Scala Back to Basics: Type Classes

Back to Basics: Type Classes
Tomer Gabel, Wix
August, 2014

THE EXPRESSION PROBLEM
“Define a datatype by cases, where one can add new cases to
the datatype and new functions over the datatype, without
recompiling existing code, and while retaining static type
safety (e.g., no casts).”
-- Philip Wadler

Let’s Build a Calculator
• Operators:
– Addition (+)
– Subtraction (-)
– Multiplication (*)
– Division (/)
– Remainder (%)
• Types:
– Integers (32-bit signed)
– Longs (64-bit signed)
– Floats (32-bit IEEE 754)
– Longs (64-bit IEEE 754)

Two Views of the Same Problem
Pattern Matching
sealed trait Operand
case class Int32(value: Int) extends Operand
case class Real32(value: Float) extends Operand
// ...
def addition[T <: Operand](left: T, right: T): T =
(left, right) match {
case (Int32 (l), Int32 (r)) => Int32 (l + r)
case (Real32(l), Real32(r)) => Real32(l + r)
// ...
}
Object Oriented
sealed trait Operand
case class Int32(value: Int) extends Operand {
def add(other: Int) = Int32(value + other)
def subtract(other: Int) = Int32(value - other)
// ...
}
case class Real32(value: Float) extends Operand {
def add(other: Float) = Real32(value + other)
def subtract(other: Float) = Real32(value - other)
// ...
}

What’s a Type Class?
• A type class:
– Enables ad-hoc polymorphism
– Statically typed (i.e. type-safe)
– Borrowed from Haskell
• Solves the expression problem:
– Behavior can be extended
– … at compile-time
– ... after the fact
– … without changing/recompiling
existing code

Example #1: Equality
• Scala inherits legacy aspects of Java
– This includes AnyRef.equals:
def equals( other: AnyRef ): Boolean
– So the following compiles:
3.14159265359 == "pi" // Evaluates to false
• What if we wanted to implement type-safe equality?
– Let’s define a type-safe isEqual function:
isEqual( 3.14159265359, "pi” ) // Does not compile!

What’s in a Type Class?
• Three components are
required:
– A signature
– Implementations for
supported types
– A function that requires
a type class This is where things get hairy.

Slight Digression
• A method in Scala can have multiple parameter lists:
def someMethod( x: Int )( y: String )( z: Double ): Unit = {
println( s"x=$x, y=$y, z=$z" )
}
scala> someMethod( 10 )( "abc" )( scala.math.Pi )
x=10, y=abc, z=3.141592653589793
• There are multiple uses for this, but the most important is…

Scala Implicits
• The last parameter list of a method can be marked implicit
• Implicit parameters are filled in by the compiler
– In effect, you require evidence of the compiler
– … such as the existence of a type class in scope
– You can also specify parameters explicitly, if needed

Putting it together
• Let’s define our type class:
trait Equality[ L, R ] {
def equals( left: L, right: R ): Boolean
}
• … and our isEqual function:
def isEqual[ L, R ]( left: L, right: R )
( implicit ev: Equality[ L, R ] ): Boolean =
ev.equals( left, right )
This is where the magic happens

Still missing something!
• We have no implementations of the Equality trait, so nothing works!
scala> isEqual( 3, 3 )
<console>:10: error: could not find implicit value for parameter ev:
Equality[Int,Int]
• We need to implement Equality[ T, T ]:
implicit def sameTypeEquality[ T ] = new Equality[ T, T ] {
def equals( left: T, right: T ) = left.equals( right )
}
• And now it works:
scala> isEqual( 3, 3 )
res1: Boolean = true

Ad-hoc Polymorphism
• Now we’ve met our original goal:
scala> isEqual( 3.14159265359, "pi" )
<console>:11: error: could not find implicit value for parameter ev: Equality[Double,String]
• But what if we wanted to equate doubles and strings?
• Well then, let’s add another implementation!
implicit object DoubleEqualsString extends Equality[ Double, String ] {
def equals( left: Double, right: String ) = left.toString == right
}
• Et voila, no recompilation or code changes needed:
scala> isEqual( 3.14159265359, "pi" )
res5: Boolean = false

Example #2: Sort Me, Maybe
• Let’s implement a sort
function (e.g. bubble sort)
• With one caveat:
– It should operate on any type
– … for which an ordering exists
• Obviously, we’ll use type
classes!

Possible Solution
trait Ordering[ T ] { def isLessThan( left: T, right: T ): Boolean }
def sort[ T ]( items: Seq[ T ] )( implicit ord: Ordering[ T ] ): Seq[ T ] = {
val buffer = mutable.ArrayBuffer( items:_* )
for ( i <- 0 until items.size;
j <- ( i + 1 ) until items.size )
if ( ord.isLessThan( buffer( j ), buffer( i ) ) ) {
val temp = buffer( i )
buffer( i ) = buffer( j )
buffer( j ) = temp
}
buffer
}

Possible Solution, cont.
• Sample implementation for integers:
implicit object IntOrdering extends Ordering[ Int ] {
def isLessThan( left: Int, right: Int ) = left < right
}
val numbers = Seq( 4, 1, 10, 8, 14, 2 )
Assert( sort( numbers ) == Seq( 1, 2, 4, 8, 10, 14 ) )

Possible Solution, cont.
• Sample implementation for a domain entity:
case class Person( name: String, age: Int )
implicit object PersonOrdering extends Ordering[ Person ] {
def isLessThan( left: Person, right: Person ) =
left.age < right.age
}
val haim = Person( "Haim", 12 )
val dafna = Person( "Dafna", 20 )
val ofer = Person( "Ofer", 1 )
assert( sort( Seq( haim, dafna, ofer ) ) ==
Seq( ofer, haim, dafna ) )

Implicit Search Order
Current Scope
• Defined implicits
• Explicit imports
• Wildcard imports
Companion
• … of T
• … of supertypes of T
Outer Scope
• Enclosing class

Example #3: Server Pipeline
• REST is good, but annoying to write. Let’s simplify:
case class DTO( message: String )
class MyServlet extends NicerHttpServlet {
private val counter = new AtomicInteger( 0 )
get( "/service" ) {
counter.incrementAndGet()
DTO( "hello, world!" )
}
get( "/count" ) {
counter.get()
}
}
Uses return value;
no direct response manipulation

• What’s in a server?
– Routing
– Rendering
– Error handling
• Let’s focus on rendering:
trait ResponseRenderer[ T ] {
def render( value : T,
request : HttpServletRequest,
response: HttpServletResponse ): Unit
}

• A couple of basic renderers:
implicit object StringRenderer extends ResponseRenderer[ String ] {
def render( value: String, request: HttpServletRequest, response: HttpServletResponse ) = {
val w = response.getWriter
try w.write( value )
finally w.close()
}
}
implicit object IntRenderer extends ResponseRenderer[ Int ] {
def render( value: Int, request: HttpServletRequest, response: HttpServletResponse ) =
implicitly[ ResponseRenderer[ String ] ].render( value.toString, request, response )
}

• Putting it together:
trait NicerHttpServlet extends HttpServlet {
private trait Handler {
type Response
def result: Response
def renderer: ResponseRenderer[ Response ]
}
private var handlers: Map[ String, Handler ] = Map.empty
protected def get[ T : ResponseRenderer ]( url: String )( thunk: => T ) =
handlers += url -> new Handler {
type Response = T
def result = thunk
def renderer = implicitly[ ResponseRenderer[ T ] ]
}

• And finally:
override def doGet( req: HttpServletRequest, resp: HttpServletResponse ) =
handlers.get( req.getRequestURI ) match {
case None =>
resp.sendError( HttpServletResponse.SC_NOT_FOUND )
case Some( handler ) =>
try handler.renderer.render( handler.result, req, resp )
catch { case e: Exception =>
resp.sendError( HttpServletResponse.SC_INTERNAL_SERVER_ERROR )
}
}

Example #4: JSON Serialization
• Assume we already have a good model for JSON
• How do we add type-safe serialization?
JsonValue
JsonObject JsonArray
JsonBoolean JsonInt
JsonDouble JsonNull
JsonField

• Let’s start with a typeclass:
trait JsonSerializer[ T ] {
def serialize( value: T ): JsonValue
def deserialize( value: JsonValue ): T
}
• And the corresponding library signature:
def serialize[ T ]( instance: T )( implicit ser: JsonSerializer[ T ] ) =
ser.serialize( instance )
def deserialize[ T ]( json: JsonValue )( implicit ser: JsonSerializer[ T ] ) =
ser.deserialize( json )

• Define a few basic serializers…
implicit object BooleanSerializer extends JsonSerializer[ Boolean ] {
def serialize( value: Boolean ) = JsonBoolean( value )
def deserialize( value: JsonValue ) = value match {
case JsonBoolean( bool ) => bool
case other => error( other )
}
}
implicit object StringSerializer extends JsonSerializer[ String ] {
def serialize( value: String ) = JsonString( value )
case JsonString( string ) => string
case other => error( other )
}
}

• We can also handle nested structures
– The compiler resolves typeclasses recursively!
• For example, Option[ T ] :
implicit def optionSerializer[ T ]( implicit ser: JsonSerializer[ T ] ) =
new JsonSerializer[ Option[ T ] ] {
def serialize( value: Option[ T ] ) =
value map ser.serialize getOrElse JsonNull
case JsonNull => None
case other => Some( ser.deserialize( other ) )
}
}
Require a serializer for T
… and delegate to it

• What about an arbitrary type?
case class Person( name: String, surname: String, age: Int )
implicit object PersonSerializer extends JsonSerializer[ Person ] {
def serialize( value: Person ) = JsonObject(
JsonField( "name", serialize( value.name ) ),
JsonField( "surname", serialize( value.surname ) ),
JsonField( "age", serialize( value.age ) )
)
case obj: JsonObject =>
Person(
name = deserialize[ String ]( obj "name" ),
surname = deserialize[ String ]( obj "surname" ),
age = deserialize[ Int ]( obj "age" )
)
case _ => error( value )
}
}

Summary
• We added serialization for Person after the fact without…
– … modifying the serialization framework
– … modifying the domain object
• We did not compromise:
– … type safety or performance
– … modularity or encapsulation
• This applies everywhere!
clients of either are unaffected!

… and we’re done
• Thank you for your time!
• Questions/comments?
– tomer@tomergabel.com
– @tomerg
– http://www.tomergabel.com
• Code samples:
– http://git.io/aWc9eQ

Scala Back to Basics: Type Classes

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