explorer is an implementation of Carbon whose primary purpose is to act as a clear specification of the language. As an extension of that goal, it can also be used as a platform for prototyping and validating changes to the language. Consequently, it prioritizes straightforward, readable code over performance, diagnostic quality, and other conventional implementation priorities. In other words, its intended audience is people working on the design of Carbon, and it is not intended for real-world Carbon programming on any scale. See the toolchain directory for a separate implementation that's focused on the needs of Carbon users.

explorer represents Carbon code using an abstract syntax tree (AST), which is defined in the ast directory. The syntax directory contains lexer and parser, which define how the AST is generated from Carbon code. The interpreter directory contains the remainder of the implementation.

explorer is an interpreter rather than a compiler, although it attempts to separate compile time from run time, since that separation is an important constraint on Carbon's design.

The class hierarchies in explorer are built to support LLVM-style RTTI, and define a kind accessor that returns an enum identifying the concrete type. explorer typically relies less on virtual dispatch, and more on using kind as the key of a switch and then down-casting in the individual cases. As a result, adding a new derived class to a hierarchy requires updating existing code to handle it. It is generally better to avoid defining default cases for RTTI switches, so that the compiler can help ensure the code is updated when a new type is added.

explorer never uses plain pointer types directly. Instead, we use the Nonnull<T*> alias for pointers that are not nullable, or std::optional<Nonnull<T*>> for pointers that are nullable.

Many of the most commonly-used objects in explorer have lifetimes that are tied to the lifespan of the entire Carbon program. We manage the lifetimes of those objects by allocating them through an Arena object, which can allocate objects of arbitrary types, and retains ownership of them. As of this writing, all of explorer uses a single Arena object, we may introduce multiple Arenas for different lifetime groups in the future.

For simplicity, explorer generally treats all errors as fatal. Errors caused by bugs in the user-provided Carbon code should be reported with the error builders in error_builders.h. Errors caused by bugs in explorer itself should be reported with CHECK or FATAL.

Many of explorer's data structures provide a Decompose method, which allows simple data types to be generically decomposed into their fields. The Decompose function for a type takes a function and calls it with the fields of that type. For example:

class MyType {
 public:
  MyType(Type1 arg1, Type2 arg2) : arg1_(arg1), arg2_(arg2) {}

  template <typename F>
  auto Decompose(F f) const { return f(arg1_, arg2_); }

 private:
  Type1 arg1_;
  Type2 arg2_;
};

Where possible, a value equivalent to the original value should be created by passing the given arguments to the constructor of the type. For example, my_value.Decompose([](auto ...args) { return MyType(args...); }) should recreate the original value.

The testdata/ subdirectory includes some example programs with expected output.

These tests make use of GoogleTest with Bazel's cc_test rules. Tests have boilerplate at the top:

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// AUTOUPDATE
// CHECK:STDOUT: result: 7

package ExplorerTest api;

fn Main() -> i32 {
  return (1 + 2) + 4;
}

To explain this boilerplate:

• The standard copyright is expected.
• The AUTOUPDATE line indicates that CHECK lines matching the output will be automatically inserted immediately below by the ./autoupdate_testdata.sh script.
• The CHECK lines indicate expected output.
  • Where a CHECK line contains text like {{.*}}, the double curly braces indicate a contained regular expression.
• The package is required in all test files, per normal Carbon syntax rules.

• ./autoupdate_testdata.sh -- Updates expected output.
  • This can be combined with git diff to see changes in output.
• bazel test ... --test_output=errors -- Runs tests and prints any errors.
• bazel test //explorer:file_test.subset --test_arg=explorer/testdata/DIR/FILE.carbon -- Runs a specific test.
• bazel run testdata/DIR/FILE.carbon.run -- Runs explorer on the file.
• bazel run testdata/DIR/FILE.carbon.verbose -- Runs explorer on the file with tracing enabled.

Please refer to Fuzzer documentation.

Explorer's Trace Output refers to a detailed record of program phases and their internal processes a program goes through when executed using the explorer. It also records things like changes in memory and action stack that describes the state of the program.

Tracing can be turned on using the --trace_file=... option. Explorer tests can be run with tracing enabled by using the <testname>.verbose test target.

By default, explorer prints the state of the program and each step that is performed during execution for the file containing the main function when tracing is enabled. Tracing for different phases and file contexts can be selected using filtering that is explained below.

Printing directly to the standard output using the --trace_file option is supported by passing - in place of a filepath (--trace_file=-).

Trace output can be filtered based on either program phase or file context.

Trace output can be filtered by selecting program phases and file contexts for which tracing should be enabled. The -trace_phase=... option is used to select program phases, while the -trace_file_context=... option is used to select file contexts.

The following options can be passed as a comma-separated list to the -trace_phase=... option to select program phases:

• source_program: Includes trace output for the source program phase.
• name_resolution: Includes trace output for the name resolution phase.
• control_flow_resolution: Includes trace output for the control flow resolution phase.
• type_checking: Includes trace output for the type checking phase.
• unformed_variables_resolution: Includes trace output for the unformed variables resolution phase.
• declarations: Includes trace output for printing declarations.
• execution: Includes trace output for program execution.
• timing: Includes timing logs indicating the time taken by each phase.
• all: Includes trace output for all phases.
• By default, tracing is only enabled for the execution phase.

The following options can be passed as a comma-separated list to the -trace_file_context=... option to select file contexts:

• main: Includes trace output for the file containing the main function.
• prelude: Includes trace output for the prelude.
• import: Includes trace output for imports.
• include: Includes trace output for all.
• By default, tracing is only enabled for the main file context.

Note (for developers): Two RAII classes SetProgramPhase and SetFileContext are provided for setting program phase and file context dynamically in the code.

The state of the program is represented by the memory and the stack. The memory is a mapping of addresses to values, and the stack is a list of actions.

The state of the program is constantly changing as the program executes. The memory is updated as objects are allocated and deallocated, and the stack is updated as actions are performed. The state of the program can be used to track the progress of the program and to debug the program.

The memory is a mapping of addresses to values. The memory is used to represent both heap-allocated objects and also mutable parts of the procedure call stack.

1. Memory Allocation is printed as

++# memory-alloc: #<allocation_index> `value` uninitialized?

2. Read Memory is printed as

<-- memory-read: #<allocation_index> `value`

3. Write Memory is printed as

--> memory-write: #<allocation_index> `value`

4. Memory Deallocation is printed as

--# memory-dealloc: #<allocation_index> `value`

allocation_index is used for locating an object within the heap. value represents the object inside heap that is accessed using allocation_index

The stack is list of actions, push and pop changes in the stack are printed in the following format

>[] stack-push: <action> (<source location>)
<[] stack-pop:  <action> (<source location>)

action is printed in the following format

ActionKind pos: <pos_count> `<syntax>` results: [<collected_results>]  scope: [<scope>]

1. ActionKind: The kind of an action. Examples: ExpressionAction, DeclarationAction, etc.
2. pos_count: The position of execution (an integer) for this action. Each action can take multiple steps to complete.
3. syntax: The syntax for the part of the program to be executed, such as an expression or statement.
4. collected_results: The results from subexpressions of this part.
5. scope: The variables whose lifetimes are associated with this part of the program.

The stack always begins with a function call to Main.

In the special case of a function call, when the function call finishes, the result value appears at the end of the results.

Each step of execution is printed in the following format:

->> step ActionKind pos: position syntax (<file-location>) --->

• The syntax is the part of the program being executed.
• The ActionKind is the kind of action for which the step is executed.
• The position says how far along explorer is in executing this action.
• The file-location gives the filename and line number for the syntax.

Each step of execution can push new actions on the stack, pop actions, increment the position number of an action, and add result values to an action.

When including syntax or code within trace messages, it should be wrapped appropriately to maintain clarity and differentiation between code elements and regular text in the trace output.

• For single-line code or syntax, use single backticks.
• For multiline code blocks, use triple backticks (```) to enclose the code.

Examples:

For single line code:
`let x: i32 = 0;`

For multi line code:
```
fn Main() -> i32 {
    return 0;
}
```

Each line of trace output starts with a prefix that indicates the nature of the information being presented. These prefixes are added using specific formatting methods in the TraceStream class.

Example usage:

trace_stream->PrefixMethod() << ... ;

The TraceStream class also have utility methods for adding formatted headings and subheadings to the trace output. These methods help structure the trace information and provide visual separation for different sections.

Heading(...) method prints the heading in following format:

* * * * * * * * * *  Heading * * * * * * * * * *
------------------------------------------------

SubHeading(...) method prints the heading in the following format:

- - - - -  Sub Heading - - - - -
--------------------------------