From 8e4b4af60cf43fb3b288c6c8843c20897b61f860 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jonatan=20K=C5=82osko?= <jonatanklosko@gmail.com>
Date: Fri, 22 Jan 2021 23:27:25 +0100
Subject: [PATCH] Add Elixir language support to the editor (#13)

---
 .../editor/elixir/language_configuration.js   |  37 ++++++
 .../on_type_formatting_edit_provider.js       | 118 ++++++++++++++++++
 assets/js/editor/monaco.js                    |  17 ++-
 3 files changed, 171 insertions(+), 1 deletion(-)
 create mode 100644 assets/js/editor/elixir/language_configuration.js
 create mode 100644 assets/js/editor/elixir/on_type_formatting_edit_provider.js

diff --git a/assets/js/editor/elixir/language_configuration.js b/assets/js/editor/elixir/language_configuration.js
new file mode 100644
index 000000000..d3dc42bf8
--- /dev/null
+++ b/assets/js/editor/elixir/language_configuration.js
@@ -0,0 +1,37 @@
+/**
+ * Defines Elixir traits to enable various editor features,
+ * like automatic bracket insertion and indentation.
+ */
+const ElixirLanguageConfiguration = {
+  comments: {
+    lineComment: "#",
+  },
+  brackets: [
+    ["{", "}"],
+    ["[", "]"],
+    ["(", ")"],
+  ],
+  surroundingPairs: [
+    ["{", "}"],
+    ["[", "]"],
+    ["(", ")"],
+    ["'", "'"],
+    ['"', '"'],
+  ],
+  autoClosingPairs: [
+    { open: "'", close: "'", notIn: ["string", "comment"] },
+    { open: '"', close: '"', notIn: ["comment"] },
+    { open: '"""', close: '"""' },
+    { open: "`", close: "`", notIn: ["string", "comment"] },
+    { open: "(", close: ")" },
+    { open: "{", close: "}" },
+    { open: "[", close: "]" },
+    { open: "<<", close: ">>" },
+  ],
+  indentationRules: {
+    increaseIndentPattern: /^\s*(after|else|catch|rescue|fn|[^#]*(do|<\-|\->|\{|\[|\=))\s*$/,
+    decreaseIndentPattern: /^\s*((\}|\])\s*$|(after|else|catch|rescue|end)\b)/,
+  },
+};
+
+export default ElixirLanguageConfiguration;
diff --git a/assets/js/editor/elixir/on_type_formatting_edit_provider.js b/assets/js/editor/elixir/on_type_formatting_edit_provider.js
new file mode 100644
index 000000000..b14c86c20
--- /dev/null
+++ b/assets/js/editor/elixir/on_type_formatting_edit_provider.js
@@ -0,0 +1,118 @@
+/**
+ * Defines custom auto-formatting behavior for Elixir.
+ *
+ * The provider is triggered when the user makes edits
+ * and it may instruct the editor to apply some additional changes.
+ */
+const ElixirOnTypeFormattingEditProvider = {
+  autoFormatTriggerCharacters: ["\n"],
+  provideOnTypeFormattingEdits(model, position, char, options, token) {
+    if (char === "\n") {
+      return closingEndTextEdits(model, position);
+    }
+
+    return [];
+  },
+};
+
+function closingEndTextEdits(model, position) {
+  const lines = model.getLinesContent();
+  const lineIndex = position.lineNumber - 1;
+  const line = lines[lineIndex];
+  const prevLine = lines[lineIndex - 1];
+  const prevIndentation = indentation(prevLine);
+
+  if (shouldInsertClosingEnd(lines, lineIndex)) {
+    // If this is the last line or the line is not empty,
+    // we have to insert a newline at the current position.
+    // Otherwise we prefer to explicitly insert the closing end
+    // in the next line, as it preserves current cursor position.
+    const shouldInsertInNextLine =
+      position.lineNumber < lines.length && isBlank(line);
+
+    const textEdit = insertClosingEndTextEdit(
+      position,
+      prevIndentation,
+      shouldInsertInNextLine
+    );
+
+    return [textEdit];
+  }
+
+  return [];
+}
+
+function shouldInsertClosingEnd(lines, lineIndex) {
+  const prevLine = lines[lineIndex - 1];
+  const prevIndentation = indentation(prevLine);
+  const prevTokens = tokens(prevLine);
+
+  if (
+    last(prevTokens) === "do" ||
+    (prevTokens.includes("fn") && last(prevTokens) === "->")
+  ) {
+    const nextLineWithSameIndentation = lines
+      .slice(lineIndex + 1)
+      .filter(line => !isBlank(line))
+      .find((line) => indentation(line) === prevIndentation);
+
+    if (nextLineWithSameIndentation) {
+      const [firstToken] = tokens(nextLineWithSameIndentation);
+
+      if (["after", "else", "catch", "rescue", "end"].includes(firstToken)) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  return false;
+}
+
+function insertClosingEndTextEdit(
+  position,
+  indentation,
+  shouldInsertInNextLine
+) {
+  if (shouldInsertInNextLine) {
+    return {
+      range: new monaco.Range(
+        position.lineNumber + 1,
+        1,
+        position.lineNumber + 1,
+        1
+      ),
+      text: `${indentation}end\n`,
+    };
+  } else {
+    return {
+      range: new monaco.Range(
+        position.lineNumber,
+        position.column,
+        position.lineNumber,
+        position.column
+      ),
+      text: `\n${indentation}end`,
+    };
+  }
+}
+
+function indentation(line) {
+  const [indentation] = line.match(/^\s*/);
+  return indentation;
+}
+
+function tokens(line) {
+  return line.replace(/#.*/, "").match(/->|[\w:]+/g) || [];
+}
+
+function last(list) {
+  return list[list.length - 1];
+}
+
+function isBlank(string) {
+  return string.trim() === "";
+}
+
+export default ElixirOnTypeFormattingEditProvider;
diff --git a/assets/js/editor/monaco.js b/assets/js/editor/monaco.js
index ad5cdb4ca..85e528996 100644
--- a/assets/js/editor/monaco.js
+++ b/assets/js/editor/monaco.js
@@ -1,5 +1,20 @@
 import * as monaco from "monaco-editor/esm/vs/editor/editor.api";
+import ElixirLanguageConfiguration from "./elixir/language_configuration";
+import ElixirOnTypeFormattingEditProvider from "./elixir/on_type_formatting_edit_provider";
 
-// TODO: add Elixir language definition
+// Register the Elixir language and add relevant configuration
+monaco.languages.register({ id: "elixir" });
+
+monaco.languages.setLanguageConfiguration(
+  "elixir",
+  ElixirLanguageConfiguration
+);
+
+monaco.languages.registerOnTypeFormattingEditProvider(
+  "elixir",
+  ElixirOnTypeFormattingEditProvider
+);
+
+// TODO: add Monarch tokenizer for syntax highlighting
 
 export default monaco;