After working 2 years in scala I stumbled upon a surprising implementation detail of scala for loops.
To be honest, to me this looks like a bug.
Concretely the scala specification of for loops states that
which essentially introduces a new (unexpected) for loop.
Let me illustrate a bit more what this means. Take the following code as an example
for{
i <- 1 to 3
_ = println(s"i = $i")
js = i to 3
_ = println(js)
j <- js
_ = println(s"j = $j")
} println((i, j))
What would you guess the output looks like? At least I myself was very surprised finding it to be
i = 1
Range(1, 2, 3)
i = 2
Range(2, 3)
i = 3
Range(3)
j = 1
j = 2
j = 3
(1,1)
(1,2)
(1,3)
j = 2
j = 3
(2,2)
(2,3)
j = 3
(3,3)
The problem is that the side-effects are executed at an non-intuitive place in time. To my honest opinion, this is 100% unwanted behaviour, as it will lead to many subtle bugs.
To get the expected behaviour the collections need to be cast to Streams
for{
i <- (1 to 3).toStream
_ = println(s"i = $i")
js = (i to 3).toStream
_ = println(js)
j <- js
_ = println(s"j = $j")
} println((i, j))
this will indeed output
i = 1
Stream(1, ?)
j = 1
(1,1)
j = 2
(1,2)
j = 3
(1,3)
i = 2
Stream(2, ?)
j = 2
(2,2)
j = 3
(2,3)
i = 3
Stream(3, ?)
j = 3
(3,3)
To finish this contribution, the change needed for this is not magic at all, it just needs a different code rewrite. Concretely, all normal value definitions need to be included in the scope of the nested code block.
The value definition must be included right after => respectively:
- [here should be images of how a normal for loop without value definitions is translated to
foreach,mapandflatMap, however I am to newbie to be able to upload more than one image per topic, awesome] - hence please look them up yourself on page 89/90 of https://www.scala-lang.org/docu/files/ScalaReference.pdf
Any comments on why this was implemented this way are highly welcome
All endorsement to get such an adaptation into Scala 3 are also highly welcome