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+# Dotc's Overall Structure
+
+The compiler code is found in package `dotty.tools`. It spans the
+following three sub-packages:
+
+    backend          Compiler backends (currently for JVM and JS)
+    dotc             The main compiler
+    io               Helper modules for file access and classpath handling.
+
+The `dotc` package contains some main classes that can be run as separate
+programs. The most important one is class `Main`. `Main` inherits from `Driver` which
+contains the highest level functions for starting a compiler and processing some sources
+`Driver` in turn is based on two other high-level classes, `Compiler` and `Run`.
+
+## Package Structure
+
+Most functionality of `dotc` is implemented in subpackages of `dotc`. Here's a list of sub-packages
+and their focus.
+
+    ast              Abstract syntax trees,
+    config           Compiler configuration, settings, platform specific definitions.
+    core             Core data structures and operations, with specific subpackages for:
+
+      core.classfile         Reading of Java classfiles into core data structures
+      core.tasty             Reading and writing of TASTY files to/from core data structures
+      core.unpickleScala2    Reading of Scala2 symbol information into core data structures
+
+    parsing          Scanner and parser
+    printing         Pretty-printing trees, types and other data
+    repl             The interactive REPL
+    reporting        Reporting of error messages, warnings and other info.
+    rewrite          Helpers for rewriting Scala 2's constructs into dotty's.
+    transform        Miniphases and helpers for tree transformations.
+    typer            Type-checking and other frontend phases
+    util             General purpose utility classes and modules.
+
+## Contexts
+
+`dotc` has almost no global state (the only significant bit of global state is the name table,
+which is used to hash strings into unique names). Instead, all essential bits of information that
+can vary over a compiler run are collected in a [Context](https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/core/Context.scala). Most methods in `dotc` take a Context value as an implicit parameter.
+
+Contexts give a convenient way to customize values in some part of the
+call-graph. To run, e.g. some compiler function `f` at a given
+phase `phase`, we invoke `f` with an explicit context parameter, like
+this
+
+    f(/*normal args*/)(ctx.withPhase(phase))
+
+This assumes that `f` is defined in way most compiler functions are:
+
+    def f(/*normal parameters*/)(implicit ctx: Context) ...
+
+Compiler code follows the convention that all implicit `Context`
+parameters are named `ctx`.  This is important to avoid implicit
+ambiguities in the case where nested methods contain each a Context
+parameters. The common name ensures then that the implicit parameters
+properly shadow each other.
+
+Sometimes we want to make sure that implicit contexts are not captured
+in closures or other long-lived objects, be it because we want to
+enforce that nested methods each get their own implicit context, or
+because we want to avoid a space leak in the case where a closure can
+survive several compiler runs. A typical case is a completer for a
+symbol representing an external class, which produces the attributes
+of the symbol on demand, and which might never be invoked. In that
+case we follow the convention that any context parameter is explicit,
+not implicit, so we can track where it is used, and that it has a name
+different from `ctx`. Commonly used is `ictx` for "initialization
+context".
+
+With these two conventions is has turned out that the use of implicit
+contexts as an dependency injection and bulk parameterization device
+worked exceptionally well. There were not very many bugs related to
+passing the wrong context by accident.
+
+## Compiler Phases
+
+Seen from a temporal perspective, the `dotc` compiler consists of a list of phases.
+The current list of phases is specified in class [Compiler] as follows:
+
+```scala
+    def phases: List[List[Phase]] = List(
+      List(new FrontEnd),           // Compiler frontend: scanner, parser, namer, typer
+      List(new PostTyper),          // Additional checks and cleanups after type checking
+      List(new Pickler),            // Generate TASTY info
+      List(new FirstTransform,      // Some transformations to put trees into a canonical form
+           new CheckReentrant),     // Internal use only: Check that compiled program has no data races involving global vars
+      List(new RefChecks,           // Various checks mostly related to abstract members and overriding
+           new CheckStatic,         // Check restrictions that apply to @static members
+           new ElimRepeated,        // Rewrite vararg parameters and arguments
+           new NormalizeFlags,      // Rewrite some definition flags
+           new ExtensionMethods,    // Expand methods of value classes with extension methods
+           new ExpandSAMs,          // Expand single abstract method closures to anonymous classes
+           new TailRec,             // Rewrite tail recursion to loops
+           new LiftTry,             // Put try expressions that might execute on non-empty stacks into their own methods
+           new ClassOf),            // Expand `Predef.classOf` calls.
+      List(new PatternMatcher,      // Compile pattern matches
+           new ExplicitOuter,       // Add accessors to outer classes from nested ones.
+           new ExplicitSelf,        // Make references to non-trivial self types explicit as casts
+           new CrossCastAnd,        // Normalize selections involving intersection types.
+           new Splitter),           // Expand selections involving union types into conditionals
+      List(new VCInlineMethods,     // Inlines calls to value class methods
+           new SeqLiterals,         // Express vararg arguments as arrays
+           new InterceptedMethods,  // Special handling of `==`, `|=`, `getClass` methods
+           new Getters,             // Replace non-private vals and vars with getter defs (fields are added later)
+           new ElimByName,          // Expand by-name parameters and arguments
+           new AugmentScala2Traits, // Expand traits defined in Scala 2.11 to simulate old-style rewritings
+           new ResolveSuper),       // Implement super accessors and add forwarders to trait methods
+      List(new Erasure),            // Rewrite types to JVM model, erasing all type parameters, abstract types and refinements.
+      List(new ElimErasedValueType, // Expand erased value types to their underlying implementation types
+           new VCElideAllocations,  // Peep-hole optimization to eliminate unnecessary value class allocations
+           new Mixin,               // Expand trait fields and trait initializers
+           new LazyVals,            // Expand lazy vals
+           new Memoize,             // Add private fields to getters and setters
+           new LinkScala2ImplClasses, // Forward calls to the implementation classes of traits defined by Scala 2.11
+           new NonLocalReturns,     // Expand non-local returns
+           new CapturedVars,        // Represent vars captured by closures as heap objects
+           new Constructors,        // Collect initialization code in primary constructors
+                                       // Note: constructors changes decls in transformTemplate, no InfoTransformers should be added after it
+           new FunctionalInterfaces,// Rewrites closures to implement @specialized types of Functions.
+           new GetClass),           // Rewrites getClass calls on primitive types.
+      List(new LambdaLift,          // Lifts out nested functions to class scope, storing free variables in environments
+                                       // Note: in this mini-phase block scopes are incorrect. No phases that rely on scopes should be here
+           new ElimStaticThis,      // Replace `this` references to static objects by global identifiers
+           new Flatten,             // Lift all inner classes to package scope
+           new RestoreScopes),      // Repair scopes rendered invalid by moving definitions in prior phases of the group
+      List(new ExpandPrivate,       // Widen private definitions accessed from nested classes
+           new CollectEntryPoints,  // Find classes with main methods
+           new LabelDefs),          // Converts calls to labels to jumps
+      List(new GenSJSIR),           // Generate .js code
+      List(new GenBCode)            // Generate JVM bytecode
+    )
+```
+
+Note that phases are grouped, so the `phases` value is a
+`List[List[Phase]]`. The idea is that all phases in a group are be
+*fused* into a single tree traversal. That way, phases can be kept
+small (most phases perform a single function) without requiring an
+excessive number of tree traversals (which are costly, because they
+have generally bad cache locality).
+
+Phases fall into 4 categories:
+
+ - Frontend phases: `Frontend`, `PostTyper` and `Pickler`. `FrontEnd` parses the source programs and generates
+   untyped abstract syntax trees, which are then typechecked and transformed into typed abstract syntax trees.
+   `PostTyper` performs checks and cleanups that require a fully typed program. In particular, it
+
+     - creates super accessors representing `super` calls in traits
+     - creates implementations of synthetic (compiler-implemented) methods
+     - avoids storing parameters passed unchanged from subclass to superclass in duplicate fields.
+
+   Finally `Pickler` serializes the typed syntax trees produced by the frontend as TASTY data structures.
+
+ - High-level transformations: All phases from `FirstTransform` to `Erasure`. Most of these phases transform
+   syntax trees, expanding high-level constructs to more primitive ones. The last phase in the group, `Erasure`
+   translates all types into types supported directly by the JVM. To do this, it performs another type checking
+   pass, but using the rules of the JVM's type system instead of Scala's.
+
+ - Low-level transformations: All phases from `ElimErasedValueType` to `LabelDefs`. These
+   further transform trees until they are just a structured version of Java bytecode.
+
+ - Code generators: These map the transformed trees to Java classfiles or Javascript files.
+
+