repl eats program output

[DarkDimius]

taken from http://scalapuzzlers.com/#pzzlr-013

Welcome to Scala.next (pre-alpha)  (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_45).
Type in expressions to have them evaluated.
Type :help for more information.
scala> implicit object RomanOrdering extends Ordering[RomanNumeral] {
         def compare(a: RomanNumeral, b: RomanNumeral) = a.value compare b.value
       }
defined module RomanOrdering
scala>
scala>
val numerals = SortedSet(RomanNumeral("M", 1000), RomanNumeral("C", 100), RomanNumeral("X", 10), RomanNumeral("I", 1), RomanNumeral("D", 500), RomanNumeral("L", 50),  RomanNumeral("V", 5))
numerals: scala.collection.immutable.SortedSet[RomanNumeral] = TreeSet(RomanNumeral(I,1), RomanNumeral(V,5), RomanNumeral(X,10), RomanNumeral(L,50), RomanNumeral(C,100), RomanNumeral(D,500), RomanNumeral(

scala>
scala> println("Roman numeral symbols for 1 5 10 50 100 500 1000:")
Roman numeral symbols for 1 5 10 50 100 500 1000:
scala> for (num <- numerals; sym = num.symbol) { print(s"${sym} ") }
scala> numerals map { _.symbol } foreach { sym => print(s"${sym} ") }
scala>

The numbers were printed, but then they were swallowed by repl and replaced by scala> invitation.

[kzys]

We probably don't need the entire puzzle to reproduce the issue.

% ./bin/dotr
Starting dotty REPL...
Welcome to Scala.next (pre-alpha)  (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_25).
Type in expressions to have them evaluated.
Type :help for more information.
scala> print("hello") 
scala>  

[nicolasstucki]

This is still an issue with the new repl

[benkobalog]

I looked into this issue. The problem seems to be that the line redraw is oblivious to the output what is printed by the interpreted Scala code, because it only follows the standard input. So the redraw overwrites anything printed in the last line.

I have two solutions, but I'm not sure if they are good solutions.

1. If something has a Unit return value print "\n" or "()". This will look a bit off in any other situations than a print without a "\n" at the end, because there will be an additional new line.

2. Watch the standard output and if the line is not empty after interpreting a code part

   1. write a "\n" character, so the "scala>" prompt will come in the next line.
   2. set the prompt offset so it doesn't start at the start of the line, but right after the last output.

If any of these are acceptable or you have a better idea, I'd like to implement it.

[smarter]

Ammonite-REPL seems to handle this correctly. Would be interesting to figure out how since our REPL UI code is based on Ammonite.

[allanrenucci]

I think a very simple fix would be to print a new line after a command is interpreted

[benkobalog]

Ammonite prints an additional new line after each command.
It also prints a new line for commands returning Unit values.