_ ++ v, v)
}}
“FP with Bananas, Lenses, Envelopes and Barbed Wire” – http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.41.125
http://en.wikipedia.org/wiki/Catamorphism
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
// .map(_ ++ v).getOrElse(v)
// .some(_ ++ v).none(v)
// .fold(v)(_ ++ v)
// .cata(_ ++ v, v)
}}
http://stackoverflow.com/questions/5328007/why-doesnt-option-have-a-fold-method
(ma /: mb) { case (result,(k, v)) =>
result + (k -> {
result.get(k) match {
case Some(vr) => vr ++ v
case None => v
}
// .map(_ ++ v).getOrElse(v)
// .some(_ ++ v).none(v)
// .fold(v)(_ ++ v)
// .cata(_ ++ v, v)
}}
http://stackoverflow.com/questions/5328007/why-doesnt-option-have-a-fold-method
(ma /: mb) { case (result, (k, v)) =>
result + (k -> result.get(k).cata(_ ++ v, v))
}
mb foreach { case (k, v) =>
result += (k -> result.get(k).cata(_ ++ v, v))
}
(ma /: mb) { case (result, (k, v)) =>
result + (k -> result.get(k).cata(_ ++ v, v))
}
mb foreach { case (k, v) =>
result += (k -> result.get(k).cata(_ ++ v, v))
}
E se volessimo unire due
mappe con type diversi?
Map[String, Set[Int]] Map[String, Map[Int, Set[Int]]]
Map[String, Set[Int]] Map[String, Map[Int, Set[Int]]]
def blend(ma: Map[String, Set[Int]],
mb: Map[String, Set[Int]])
: Map[String, Set[Int]] = ???
def blend(ma: Map[String, Map[Int, Set[Int]]],
mb: Map[String, Map[Int, Set[Int]]])
: Map[String, Map[Int, Set[Int]]] = ???
Map[String, Set[Int]] Map[String, Map[Int, Set[Int]]]
(ma /: mb) { case (result, (k, v)) =>
result + (k -> result.get(k).cata(_ ++ v, v))
}
(ma /: mb) { case (result, (k, v)) =>
result + ??? // { ??? => { ??? } }
}
Map[String, Set[Int]] Map[String, Map[Int, Set[Int]]]
trait Blendable[A] {
def blend(ma: A, mb: A): A
}
Map[String, Set[Int]] Map[String, Map[Int, Set[Int]]]
trait Blendable[A] {
def blend(ma: A, mb: A): A
}
new Blendable[...] {
def blend(ma: ..., mb: ...): ... = ???
}
Map[String, Set[Int]] Map[String, Map[Int, Set[Int]]]
trait Blendable[A] {
def blend(ma: A, mb: A): A
}
new Blendable[...] {
def blend(ma: ..., mb: ...): ... = ???
}
x10 Developer
Cosa intendiamo
veramente per “blend”
List utilizzando l’operatore binario ++
Set utilizzando l’operatore binario ++
List(1, 2, 3) ++ List(4, 5, 6) == List(1, 2, 3, 4, 5, 6)
Set(1, 2) ++ Set(2, 3) == Set(1, 2, 3)
(List, ++)
(Set, ++)
(1 blend 2) == ???
(List, ++)
(Set, ++)
(Int, +)
(1 blend 2) == 1 + 2 == 3
(List, ++)
(Set, ++)
(Int, +)
(String, +)("ab" blend "cd") == ("ab" + "cd") == "abcd"
(List, ++)
(Set, ++)
(Int, +)
(String, +)
(Map[...], Blendable[...].blend)
Blendable[Map[String, Set[Int]]].blend(ma, mb)
Cosa potrebbe
consigliarci un
Matematico?
WARNING
Algebra ahead
☣
https://it.wikipedia.org/wiki/Semigruppo
“Un semigruppo è un insieme S munito di una
operazione binaria associativa m: S × S → S”
https://it.wikipedia.org/wiki/Propriet%C3%A0_di_chiusura
Proprietà di chiusura ≝ ∀a, b ∈ T : a∙b ∈ T
Per ogni a, b in T, il risultato dell’operazione a⋅b è in T:
trait Semigroup[T] {
def op(a: T, b: T): T
}
def op(a: Boolean, b: Boolean): Boolean
def op(a: Int, b: Int): Boolean
✓
✘
https://it.wikipedia.org/wiki/Associativit%C3%A0
Legge Associativa ≝ ∀a, b, c ∈ T : (a∙b)∙c = a∙(b∙c)
Ogni a, b e c in T soddisfano (a∙b)∙c = a∙(b∙c)
trait Semigroup[T] {
def op(a: T, b: T): T
}
((a op b) op c) == (a op (b op c))
https://it.wikipedia.org/wiki/Semigruppo
“Un semigruppo è un insieme S munito di una
operazione binaria associativa m: S × S → S”
Scalaz e Semigruppi
import scalaz.std.set._
implicit def setSemigroup[A]:Semigroup[Set[A]] =
new Semigroup[Set[A]] {
def append(f1: Set[A], f2: => Set[A]) = f1 ++ f2
}
implicit def setSemigroup[A]:Semigroup[Set[A]] =
new Semigroup[Set[A]] {
def append(f1: Set[A], f2: => Set[A]) = f1 ++ f2
}
op
import scalaz.syntax.semigroup._
import scalaz.std.list._
List(1, 2) |+| List(3, 4)
res: List[Int] = List(1, 2, 3, 4)
import scalaz.syntax.semigroup._
import scalaz.std.set._
Set(1, 2) |+| Set(2, 3)
res: Set[Int] = Set(1, 2, 3)
import scalaz.syntax.semigroup._
import scalaz.std.anyVal._
1 |+| 2 |+| 3
res: Int = 6
import scalaz.syntax.semigroup._
import scalaz.std.string._
"a" |+| "b" |+| "c"
res: String = "abc"
/**
* A semigroup in type F must satisfy two laws:
*
* - '''closure''': `∀ a, b in F, append(a, b)` is also in `F`.
* - '''associativity''': `∀ a, b, c` in `F`, the equation
* `append(append(a, b), c) = append(a, append(b , c))` holds.
*/
trait SemigroupLaw {
def associative(f1: F, f2: F, f3: F)(implicit F: Equal[F]): Boolean =
F.equal(append(f1, append(f2, f3)), append(append(f1, f2), f3))
}
import scalaz.scalacheck.ScalazProperties._
import scalaz.std.anyVal._
semigroup.laws[Int].check
+ semigroup.associative: OK, passed 100 tests.
semigroup.laws[String].check
semigroup.laws[Set[Int]].check
semigroup.laws[List[String]].check
semigroup.laws[Map[Int, Int]].check
+ semigroup.associative: OK, passed 100 tests.
+ semigroup.associative: OK, passed 100 tests.
+ semigroup.associative: OK, passed 100 tests.
+ semigroup.associative: OK, passed 100 tests.
La nostra Map[_, Set[Int]]
è un semigruppo?
Map("a" -> 1, "b" -> 4) |+| Map("a" -> 2)
res: Map[…] = Map(a -> 3, b -> 4)
“Some data structures form interesting semigroups as long as
the types of the elements they contain also form semigroups.”
import scalaz.scalacheck.ScalazProperties._
semigroup.laws[Map[String, Set[Int]]].check
+ semigroup.associative: OK, passed 100 tests. ✓
Map("a" -> Set(1, 2)) |+| Map("a" -> Set(2, 3))
res: Map[…] = Map(a -> Set(1, 2, 3))
“adattato” da: Functional Programming in Scala - Part 3 - Chapter 10 Monoids
✓
Ma nel mio codebase
non ho Map così semplici…
Map("a" ->
Map("aa" ->
Map("aaa" ->
Map("aaaa" -> List(1, 3),
"aaab" -> List(2, 4))))) |+| Map("a" ->
Map("aa" ->
Map("aaa" ->
Map("aaaa" -> List(5, 7),
"aaab" -> List(6, 8)))))
Map(a->Map(aa->Map(aaa->Map(aaaa->List(1, 3, 5, 7), aaab->List(2, 4, 6, 8)))))
Benchmarking
“Experience indicates that nearly everybody
has the wrong idea about the real bottlenecks
in his programs” – Donald Knuth
Computer programming as an art (1974) – http://dl.acm.org/citation.cfm?id=361612
Map[String, Set[Int]]
Settembre 2015
@filippovitale
$ tail -f domande

Polimorfismo cosa?