Strange behavior of && operator (maybe also exists with other logic operation) #37

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1. Atry commented on Nov 2, 2015

   

   Atry

   on Nov 2, 2015

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   Given a method that accepts a call-by-name parameter, it means the method decides when and how many times the parameter evaluates.

   Boolean.&& is one of that kind of methods. Technically, there is no way to pass a monadic expression to methods like that. You could not pass an Option[Boolean] to a => Boolean typed method because type mismatch.

   Instead, the only thing monadic macro can do is evaluating the monadic expression before passing it to a method. The following code is generated by monadic for you example code.

   {
     val monad$macro$1: scalaz.Traverse[Option] with scalaz.MonadPlus[Option] with scalaz.Each[Option] with scalaz.Index[Option] with scalaz.Length[Option] with scalaz.Cozip[Option] with scalaz.Zip[Option] with scalaz.Unzip[Option] with scalaz.Align[Option] with scalaz.IsEmpty[Option] with scalaz.Cobind[Option] with scalaz.Optional[Option]{def point[A](a: => A): Some[A]; def cojoin[A](a: Option[A]): Option[Some[A]]} = scalaz.std.option.optionInstance;
     monad$macro$1.bind(com.thoughtworks.each.Monadic.EachOps[Some, Int](scala.Some.apply[Int](1)).underlying)(((element$macro$2: Int) => {
       val i: Boolean = element$macro$2.<(0);
       monad$macro$1.map({
         scala.this.Predef.println("foo");
         monad$macro$1.map(com.thoughtworks.each.Monadic.EachOps[Some, Int](scala.Some.apply[Int](2)).underlying)(((element$macro$3: Int) => element$macro$3.>(0)))
       })(((element$macro$4: Boolean) => i.&&(element$macro$4)))
     }))
   }

   In brief, there is no way to pass a monadic expression to a method that accept call-by-name parameters and still keep the same behavior as the behavior when passing a direct expression. No way, even if you manually create the monadic expression without using Each library.
2. Atry commented on Nov 2, 2015

   

   Atry

   on Nov 2, 2015

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   In order to achieve the correspond behavior with a monadic expression, you must replace call-by-name parameter => T to a explicit function () => M[T], for example

   // Monadic version of Boolean.&&
   def monadicBooleanAnd[M[_] : Monad](leftHandSide: Boolean, rightHandSide: () => M[Boolean]): M[Boolean] = {
     if (leftHandSide) {
       rightHandSide()
     } else {
       Monad[M].point(false)
     }
   }
   
   monadic[Option] { val i = Some(1).each < 0; monadicBooleanAnd(i, { () => monadic[Option] { println("foo"); Some(2).each > 0 } }).each }
3. jilen commented on Dec 10, 2015

   

   jilen

   on Dec 10, 2015

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   We should point this out in the README file.
   For a trade system like this will lead to terrible behavior

   val decreased = account.decrease(xxx).each
   val recordAdded = decreased && transfer.addRecord(xxx).each
4. Atry commented on Dec 10, 2015

   

   Atry

   on Dec 10, 2015

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   Could you create a Pull Request to the README.md?
5. Atry commented on Jan 8, 2016

   

   Atry

   on Jan 8, 2016

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   How do you think if we translate Boolean.&& to a if expressions to enable behavior like a call-by-name parameter?
6. jilen commented on Jan 8, 2016

   

   jilen

   on Jan 8, 2016

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   That seems reasonable. And it will be better if we can raise an warning for other call-by-name parameter.
7. Atry commented on Mar 20, 2017

   

   Atry

   on Mar 20, 2017

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   I think it's better to disable .each magic in a call-by-value parameters.

   The following code should not compile any more

   monadic[Option] { val i = Some(1).each < 0; i && {println("foo"); Some(2).each > 0} }
   
8. jilen commented on Mar 20, 2017

   

   jilen

   on Mar 20, 2017

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   @Atry Can't agree anymore
9. MarisaKirisame commented on Mar 20, 2017

   

   MarisaKirisame

   on Mar 20, 2017

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   👍
10. Atry

    added a commit that references this issue on Feb 3, 2023

    Update sbt-sonatype to 3.9.16 in template (ThoughtWorksInc#37)

    ...

    43ef7e9