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A macro library that converts native imperative syntax to scalaz's monadic expressions
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scala-steward/each
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ThoughtWorks Each is a macro library that converts native imperative syntax toScalaz's monadic expression. See theobject cats inDsl.scala for the similar feature forCats.
There is a macro libraryStateless Future that providesawait for asynchronous programming.await is a mechanism that transform synchronous-like code into asynchronous expressions. C# 5.0, ECMAScript 7 and Python 3.5 also support the mechanism.
Theawait mechanism in Stateless Future is implemented by an algorithm calledCPS transform. When learningscalaz, we found that the same algorithm could be applied for any monadic expression, includingOption monad,IO monad, andFuture monad. So we started this project, Each.
Each is a superset ofawait syntax. Each supports multiple types of monads, whileawait only works withFuture. When we perform a CPS transform for monadic expression with theFuture monad, the use case looks almost the same as theawait syntax inStateless Future. Each is like F#'sComputation Expressions, except Each reuses the normal Scala syntax instead of reinventing new syntax.
For example:
importcom.thoughtworks.each.Monadic._importscalaz.std.scalaFuture._// Returns a Future of the sum of the length of each string in each parameter Future,// without blocking any thread.defconcat(future1:Future[String],future2:Future[String]):Future[Int]= monadic[Future] { future1.each.length+ future2.each.length}
The similar code works for monads other thanFuture:
importcom.thoughtworks.each.Monadic._importscalaz.std.option._defplusOne(intOption:Option[Int])= monadic[Option] { intOption.each+1}assert(plusOne(None)==None)assert(plusOne(Some(15))==Some(16))
importcom.thoughtworks.each.Monadic._importscalaz.std.list._defplusOne(intSeq:List[Int])= monadic[List] { intSeq.each+1}assert(plusOne(Nil)==Nil)assert(plusOne(List(15))==List(16))assert(plusOne(List(15,-2,9))==List(16,-1,10))
libraryDependencies+="com.thoughtworks.each"%%"each"%"latest.release"addCompilerPlugin("org.scalamacros"%"paradise"%"2.1.0" crossCrossVersion.full)
or%%% for Scala.js projects:
libraryDependencies+="com.thoughtworks.each"%%%"each"%"latest.release"addCompilerPlugin("org.scalamacros"%"paradise"%"2.1.0" crossCrossVersion.full)
Note that ThoughtWorks Each requires Scalaz 7.2.x and does not compatible with Scala 7.1.x .
Seehttps://repo1.maven.org/maven2/com/thoughtworks/each/ for a list of available versions.
importcom.thoughtworks.each.Monadic._
Scalaz has providedOption monad, so you just import it.
importcom.thoughtworks.each.Monadic._importscalaz.std.option._
Please import other monad instances if you need other monads.
importcom.thoughtworks.each.Monadic._importscalaz.std.option._valresult:Option[String]= monadic[Option] {"Hello, Each!"}
importcom.thoughtworks.each.Monadic._importscalaz.std.option._valname=Option("Each")valresult:Option[String]= monadic[Option] {"Hello,"+ name.each+"!"}
monadic blocks do not supporttry,catch andfinally. If you want these expressions, usethrowableMonadic orcatchIoMonadic instead, for example:
varcount=0valio= catchIoMonadic[IO] { count+=1// Evaluates immediatelyval_=IO(()).each// Pauses until io.unsafePerformIO()try { count+=1 (null:Array[Int])(0)// Throws a NullPointerException }catch {casee:NullPointerException=> { count+=1100 } }finally { count+=1 }}assertEquals(1, count)assertEquals(100, io.unsafePerformIO())assertEquals(4, count)
Note thatcatchIoMonadic requires an implicit parameterscalaz.effect.MonadCatchIO[F] instead ofMonad[F].scalaz.effect.MonadCatchIO[F] is only provided forscalaz.effect.IO by default.
Each supports.each magic in afor loop on any instances that supportFoldable type class. For example, you couldimport scalaz.std.list._ to enable theFoldable type class forList.
importcom.thoughtworks.each.Monadic._importscalaz.std.list._importscalaz.std.option._valn=Some(10)@monadic[Option]valresult= {varcount=1for (i<-List(300,20)) { count+= i* n.each } count}Assert.assertEquals(Some(3201), result)
Note that you need to use@monadic[Option] annotation instead ofmonadic[Option] block to in order to enable thefor loop syntax.
Each also supports.each magic in afor comprehension on any instances that supportTraverse andMonadPlus type class.
importcom.thoughtworks.each.Monadic._importscalaz.std.list._valn=Some(4000)@monadic[Option]valresult= {for { i<-List(300,20) (j, k)<-List(50000->"1111",600000->"yyy")if i> n.each-3900 a= i+ j }yield { a+ n.each* k.length }}Assert.assertEquals(Some(List(66300,612300)), result)
Note that you need to use@monadic[Option] annotation instead ofmonadic[Option] block to in order to enable thefor comprehension syntax.
If acall-by-name parameter of a method call is a monadic expression,Each will transform the monadic expression before the method call. The behavior was discussed at#37.
definnerFailureFuture=Future.failed(newException("foo"))valsomeValue=Some("value")valresult= monadic[Future] { someValue.getOrElse(innerFailureFuture.each)}
result will be a future of failure because the above example equals to
definnerFailureFuture=Future.failed(newException("foo"))valsomeValue=Some("value")valresult= innerFailureFuture.map(someValue.getOrElse)
innerFailureFuture.each is evaluated before being passed togetOrElse method call, even ifgetOrElse accepts a call-by-name parameter.
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A macro library that converts native imperative syntax to scalaz's monadic expressions
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