Improve REPL display

[scabug]

The current REPL output for definitions is very confusing because it is almost, but not quite valid Scala and also inconsistent.

Some examples:

scala> case class Foo()
defined class Foo

scala> trait Bar
defined trait Bar

scala> type Qux = Nothing
defined type alias Qux

scala> val a = 1
a: Int = 1

scala> var b = 1
b: Int = 1

The Scala REPL's response should be more consistent reporting about definitions.
Possible solutions

• no output in the success case (as does the Python REPL)
• display the type definition (as it is currently the case but without "defined" )
• display the type definition as valid Scala
• ...

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10024?orig=1
Reporter: Raphael Bosshard (esarbe)
Blocks #7960, #10023

[scabug]

@som-snytt said:
There's a PR underway for pretty-printing. Also consider output such as

scala> val (x, y) = (42, 17)
x: Int = 42
y: Int = 17

Customizing output is not amenable to piece-meal tweaks.

[scabug]

Raphael Bosshard (esarbe) said:
What's the pretty-printing PR's number?

[scabug]

@dwijnand said:
[~esarbe]: scala/scala#5222

[eed3si9n]

This and related REPL display issues might be good first issue if they were defined clearly on what are expected at the end.

For example, this issue says "no output in the success case (as does the Python REPL)" as possible solution while #7960 says it would be better to include val or val:

scala> var x = 5
var x: Int = 5

[SethTisue]

no output in the success case (as does the Python REPL)

I agree with #7960. I think it's valuable to show the types, and it's valuable to show the computed value, too:

scala> val x = 2 + 2
x: Int = 4

[SethTisue]

closed #7960 and #10023, consolidating here.

[SethTisue]

note that anyone doing nontrivial REPL work should target 2.13, since the REPL code changed around between 2.12 and 2.13 so merging changes forward is extra work we'd rather not do.