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I have following application that I could not figure it out, why running it twice it breaks RT: 

val program = for {
  _ <- IO { println("Welcome to Scala!  What's your name?") }
  _ <- IO { println(s"Well hello, foo") }
} yield ()

program.unsafeRunSync()
program.unsafeRunSync()

I run it twice and got the same result twice, why it breaks RT?

softshipper
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1 Answers1

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It's because val a = program.unsafeRunSync(); val b = program.unsafeRunSync() is not the same program as val a = program.unsafeRunSync(); val b = a. If an expression is RT then you can inline it or factor it out freely. You can't do that here.

tpolecat
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  • Could you please make an example about `inline it or factor it out`? And why it is not the same? – softshipper Nov 11 '17 at 08:45
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    The first is an inlining of the second; the second is a factoring of the first. They're not the same because the first prints 4 lines of text and the second prints 2 lines. – tpolecat Nov 11 '17 at 20:45
  • Could you please explain, what are `inlining` and `factoring`? Could you please provide an example? – softshipper Nov 11 '17 at 20:58
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    Inlining is replacing a reference to a variable with the right-hand side of its definition. Factoring out means removing duplicate code by saving the result of an expression in a variable and replacing every instance of that expression with a reference to that variable. – Jasper-M Nov 11 '17 at 23:35