Compiler crash on leftover do-call in FFI after capability passing

[leonfuss]

During language exploration I ran into an unexpected compiler crash. The compiler (version: 0.2.2) always crashed when trying to compile the following code:

import io/file

record File (
    path: String
)

def main() = {
    try {
       val file = open("test.meff")
       println(file)
    } with IOException[A] { (msg) =>
        println("Error opening file")
    }
}

def open(path: String) : File / IOException = {
    File(path)
}

Stacktrace

[error] Should have been translated away (to explicit selection `@CAP.op()`) by capability passing.
[info]   at effekt.util.messages$CompilerPanic$.apply(Messages.scala:37)
[info]   at effekt.util.messages$ErrorReporter.panic(Messages.scala:111)
[info]   at effekt.util.messages$ErrorReporter.panic$(Messages.scala:90)
[info]   at effekt.context.Context.panic(Context.scala:39)
[info]   at effekt.util.messages$ErrorReporter.panic(Messages.scala:112)
[info]   at effekt.core.Transformer$.transformAsExpr(Transformer.scala:479)
[info]   at effekt.core.Transformer$.transformAsPure(Transformer.scala:310)
[info]   at effekt.core.Transformer$.transform$$anonfun$6(Transformer.scala:148)
[info]   at effekt.core.Transformer$.$anonfun$59(Transformer.scala:659)
[info]   at effekt.core.TransformerOps.withBindings$$anonfun$1(Transformer.scala:786)
[info]   at effekt.context.ContextOps.in(Context.scala:29)
[info]   at effekt.context.ContextOps.in$(Context.scala:18)
[info]   at effekt.context.Context.effekt$namer$NamerOps$$super$in(Context.scala:39)
[info]   at effekt.namer.NamerOps.in(Namer.scala:739)
[info]   at effekt.namer.NamerOps.in$(Namer.scala:724)
[info]   at effekt.context.Context.in(Context.scala:82)
[info]   at effekt.core.TransformerOps.withBindings(Transformer.scala:789)
[info]   at effekt.core.TransformerOps.withBindings$(Transformer.scala:737)
[info]   at effekt.context.Context.withBindings(Context.scala:39)
[info]   at effekt.core.Transformer$.insertBindings(Transformer.scala:659)
[info]   at effekt.core.Transformer$.transform(Transformer.scala:148)
[info]   at effekt.core.Transformer$.transformAsControlBlock(Transformer.scala:302)
[info]   at effekt.core.Transformer$.transformAsBlock(Transformer.scala:212)
[info]   at effekt.core.Transformer$.transformBox(Transformer.scala:200)
[info]   at effekt.core.Transformer$.transformAsExpr(Transformer.scala:336)
[info]   at effekt.core.Transformer$.$anonfun$13(Transformer.scala:105)
[info]   at scala.collection.immutable.List.map(List.scala:250)
[info]   at effekt.core.Transformer$.transformToplevel(Transformer.scala:105)
[info]   at effekt.core.Transformer$.$anonfun$1(Transformer.scala:33)
[info]   at scala.collection.immutable.List.flatMap(List.scala:293)
[info]   at effekt.core.Transformer$.transform$$anonfun$1(Transformer.scala:33)
[info]   at effekt.context.Context.using$$anonfun$1(Context.scala:73)
[info]   at effekt.context.Context.using(Context.scala:74)
[info]   at effekt.core.Transformer$.transform(Transformer.scala:41)
[info]   at effekt.core.Transformer$.run(Transformer.scala:26)
[info]   at effekt.core.Transformer$.run(Transformer.scala:19)
[info]   at effekt.Phase$$anon$1.run$$anonfun$1(Phase.scala:51)
[info]   at scala.Option.flatMap(Option.scala:283)
[info]   at effekt.Phase$$anon$1.run(Phase.scala:51)
[info]   at effekt.Phase$task$2$.run(Phase.scala:87)
[info]   at effekt.util.Task$.$anonfun$1(Task.scala:144)
[info]   at effekt.context.Context.withMessages(Context.scala:97)
[info]   at effekt.util.Task$.compute(Task.scala:145)
[info]   at effekt.util.Task$.need(Task.scala:174)
[info]   at effekt.util.Task$.need(Task.scala:165)
[info]   at effekt.util.Task.apply(Task.scala:29)
[info]   at effekt.util.Task.apply$(Task.scala:9)
[info]   at effekt.Phase$task$2$.apply(Phase.scala:85)
[info]   at effekt.Phase$$anon$4.run(Phase.scala:94)
[info]   at effekt.Phase.apply(Phase.scala:41)
[info]   at effekt.Phase.apply$(Phase.scala:34)
[info]   at effekt.Phase$$anon$4.apply(Phase.scala:92)
[info]   at effekt.Compiler$$anon$3.run$$anonfun$3$$anonfun$1(Compiler.scala:271)
[info]   at scala.collection.immutable.List.foldRight(List.scala:352)
[info]   at effekt.Compiler$$anon$3.run$$anonfun$3(Compiler.scala:273)
[info]   at scala.Option$WithFilter.flatMap(Option.scala:283)
[info]   at effekt.Compiler$$anon$3.run(Compiler.scala:274)
[info]   at effekt.Compiler$$anon$3.run(Compiler.scala:268)
[info]   at effekt.Phase$$anon$2.run(Phase.scala:56)
[info]   at effekt.Phase$$anon$2.apply(Phase.scala:54)
[info]   at effekt.generator.js.JavaScript.compile(JavaScript.scala:32)
[info]   at effekt.Driver.compile$1(Driver.scala:71)
[info]   at effekt.Driver.compileSource(Driver.scala:82)
[info]   at effekt.Driver.compileSource$(Driver.scala:21)
[info]   at effekt.Server$.compileSource(Server.scala:263)
[info]   at kiama.util.Compiler.compileFile(Compiler.scala:102)
[info]   at kiama.util.Compiler.compileFile$(Compiler.scala:22)
[info]   at effekt.Server$.compileFile(Server.scala:263)
[info]   at effekt.Driver.run$$anonfun$1(Driver.scala:41)
[info]   at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[info]   at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[info]   at scala.collection.immutable.List.foreach(List.scala:333)
[info]   at effekt.Driver.run(Driver.scala:42)
[info]   at effekt.Driver.run$(Driver.scala:21)
[info]   at effekt.Server$.run(Server.scala:263)
[info]   at kiama.util.Compiler.driver(Compiler.scala:86)
[info]   at kiama.util.Compiler.driver$(Compiler.scala:22)
[info]   at effekt.Server$.driver(Server.scala:263)
[info]   at kiama.util.Compiler.main(Compiler.scala:50)
[info]   at kiama.util.Compiler.main$(Compiler.scala:22)
[info]   at effekt.Server$.main(Server.scala:263)
[info]   at effekt.Server.main(Server.scala)

Please don’t hesitate to reach out if any additional informations are required!

[jiribenes]

Hi @leonfuss, thanks for reporting!
I was able to reproduce this on 0.2.2 (with nix run github:jiribenes/effekt-nix -- <reprofile>) and I'll take a look if we fixed the underlying problem (an extra do <something> still existing after capability passing)

effekt/effekt/shared/src/main/scala/effekt/core/Transformer.scala

Lines 527 to 528 in 68d4037

	case source.Do(effect, id, targs, vargs, bargs) =>
	Context.panic("Should have been translated away (to explicit selection `@CAP.op()`) by capability passing.")

I'll rename the title to make the exact nature of the issue more apparent.

The current state on master is some +500 new commits since the last release with a lot of new features and libraries (a much better IO library for example). We're aiming to release newer versions more often, see #538 (ideally starting this/next week).

Workarounds

If you just want to get IO working, I think that the file API on 0.2.2 required some hard to explain AsyncIO magic:

import io/async

def process(fileName: String): Unit / {AsyncIO} = {
    val promise = do read(fileName)
    val contents: String = promise.block()

    // ...
    ()
}

def main() = {
    asyncIO(box { process("input") })
}

It's somewhat more intuitive to do this on master:

import io
import io/files
import io/error

def process(fileName: String): Unit / {Files} = {
  val contents: String = do readFile(fileName)

  // ...
  ()
}

// Note: the eventloop currently must be encompassing the whole program
def main() = eventloop(box {
    with on[IOError].panic;
    with filesystem;

    process("input")
  })
}

[jiribenes]

Yeah, in v0.2.2 just the import already produces the same crash:

import io/file
def main() = ()

... and if we minimise io/file, we find the actual culprit:

def await[T](p: Promise[T]): T / { IOException } =
  try { awaitImpl(p){exc} } with exc : IOException[A] {
    msg => do IOException(msg)
  }

extern io def awaitImpl[T](p: Promise[T]){exc: IOException}: T =
  "$effekt.callcc(k => ${p}.then(res => k($effekt.pure(res)), err => k(${box { (msg: String) => do IOException[T](msg) }}(err.message)))).then(c => c)"
  // here instead of `do IOException`, we probably wanted to do `exc.IOException` instead!
  // ... if you do that, the issue goes away completely :)

[jiribenes]

OK, here's a very tiny version that still fails on master:

effect IOException(msg: String): Nothing
extern io def three {exc: IOException}: Nothing =
  js "${box { (msg: String) => do IOException(msg) }}('oops')}"

with the original error message:

[error] Should have been translated away (to explicit selection `@CAP.op()`) by capability passing.
[info]   at effekt.util.messages$CompilerPanic$.apply(Messages.scala:37)
...

... which means that this is still a relevant bug, even though the io/file library no longer exists.

I haven't been able to remove the weird indirection through FFI, but I'm running out of time and ideas :)
(my guess is something like "capability passing doesn't see through FFI splices" which makes sense, we don't even allow unhandled control effects on extern defs anyway)