REPL: values that fail to initialise should not be accessible in later runs

[allanrenucci]

scala> val x = 1 / 0 
java.lang.ArithmeticException: / by zero
	at rs$line$5$.<init>(rs$line$5:1)
	at rs$line$5$.<clinit>(rs$line$5)
	at rs$line$5.xShow(rs$line$5)
        ...

scala> def foo = x // should not compile
def foo: Int

scala> x // should not compile
java.lang.NoClassDefFoundError: Could not initialize class rs$line$5$
	at rs$line$8$.<init>(rs$line$8:1)
	at rs$line$8$.<clinit>(rs$line$8)
	at rs$line$8.res3Show(rs$line$8)

[ankitson]

I want to try this.

It looks like the problem is that a successful compile adds definitions to the REPL state, but they are evaluated only after the compile by the renderer when it tries to show them. I am thinking of patching the compile method to evaluate all values added to definitions (like in Rendering.renderVal) before returning success. Does that make sense?

[allanrenucci]

Good analysis! I suggest you patch displayDefinitions
https://github.com/lampepfl/dotty/blob/17c4497455de59f56c4704053608599fa4f225b4/compiler/src/dotty/tools/repl/ReplDriver.scala#L249

It should probably return the old state if a failure occurs during evaluation. I think renderVal should return a Try[String]
https://github.com/lampepfl/dotty/blob/17c4497455de59f56c4704053608599fa4f225b4/compiler/src/dotty/tools/repl/Rendering.scala#L73

[allanrenucci]

Ideally, I think we should decouple rendering and evaluation. But it's probably best implemented as another PR

[Blaisorblade]

Just run today into a likely variant:

scala> val x = ???; val y = x
scala.NotImplementedError: an implementation is missing
	at scala.Predef$.$qmark$qmark$qmark(Predef.scala:284)
	at rs$line$1$.<init>(rs$line$1:1)
	at rs$line$1$.<clinit>(rs$line$1)
	at rs$line$1.x(rs$line$1)
	...

java.lang.NoClassDefFoundError: Could not initialize class rs$line$1$
	at rs$line$1.y(rs$line$1)
	...

Edited(@allanrenucci) for simplicity: REPL shouldn't try to initialise y if it failed to initialise x