Docs: add ADR010: LSP server

Author: strager

docs/architecture/ADR008-Exceptions.md

@@ -30,3 +30,8 @@ disabling C++ exceptions.
 
 `setjmp`/`longjmp` is foreign to many C++ programmers and is harder to use than
 C++ exceptions.
+
+simdjson's On Demand API exposes parse errors in every API call. This means that
+error handling code needs to be written all over. simdjson's exception-based On
+Demand API would simplify error handling significantly. We haven't determined
+yet whether `setjmp`/`longjmp` can be used.

docs/architecture/ADR010-LSP-server.md

@@ -0,0 +1,46 @@
+# ADR010: LSP server
+
+**Status**: Accepted and active.
+
+## Context
+
+The [Language Server Protocol (LSP)][LSP] makes code editor plugins easier to
+write by standardizing several aspects of editor<->language interaction. In an
+LSP world, there are three major components components:
+
+* editor (language-agnostic)
+* editor plugin (LSP client) (editor-specific; language-specific)
+* language server (LSP server) (editor-agnostic; language-specific)
+
+Developers of language tooling are expected to implement the language server.
+
+[LSP]: https://langserver.org/
+
+## Decision
+
+quick-lint-js develops its own LSP server. It is integrated into the same
+executable as the CLI and is accessible with the `--lsp-server` option.
+
+quick-lint-js develops its own editor plugins. Some of these editor plugins
+utilize the LSP server.
+
+JSON parsing is done with simdjson. JSON serializing is done manually.
+
+## Consequences
+
+LSP has some accidental complexities. For example, locations count the number of
+UTF-16 code units since the beginning of a line. Counting in this way is
+non-trivial for the LSP server and is complicated for some LSP clients (editors)
+too. Non-LSP editor plugins don't necessarily have this complexity;
+quick-lint-js can use the editor's native location scheme without multiple
+layers of translation.
+
+Error handling with simdjson, particularly with its On Demand API, is tedious.
+This is because of [ADR008: Exceptions][ADR008] banning exception in
+quick-lint-js. simdjson's exception-based interface would be much easier to use.
+
+Contrary to what LSP marketing might imply, LSP still requires editor plugins to
+be written for the specific LSP server. Developing and maintaining editor
+plugins for popular editors is still necessary with LSP.
+
+[ADR008]: ./ADR008-Exceptions.md

docs/architecture/README.md

@@ -199,6 +199,7 @@ Nygard][ADR-bible].
 * [ADR006: Location tracking](ADR006-Location-tracking.md)
 * [ADR008: Exceptions](ADR008-Exceptions.md)
 * [ADR009: Website](ADR009-Website.md)
+* [ADR010: LSP server](ADR010-LSP-server.md)
 
 ### Rejected ADRs