carbon-lang/explorer/README.md

# Explorer

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Part of the Carbon Language project, under the Apache License v2.0 with LLVM
Exceptions. See /LICENSE for license information.
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`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`](/toolchain/) directory for a separate implementation that's
focused on the needs of Carbon users.

## Overview

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

## Programming conventions

The class hierarchies in `explorer` are built to support
[LLVM-style RTTI](https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html), 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*>`](common/nonnull.h) 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`](common/arena.h) 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 `Arena`s 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`](common/error_builders.h). Errors caused by bugs
in `explorer` itself should be reported with
[`CHECK` or `FATAL`](../common/check.h).

### `Decompose` functions

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.

## Example Programs (Regression Tests)

The [`testdata/`](testdata/) subdirectory includes some example programs with
expected output.

These tests make use of LLVM's
[lit](https://llvm.org/docs/CommandGuide/lit.html) and
[FileCheck](https://llvm.org/docs/CommandGuide/FileCheck.html). Tests have
boilerplate at the top:

```carbon
// 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
// RUN: %{explorer-run}
// RUN: %{explorer-run-trace}
// CHECK:result: 0

package ExplorerTest api;
```

To explain this boilerplate:

-   The standard copyright is expected.
-   The `AUTOUPDATE` line indicates that `RUN` and `CHECK` lines will be
    automatically inserted immediately below by the `./lit_autoupdate.py`
    script.
-   The `RUN` lines indicate two commands for `lit` to execute using the file:
    one without trace and debug output, one with.
    -   `RUN:` will be followed by the `not` command when failure is expected.
        In particular, `RUN: not explorer ...`.
    -   The full command is in `lit.cfg.py`; it will run explorer and pass
        results to
        [`FileCheck`](https://llvm.org/docs/CommandGuide/FileCheck.html).
-   The `CHECK` lines indicate expected output, verified by `FileCheck`.
    -   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.

### Useful commands

-   `./lit_autodupate.py` -- 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 run testdata/DIR/FILE.carbon.run` -- Runs explorer on the file.

### Updating fuzzer logic after making AST changes

Please refer to
[Fuzzer documentation](https://github.com/carbon-language/carbon-lang/blob/trunk/explorer/fuzzing/README.md).

## Trace Program Execution

When tracing is turned on (using the `--trace_file=...` option), `explorer`
prints the state of the program and each step that is performed during
execution.

### State of the Program

The state of the program is printed in the following format, which consists of
two components: (1) a stack of actions and (2) a memory.

    {
    stack: action1 ## action2 ## ...
    memory: 0: valueA, 1: valueB, 2: valueC, ...
    }

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,
for example, for local variables. When an address is deallocated, it stays in
memory but `!!` is printed before its value.

The stack is list of actions separated by double pound signs (`##`). Each action
has the format:

    syntax .position. [[ results ]] { scope }

which can have up to four parts.

1. The `syntax` for the part of the program to be executed such as an expression
   or statement.
2. The `position` of execution (an integer) for this action (each action can
   take multiple steps to complete).
3. The `results` from subexpressions of this part.
4. The `scope` is 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`.

### Step of Execution

Each step of execution is printed in the following format:

    --- step kind syntax .position. (file-location) --->

-   The `syntax` is the part of the program being executed.
-   The `kind` is the syntactic category of the part, such as `exp`, `stmt`, or
    `decl`.
-   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.