scala · anatoliykmetyuk · Jul 5, 2019 · Jul 2, 2019 · Jul 2, 2019 · Jul 2, 2019
diff --git a/docs/docs/contributing/debugging.md b/docs/docs/contributing/debugging.md
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+# Debugging Techniques
+- [Setting up the playground](#setting-up-the-playground)
+- [Show for human readable output](#show-for-human-readable-output)
+- [How to disable color](#how-to-disable-color)
+- [Reporting as a non-intrusive println](#reporting-as-a-non-intrusive-println)
+- [Printing out trees after phases](#printing-out-trees-after-phases)
+- [Configuring the printer output](#configuring-the-printer-output)
+- [Figuring out an object creation site](#figuring-out-an-object-creation-site)
+  * [Via ID](#via-id)
+  * [Via tracer](#via-tracer)
+- [Built-in Logging Architecture](#built-in-logging-architecture)
+  * [Printers](#printers)
+  * [Tracing](#tracing)
+  * [Reporter](#reporter)
+
+<small><i><a href='http://ecotrust-canada.github.io/markdown-toc/'>Table of contents generated with markdown-toc</a></i></small>
+
+## Setting up the playground
+Consider the `../issues/Playground.scala` (relative to the Dotty directory) file is:
+
+```scala
+object Playground {
+  def main(args: Array[String]) = {
+    println("Hello World")
+  }
+}
+```
+
+Then, you can debug Dotty by compiling this file via `dotc ../issues/Playground.scala` (from the SBT console) and collecting various debug output in process. This section documents techniques you can use to collect the debug info.
+
+[This](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/typer/Typer.scala#L2231) is the entry point to the Typer. The job of the Typer is to take an untyped tree, compute its type and turn it into a typed tree by attaching the type information to that tree (in an immutable way of course). We will use this entry point to practice debugging techniques. E.g.:
+
+```scala
+  def typed(tree: untpd.Tree, pt: Type, locked: TypeVars)(implicit ctx: Context): Tree =
+    trace(i"typing $tree", typr, show = true) {
+      println("Hello Debug!")
+      /*...*/
+```
+
+Then:
+
+```
+dotc ../issues/Playground.scala
+```
+
+The techniques discussed below can be tried out in place of `println("Hello Debug")` in that location. They are of course applicable thoughout the codebase.
+
+## Show for human readable output
+Many objects in the compiler have a `show` method available on them via implicit rich wrapper:
+
+```scala
+println(tree.show)
+```
+
+This will output every single tree passing through the typer (or wherever else you inject it) in a human readable form. Try calling `show` on anything you want to be human-readable, and chances are it will be possible to do so.
+
+## How to disable color
+Note that the `show` command above outputs the code in color. This is achieved by injecting special characters into the strings which terminals interpret as commands to change color of the output. This however may not be what you want, e.g. if you want to zero-in on a particular tree:
+
+```scala
+if (tree.show == """println("Hello World")""")
+  println(s"${tree.show}\n${pt.show}\n${tree.uniqueId}\n===\n")
+```
+
+The intention above is to output an extended debug info on a tree that matches a particular human-readable representation. However, because of the color characters, the comparison will fail.
+
+To disable color output from `show`, run `dotc` as follows:
+
+`dotc -color:never ../issues/Playground.scala`
+
+## Reporting as a non-intrusive println
+Consider you want to debug the `tree` that goes into `assertPositioned(tree)` in the `typed` method. You can do:
+
+```scala
+println(tree.show)
+assertPositioned(tree)
+```
+
+But you can also do:
+
+```scala
+assertPositioned(tree.reporting(t => t.show))
+```
+
+`(a: A).reporting(f: A => String): A` is defined on all types. Its job is to print a `String` produced by the argument function and return the value it is called on unchanged.
+
+## Printing out trees after phases
+To print out the trees you are compiling after Frontend (scanner, parser, namer, typer) phase:
+
+```
+dotc -Xprint:frontend ../issues/Playground.scala
+```
+
+To print out the trees after Frontend and CollectSuperCalls phases:
+
+```
+dotc -Xprint:frontend,collectSuperCalls ../issues/Playground.scal
+```
+
+To print out the trees after all phases:
+
+```
+dotc -Xprint:all ../issues/Playground.scala
+```
+
+To find out the list of all the phases and their names, check out [this](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/Compiler.scala#L34) line in `Compiler.scala`. Each `Phase` object has `phaseName` defined on it, this is the phase name.
+
+## Configuring the printer output
+Printing from the `show` and `-Xprint` is done from the Printers framework (discussed in more details below). The following settings influence the output of the printers:
+
+```scala
+val printLines        = BooleanSetting("-print-lines"        , "Show source code line numbers.") withAbbreviation "--print-lines"
+val uniqid            = BooleanSetting("-uniqid"             , "Uniquely tag all identifiers in debugging output.") withAbbreviation "--unique-id"
+val XprintInline      = BooleanSetting("-Xprint-inline"      , "Show  where inlined code comes from")
+val XprintTypes       = BooleanSetting("-Xprint-types"       , "Print tree types (debugging option).")
+val Ydebug            = BooleanSetting("-Ydebug"             , "Increase the quantity of debugging output.")
+val YdebugFlags       = BooleanSetting("-Ydebug-flags"       , "Print all flags of definitions")
+val YdebugMissingRefs = BooleanSetting("-Ydebug-missing-refs", "Print a stacktrace when a required symbol is missing")
+val YdebugNames       = BooleanSetting("-Ydebug-names"       , "Show internal representation of names")
+val YdebugPos         = BooleanSetting("-Ydebug-pos"         , "Show full source positions including spans")
+val YdebugTrace       = BooleanSetting("-Ydebug-trace"       , "Trace core operations")
+val YdebugTreeWithId  = IntSetting    ("-Ydebug-tree-with-id", "Print the stack trace when the tree with the given id is created", Int.MinValue)
+val YprintDebug       = BooleanSetting("-Yprint-debug"       , "when printing trees, print some extra information useful for debugging.")
+val YprintDebugOwners = BooleanSetting("-Yprint-debug-owners", "when printing trees, print owners of definitions.")
+val YprintPos         = BooleanSetting("-Yprint-pos"         , "show tree positions.")
+val YprintPosSyms     = BooleanSetting("-Yprint-pos-syms"    , "show symbol definitions positions.")
+val YprintSyms        = BooleanSetting("-Yprint-syms"        , "when printing trees print info in symbols instead of corresponding info in trees.")
+val YshowTreeIds      = BooleanSetting("-Yshow-tree-ids"     , "Uniquely tag all tree nodes in debugging output.")
+val YshowVarBounds    = BooleanSetting("-Yshow-var-bounds"   , "Print type variables with their bounds")
+val YtestPickler      = BooleanSetting("-Ytest-pickler"      , "self-test for pickling functionality; should be used with -Ystop-after:pickler")
+```
+
+They are defined in [ScalaSettings.scala](https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala). E.g. `YprintPos` is defined as:
+
+```scala
+val YprintPos: Setting[Boolean] = BooleanSetting("-Yprint-pos", "show tree positions.")
+```
+
+And is to be used as:
+
+```scala
+dotc -Yprint-pos  ../issues/Playground.scala
+```
+
+If used, all the trees output with `show` or via `-Xprint:frontend` will also have positions attached to them, e.g.:
+
+```scala
+package <empty>@<Playground.scala:1> {
+  module object Playground {
+    def main(
+      args:
+        Array@<Playground.scala:2>[String@<Playground.scala:2>]@<
+          Playground.scala:2
+        >
+      @<Playground.scala:2>
+    ) =
+      {
+        println@<Playground.scala:3>("Hello World"@<Playground.scala:3>)@<
+          Playground.scala:3
+        >
+      }@<Playground.scala:2>
+    @<Playground.scala:2>
+  }@<Playground.scala:1>
+}@<Playground.scala:1>
+<empty>@<Playground.scala:1>
+```
+
+## Figuring out an object creation site
+### Via ID
+Every [Positioned](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/ast/Positioned.scala) (a trait of a `Tree`) object has `uniqueId` field. It is an integer that is unique for that tree and doesn't change from compile run to compile run. You can output these IDs from the printers (= from `show` and `-Xprint`) framework via `-Yshow-tree-ids` flag, e.g.:
+
+```
+dotc -Xprint:frontend -Yshow-tree-ids  ../issues/Playground.scala
+```
+
+Gives:
+
+```scala
+package <empty>#1047 {
+  final lazy module val Playground: Playground$#1049 =
+    new Playground$#1049#1050#1051()#1052
+  #1053
+  final module class Playground$() extends Object#1090#1091#1092()#1093, _root_#
+    1061
+  .scala#1062.Serializable#1063 { this: Playground#1054.type#1055 =>
+    def main(args: Array#1028[String#1033]#1034#1038): Unit#1039 =
+      {
+        println#1094("Hello World"#1041)#1095
+      }#1096
+    #1097
+  }#1099
+}#1100
+```
+
+You can then use these IDs to locate the creation site of a given tree using that ID via `-Ydebug-tree-with-id`, e.g.:
+
+```
+dotc -Ydebug-tree-with-id 1049 ../issues/Playground.scala
+```
+
+When requested what you want to do:
+
+```
+Debug tree (id=1049) creation
+Ident(Playground$)
+
+
+a)bort, s)tack, r)esume
+```
+
+Input `s` for stack trace. You will get:
+
+```
+java.lang.Throwable
+  at dotty.tools.dotc.reporting.Reporter$.loop$1(Reporter.scala:55)
+  at dotty.tools.dotc.reporting.Reporter$.displayPrompt(Reporter.scala:63)
+  at dotty.tools.dotc.ast.Positioned.printTrace$1(Positioned.scala:32)
+  at dotty.tools.dotc.ast.Positioned.uniqueId_$eq(Positioned.scala:34)
+  at dotty.tools.dotc.ast.Positioned.<init>(Positioned.scala:45)
+  at dotty.tools.dotc.ast.Trees$Tree.<init>(Trees.scala:53)
+  at dotty.tools.dotc.ast.Trees$DenotingTree.<init>(Trees.scala:266)
+  at dotty.tools.dotc.ast.Trees$NameTree.<init>(Trees.scala:292)
+  at dotty.tools.dotc.ast.Trees$RefTree.<init>(Trees.scala:298)
+  at dotty.tools.dotc.ast.Trees$Ident.<init>(Trees.scala:375)
+  at dotty.tools.dotc.ast.untpd$.Ident(untpd.scala:301)
+  at dotty.tools.dotc.ast.desugar$.moduleDef(Desugar.scala:804)
+  at dotty.tools.dotc.ast.desugar$.defTree(Desugar.scala:1038)
+  at dotty.tools.dotc.typer.Namer.expand(Namer.scala:441)
+  at dotty.tools.dotc.typer.Namer.index$$anonfun$1(Namer.scala:722)
+  at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
+  at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
+  at scala.collection.immutable.List.foreach(List.scala:392)
+  at dotty.tools.dotc.typer.Namer.index(Namer.scala:722)
+  at dotty.tools.dotc.typer.Namer.recur$1(Namer.scala:484)
+  at dotty.tools.dotc.typer.Namer.indexExpanded(Namer.scala:501)
+  at dotty.tools.dotc.typer.Namer.index(Namer.scala:474)
+  at dotty.tools.dotc.typer.FrontEnd.enterSyms$$anonfun$1(FrontEnd.scala:69)
+  at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
+  at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:41)
+  at dotty.tools.dotc.typer.FrontEnd.enterSyms(FrontEnd.scala:71)
+  at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:100)
+  at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:158)
+  at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
+  at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
+  at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36)
+  at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
+  at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198)
+  at dotty.tools.dotc.Run.runPhases$5(Run.scala:170)
+  at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:178)
+  at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
+  at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:102)
+  at dotty.tools.dotc.Run.compileUnits(Run.scala:185)
+  at dotty.tools.dotc.Run.compileSources(Run.scala:120)
+  at dotty.tools.dotc.Run.compile(Run.scala:104)
+  at dotty.tools.dotc.Driver.doCompile(Driver.scala:34)
+  at dotty.tools.dotc.Driver.process(Driver.scala:172)
+  at dotty.tools.dotc.Driver.process(Driver.scala:141)
+  at dotty.tools.dotc.Driver.process(Driver.scala:153)
+  at dotty.tools.dotc.Driver.main(Driver.scala:180)
+  at dotty.tools.dotc.Main.main(Main.scala)
+```
+
+So that tree was created at:
+
+```
+  at dotty.tools.dotc.ast.desugar$.moduleDef(Desugar.scala:804)
+```
+
+Since all the stack frames above it are technical frames executing the tree creation command, and the frame in question is the location where the intent of the tree creation was expressed.
+
+### Via tracer
+Some objects may not be `Positioned` and hence their creation site is not debuggable via the technique in the section above. Say you target a tree at `Typer`'s `typed` method as follows:
+
+```scala
+if (tree.show == """println("Hello World")""") {
+  val creationSite = "<creation site stack here>"
+  println(creationSite)
+}
+```
+
+In other words, you have a reference to the object and want to know were it was created. To do so, go to the class definition of that object. In our case, `tree` is a [`Tree`](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/ast/Trees.scala#L52). Now, create a new `val` member of that type:
+
+```scala
+val tracer = Thread.currentThread.getStackTrace.mkString("\n")
+```
+
+Then, from the `typed` method (or wherever you want to access the trace):
+
+```scala
+if (tree.show == """println("Hello World")""") {
+  val creationSite = tree.tracer
+  println(creationSite)
+}
+```
+
+## Built-in Logging Architecture
+Dotty has a lot of debug calls scattered throughout the code, vast majority of them disabled. At least three (possibly intertwined) architectures for logging are used for that:
+
+- Printer
+- Tracing
+- Reporter
+
+These do not follow any particular system and so probably it will be easier to go with `println` most of the times instead.
+
+### Printers
+Defined in [Printers.scala](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/config/Printers.scala) as a set of variables, each responsible for its own domain. To enable them, replace `noPrinter` with `default`. [Example](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/typer/Typer.scala#L2226) from the code:
+
+```scala
+typr.println(i"make contextual function $tree / $pt ---> $ifun")
+```
+
+`typr` is a printer.
+
+### Tracing
+Defined in [trace.scala](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/reporting/trace.scala). [Example](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/typer/Typer.scala#L2232) from the code:
+
+```scala
+trace(i"typing $tree", typr, show = true) { // ...
+```
+
+To enable globally, change [tracingEnabled](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/config/Config.scala#L164) to `true` (will recompile a lot of code).
+
+You also need to set the printer referenced in the call (in the example, `typr`) to `default` as explained in the section on printers.
+
+To enable for a single trace, do the following:
+
+```scala
+trace.force(i"typing $tree", typr, show = true) { // ...
+```
+
+### Reporter
+Defined in [Reporter.scala](https://github.com/lampepfl/dotty/blob/10526a7d0aa8910729b6036ee51942e05b71abf6/compiler/src/dotty/tools/dotc/reporting/Reporter.scala). Enables calls such as `ctx.log`, `ctx.error` etc. To enable, run dotc with `-Ylog:frontend` option.