Scala variable argument list with call-by-name possible? - scala

I've got some code like this:
def foo (s: => Any) = println(s)
But when I want to transform this to an argument list with variable length, it won't compile anymore (tested on Scala 2.10.0-RC2):
def foo (s: => Any*) = println(s)
What must I write, that it works like this?

You have to use zero-argument functions instead. If you want, you can
implicit def byname_to_noarg[A](a: => A) = () => a
and then
def foo(s: (() => Any)*) = s.foreach(a => println(a()))
scala> foo("fish", Some(7), {println("This still happens first"); true })
This still happens first
fish
Some(7)
true

There is an issue:
https://issues.scala-lang.org/browse/SI-5787
For the accepted answer, to recover the desired behavior:
object Test {
import scala.language.implicitConversions
implicit def byname_to_noarg[A](a: => A) = () => a
implicit class CBN[A](block: => A) {
def cbn: A = block
}
//def foo(s: (() => Any)*) = s.foreach(a => println(a()))
def foo(s: (() => Any)*) = println(s(1)())
def goo(a: =>Any, b: =>Any, c: =>Any) = println(b)
def main(args: Array[String]) {
foo("fish", Some(7), {println("This still happens first"); true })
goo("fish", Some(7), {println("This used to happens first"); true })
foo("fish", Some(7), {println("This used to happens first"); true }.cbn)
}
}
Excuse the lolcats grammar.

Related

Conditional Behavior With Free Monads

I'm following the tutorial here: http://typelevel.org/cats/datatypes/freemonad.html and trying to modify it to work with a cache in front of the key value store. This is what I've come up with so far but I'm getting a compiler error with valueGetOperation. I understand why I get the compile error, I just don't understand how to work around it. What's the best practice for conditional behavior when using a free monad?
import cats.data.Coproduct
import cats.free.{Free, Inject}
object KvStore {
sealed trait KvOp[A]
case class Get[T](key: String) extends KvOp[Option[T]]
case class Put[T](key: String, value: T) extends KvOp[Unit]
case class Delete[T](key: String) extends KvOp[Unit]
}
object CacheStore {
sealed trait CacheOp[A]
case class Get[T](key: String) extends CacheOp[Option[T]]
case class Put[T](key: String, value: T) extends CacheOp[Unit]
case class Delete[T](key: String) extends CacheOp[Unit]
}
type WriteThruCache[A] = Coproduct[KvStore.KvOp, CacheStore.CacheOp, A]
class KvOps[F[_]](implicit I: Inject[KvStore.KvOp, F]) {
import KvStore._
def get[T](key: String): Free[F, Option[T]] = Free.inject[KvOp, F](Get(key))
def put[T](key: String, value: T): Free[F, Unit] = Free.inject[KvOp, F](Put(key, value))
def delete[T](key: String): Free[F, Unit] = Free.inject[KvOp, F](Delete(key))
}
object KvOps {
implicit def kvOps[F[_]](implicit I: Inject[KvStore.KvOp, F]): KvOps[F] = new KvOps[F]
}
class CacheOps[F[_]](implicit I: Inject[CacheStore.CacheOp, F]) {
import CacheStore._
def get[T](key: String): Free[F, Option[T]] = Free.inject[CacheOp, F](Get(key))
def put[T](key: String, value: T): Free[F, Unit] = Free.inject[CacheOp, F](Put(key, value))
def delete[T](key: String): Free[F, Unit] = Free.inject[CacheOp, F](Delete(key))
}
object CacheOps {
implicit def cacheOps[F[_]](implicit I: Inject[CacheStore.CacheOp, F]): CacheOps[F] = new CacheOps[F]
}
def valueWriteOperation[T](implicit Kv: KvOps[WriteThruCache], Cache: CacheOps[WriteThruCache]): ((String, T) => Free[WriteThruCache, Unit]) = {
(key: String, value: T) =>
for {
_ <- Kv.put(key, value)
_ <- Cache.put(key, value)
} yield ()
}
// This is where I'm stuck
// desired behavior: If the value isn't in the cache, load it from the kv store and put it in the cache
def valueGetOperation[T](implicit Kv: KvOps[WriteThruCache], Cache: CacheOps[WriteThruCache]): ((String) => Free[WriteThruCache, Option[T]]) = {
(key: String) =>
for {
cacheOption <- Cache.get[T](key)
kvOption <- Kv.get[T](key) if cacheOption.isEmpty // value withFilter is not a member of cats.free.Free[A$A39.this.WriteThruCache,Option[T]]
} yield cacheOption.orElse(kvOption)
}
As you know in for comprehension, when you use if it is desugared by compiler to calling withFilter method, and if it's not accessible it falls back to filter method. If they are not implemented you will receive compiler error.
However you can simply use if else!
for {
booleanValue <- myfreeAlbebra.checkCondidtion(arg1, arg2)
valueToReturn <- if (booleanValue) {
myfreeAlbebra.someValue
} else {
myfreeAlbebra.someOtherValue
}
} yield valueToReturn
alternatively you can do something like:
for {
booleanValue <- myfreeAlbebra.checkCondidtion(arg1, arg2)
valueToReturnOpt <- myfreeAlbebra.someValue
fallbackValue <- myfreeAlbebra.someOtherValue
} yield valueToReturnOpt.getOrElse(fallbackValue)
The formar one will assign value to valueToReturn depending on booleanValue. As such only one branch will be interpreted. The later will evaluate both values and return one of them depending on whether or not valueToReturnOpt will be empty.
Personally I would try something like:
def valueGetOperation[T](implicit Kv: KvOps[WriteThruCache], Cache: CacheOps[WriteThruCache]): ((String) => Free[WriteThruCache, Option[T]]) = {
(key: String) =>
for {
cacheOption <- Cache.get[T](key)
returnedValue <- if (cacheOption.isEmpty) Cache.get[T](key) else Kv.get[T](key)
} yield returnedValue
}
Following Mateusz' suggestions, this is what I came up with:
def withFallback[A[_], T](loadedValue: Option[T], fallback: => Free[A, Option[T]]): Free[A, Option[T]] = {
if(loadedValue.isDefined) {
Free.pure[A, Option[T]](loadedValue)
} else {
fallback
}
}
def valueGetOperation[T](implicit Kv: KvOps[WriteThruCache], Cache: CacheOps[WriteThruCache]): ((String) => Free[WriteThruCache, Option[T]]) = {
(key: String) =>
for {
cachedOption <- Cache.get[T](key)
actualValue <- withFallback[WriteThruCache, T](cachedOption, fallback = Kv.get[T](key))
} yield actualValue
}
If there's a standard construct to implement withFallback I'd be glad to know about it.
You could also use OptionT#orElse.
import cats.data.OptionT
type KV[A] = Free[WriteThruCache, A]
def valueGetOperation[T](
implicit
Kv: KvOps[WriteThruCache],
Cache: CacheOps[WriteThruCache]
): String => KV[Option[T]] =
key => OptionT[KV, T](Cache.get[T](key)).orElse(OptionT[KV, T](Kv.get[T](key))).value
Or OptionT#orElseF :
def valueGetOperation[T](
implicit
Kv: KvOps[WriteThruCache],
Cache: CacheOps[WriteThruCache]
): String => KV[Option[T]] =
key => OptionT[KV, T](Cache.get[T](key)).orElseF(Kv.get[T](key)).value
Note that with the -Ypartial-unification flag in Scala 2.12 you don't need the KV type alias and you can write OptionT(...) instead of OptionT[KV, T](...).

scala found Unit required Unit.type

I have the following:
object T {
abstract class First {
def doSomething= (s:String) => Unit
}
class Second extends First {
override def doSomething = {
(s:String) => ()
}
}
def main(args: Array[String]): Unit = {
new Second().doSomething
}
}
but this fails to compile with the error:
Error:(8, 21) type mismatch;
found : Unit
required: Unit.type
(s:String) => ()
Why isn't the override from class Second valid? How could I make it work?
The problem is that (s:String) => Unit is returning Unit.type - instead change it to (s:String) => () (or to a method if you didn't mean to be returning Function1[String, Unit] from your method.)
To put it another way:
def doSomething = (s:String) => Unit
is really:
def doSomething: (String) => Unit.type =
// A function that takes a string and returns the companion type of Unit
(s: String) => Unit
While what you probably wanted was:
def doSomething: (String) => Unit =
// A function that takes a string and returns Unit
(s: String) => ()
Or maybe:
def doSomething(s: String): Unit
// An abstract method that takes a string and returns nothing.
(s:String) => Unit returns Unit.type ie it returns the type rather than a value of that type. You want to do (s:String) => (), which will return a value, whose type is Unit.
' trait Processor0 {
def process = (x:String) => ()
}
case class Processor2() extends Processor0 {
override def process = (x:String) => println("Processor2 x :" + x)
}
case class Processor3() extends Processor0 {
override def process = (x:String) => println("Processor3 x :" + x)
}
object DemoProc2 {
def main( args : Array[String]):Unit ={
val s:String = "yes"
val myFuncs: Map[String,(String) => () ]= Map( //Error 1. '=>' expected but ']' found. 2.identifier expected but ';' found.
"string2" -> (() => ProcessorTwo().process(s)),
"string3" -> (() => ProcessorThree().process(s))
)
myFuncs.values.foreach(v => v());
}
} '

Can function parameter be set before it's invoked?

Here I create a List of objects where each element is of type (String , () => Unit)
case class FunctionDesc(label: String, fun: () => Unit)
def f() = {
println("in f")
}
val functionList = List[FunctionDesc](FunctionDesc("a1", f), FunctionDesc("a2", f))
functionList.foreach(f => f.fun())
This works fine but if I want to modify the function List to contain a parameter then have to decide what the parameter value
should be when the function is being implemented :
case class FunctionDesc2(label: String, fun: (String) => Unit)
def f2(str: String) = {
println("in f2")
}
def f3(str: String) = {
println("in f3")
}
val functionList2 = List[FunctionDesc2](FunctionDesc2("a3", f2), FunctionDesc2("a4", f3))
functionList2.foreach(f => f.fun("param value"))
Can decide what the function parameter type should before it's invoked ?
So instead of
val functionList2 = List[FunctionDesc2](FunctionDesc2("a3", f2), FunctionDesc2("a4", f3))
use something like :
val functionList2 = List[FunctionDesc2](FunctionDesc2("a3", f2("f5")), FunctionDesc2("a4", f2("f6"))
You can achieve this by making f2 and f3 return a function. Here's a simplified version:
case class FunctionDesc2(label: String, fun: (String) => Unit)
// `f` closes over `str` and returns a function from `String` to `Unit`
def f(str: String) = (s: String) => println(s"$str: $s")
val functionList = List(FunctionDesc2("a", f("foo")), FunctionDesc2("b", f("bar")))
functionList.foreach(_.fun("value"))
// foo: value
// bar: value
You can use currying:
scala> case class FunctionDesc2(label: String, fun: () => Unit)
defined class FunctionDesc2
scala> def f2(str: String)(): Unit = {
| println("in f2")
| }
f2: (str: String)()Unit
scala> def f3(str: String)(): Unit = {
| println("in f3")
| }
f3: (str: String)()Unit
scala> val functionList2 = List[FunctionDesc2](FunctionDesc2("a3", f2("f5")), FunctionDesc2("a4", f3("f6")))
functionList2: List[FunctionDesc2] = List(FunctionDesc2(a3,<function0>), FunctionDesc2(a4,<function0>))
scala> functionList2.foreach(desc => desc.fun())
in f2
in f3

Safe partial function

I want to convert a partial function into a "safe" partial function, i.e. PartialFunction[T,R] into a PartialFunction[T,Try[R]], is there any better way than the following implementation?
def safe[T,R](pf:PartialFunction[T,R]):PartialFunction[T,Try[R]]=new PartialFunction[T, Try[R]]{
def isDefinedAt(t:T) = pf.isDefinedAt(t)
def apply(t:T) = Try(pf.apply(t))
}
You can define safe using a for expression:
def safe[T,R](pf:PartialFunction[T,R]) = PartialFunction {
(t:T) => for(a <- Try(pf(t))) yield a
}
Or using an implicit class:
object SafeImplicit {
implicit class Safe[T,R](pf: PartialFunction[T,R]){
def safe(t: =>T) = for(a<-Try(pf(t))) yield a
}
}
import SafeImplicit.Safe
def f: PartialFunction[Int, String] = {case 1 => "one"}
def g: PartialFunction[Int, String] = {case 1 => "a".toInt;"one"}
f.safe(1) // Success(one)
f.safe(2) // Failure(scala.MatchError: 2 ...)
f.safe("a".toInt) // Failure(java.lang.NumberFormatException ...)
g.safe(1) // Failure(java.lang.NumberFormatException ...)

how to implement this future/state concept as a monad in scala

I'm trying to implement a container for a match (like in sports) result so that I can create matches between the winners of other matches. This concept is close to what a future monads is as it contains a to be defined value, and also close to a state monad as it hides state change. Being mostly a begginer on the topic I have implemented an initial version in scala that is surely improvable. I added a get method that I'm not sure was a good idea, and so far the only way to create a value would be Unknown(null) which is not as elegant as I'd hoped. What do you think I could do to improve this design?
case class Unknown[T](t : T) {
private var value : Option[T] = Option(t)
private var applicatives: List[T => Unit] = Nil
def set(t: T) {
if (known) {
value = Option(t)
applicatives.foreach(f => f(t))
applicatives = Nil
} else {
throw new IllegalStateException
}
}
def get : T = value.get
def apply(f: T => Unit) = value match {
case Some(x) => f(x);
case None => applicatives ::= f
}
def known = value == None
}
UPDATE: a usage example of the current implementation follows
case class Match(val home: Unknown[Team], val visit: Unknown[Team], val result: Unknown[(Int, Int)]) {
val winner: Unknown[Team] = Unknown(null)
val loser: Unknown[Team] = Unknown(null)
result.apply(result => {
if (result._1 > result._2) {
home.apply(t => winner.set(t))
visit.apply(t => loser.set(t))
} else {
home.apply(t => loser.set(t))
visit.apply(t => winner.set(t))
}
})
}
And a test snippet:
val definedUnplayedMatch = Match(Unknown(Team("A")), Unknown(Team("B")), Unknown(null));
val definedPlayedMatch = Match(Unknown(Team("D")), Unknown(Team("E")), Unknown((1,0)));
val undefinedUnplayedMatch = Match(Unknown(null), Unknown(null), Unknown(null));
definedUnplayedMatch.winner.apply(undefinedUnplayedMatch.home.set(_))
definedPlayedMatch.winner.apply(undefinedUnplayedMatch.visit.set(_))
undefinedUnplayedMatch.result.set((3,1))
definedUnplayedMatch.result.set((2,4))
undefinedUnplayedMatch.winner.get must be equalTo(Team("B"));
undefinedUnplayedMatch.loser.get must be equalTo(Team("D"));
UPDATE - CURRENT IDEA : I haven't had much time to work on this because my laptop broke down, but I though it would be useful to write the monad I have so far for those who are interested:
sealed abstract class Determine[+A] {
def map[B](f: A => B): Determine[B]
def flatMap[B](f: A => Determine[B]): Determine[B]
def filter(p: A => Boolean): Determine[A]
def foreach(b: A => Unit): Unit
}
final case class Known[+A](value: A) extends Determine[A] {
def map[B](f: A => B): Determine[B] = Known(f(value))
def flatMap[B](f: A => Determine[B]): Determine[B] = f(value)
def filter(p: A => Boolean): Determine[A] = if (p(value)) this else Unknown
def foreach(b: A => Unit): Unit = b(value)
}
final case class TBD[A](definer: () => A) extends Determine[A] {
private var value: A = _
def map[B](f: A => B): Determine[B] = {
def newDefiner(): B = {
f(cachedDefiner())
}
TBD[B](newDefiner)
}
def flatMap[B](f: A => Determine[B]): Determine[B] = {
f(cachedDefiner())
}
def filter(p: A => Boolean): Determine[A] = {
if (p(cachedDefiner()))
this
else
Unknown
}
def foreach(b: A => Unit): Unit = {
b(cachedDefiner())
}
private def cachedDefiner(): A = {
if (value == null)
value = definer()
value
}
}
case object Unknown extends Determine[Nothing] {
def map[B](f: Nothing => B): Determine[B] = this
def flatMap[B](f: Nothing => Determine[B]): Determine[B] = this
def filter(p: Nothing => Boolean): Determine[Nothing] = this
def foreach(b: Nothing => Unit): Unit = {}
}
I got rid of the set & get and now the TBD class receives instead a function that will define provide the value or null if still undefined. This idea works great for the map method, but the rest of the methods have subtle bugs.
For a simple approach, you don't need monads, with partial application is enough:
//some utilities
type Score=(Int,Int)
case class MatchResult[Team](winner:Team,loser:Team)
//assume no ties
def playMatch[Team](home:Team,away:Team)(score:Score)=
if (score._1>score._2) MatchResult(home,away)
else MatchResult(away,home)
//defined played match
val dpm= playMatch("D","E")(1,0)
//defined unplayed match, we'll apply the score later
val dum= playMatch("A","B")_
// a function that takes the dum score and applies it
// to get a defined played match from an undefined one
// still is a partial application of match because we don't have the final result yet
val uumWinner= { score:Score => playMatch (dpm.winner,dum(score).winner) _ }
val uumLoser= { score:Score => playMatch (dpm.loser,dum(score).loser) _}
//apply the scores
uumWinner (2,4)(3,1)
uumLoser (2,4)(0,1)
//scala> uumWinner (2,4)(3,1)
//res6: MatchResult[java.lang.String] = MatchResult(D,B)
//scala> uumLoser (2,4)(0,1)
//res7: MatchResult[java.lang.String] = MatchResult(A,E)
This is a starting point, I'm pretty sure it can be further refined. Maybe there we'll find the elusive monad. But I think an applicative functor will be enough.
I'll give another pass later...