---
layout: developer-doc
title: Enso Debugger Protocol Message Specification
category: debugger
tags: [repl, debugger, protocol, specification]
order: 1
---

# Enso Debugger Protocol Message Specification

Binary Protocol for the Debugger is used in communication between the runtime
and tools exploiting the REPL/debugger functionalities. It can be used to
implement a simple REPL or add debugging capabilities to the editor.

<!-- MarkdownTOC levels="2,3" autolink="true" -->

- [Types](#types)
  - [`ObjectRepr`](#objectrepr)
  - [`StackTraceElement`](#stacktraceelement)
  - [`Exception`](#exception)
  - [`Binding`](#binding)
- [Messages](#messages)
  - [Session Start](#session-start)
  - [Evaluation](#evaluation)
  - [List Bindings](#list-bindings)
  - [Session Exit](#session-exit)

<!-- /MarkdownTOC -->

## Types

There are some helper types used within the debugger's protocol. These are
specified here.

### `ObjectRepr`

External representation of arbitrary values returned by the REPL (internally
these are of type `Object`).

As these values are only used for presentation, they are represented by String.

```typescript
interface ObjectRepr {
  representation: String;
}
```

### `StackTraceElement`

Represents a line of the stack trace. Corresponds to
`java.lang.StackTraceElement`.

```typescript
interface StackTraceElement {
  declaringClass: String;
  methodName: String;
  fileName: String;
  lineNumber: Int;
}
```

### `Exception`

Represents an exception that may have been raised during requested execution.

```typescript
interface Exception {
  message: String;
  stackTrace: [StackTraceElement];
  cause: Exception;
}
```

### `Binding`

Represents a single binding in the current scope.

```typescript
interface Binding {
  name: String;
  value: ObjectRepr;
}
```

## Messages

All endpoints accept messages of type `Request` and return a `Response`. These
messages contain unions that contain the actual payload specified for each type
of message.

```idl
namespace org.enso.polyglot.debugger.protocol;

union RequestPayload {
    EVALUATE: EvaluationRequest,
    LIST_BINDINGS: ListBindingsRequest,
    SESSION_EXIT: SessionExitRequest
}

table Request {
    payload: RequestPayload (required);
}

union ResponsePayload {
    EVALUATION_SUCCESS: EvaluationSuccess,
    EVALUATION_FAILURE: EvaluationFailure,
    LIST_BINDINGS: ListBindingsResult,
    SESSION_START: SessionStartNotification
}

table Response {
    payload: ResponsePayload (required);
}
```

### Session Start

When a breakpoint is reached, the debugger sends a notification to the client
indicating that a REPL session should be started.

The whole REPL session should live inside the endpoint's function handling this
notification. This means that this function should not return before sending the
session exit request.

#### Notification

```idl
namespace org.enso.polyglot.protocol.debugger;

table SessionStartNotification {}
```

### Evaluation

Evaluates an arbitrary expression in the current execution context.

Responds with either a message with the value of successfully evaluated
expression or a message with an exception that has been raised during
evaluation.

#### Request

```idl
namespace org.enso.polyglot.protocol.debugger;

table ReplEvaluationRequest {
  expression: String (required);
}
```

#### Response

```idl
namespace org.enso.polyglot.protocol.debugger;

table ReplEvaluationSuccess {
  result: ObjectRepr (required);
}

table ReplEvaluationFailure {
  exception: Exception (required);
}
```

### List Bindings

Lists all the bindings available in the current execution scope and sends them
back.

#### Request

```idl
namespace org.enso.polyglot.protocol.debugger;

table ReplListBindingsRequest {}
```

#### Response

```idl
namespace org.enso.polyglot.protocol.debugger;

table ReplListBindingsResult {
  bindings: [Binding];
}
```

### Session Exit

Terminates this REPL session (and resumes normal program execution).

The last result of Evaluation will be returned from the instrumented node or if
no expressions have been evaluated, unit is returned.

This request must always be sent at the end of REPL session, as otherwise the
program will never resume. It does not return any response. It is important to
note that a thread calling `sendBinary` with this message will never return, as
control will be passed to the interpreter.

#### Request

```idl
namespace org.enso.polyglot.protocol.debugger;

table ReplExitRequest {}
```