refactor(language): name disambiguation

* Use fully qualified names starting with :: (as in C++) to unambiguously refer
  to an element.
* Name shadowing within modules.

3 files changed, 57 insertions, 4 deletions

diff --git a/subprojects/frontend/src/language/problem.grammar b/subprojects/frontend/src/language/problem.grammar
index ac451e7a..e39ce102 100644
--- a/subprojects/frontend/src/language/problem.grammar
+++ b/subprojects/frontend/src/language/problem.grammar
@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2021-2023 The Refinery Authors <https://refinery.tools/>
+ * SPDX-FileCopyrightText: 2021-2024 The Refinery Authors <https://refinery.tools/>
  *
  * SPDX-License-Identifier: EPL-2.0
  */
@@ -22,6 +22,9 @@
 @top Problem { statement* }
 
 statement {
+  ImportStatement {
+    kw<"import"> QualifiedName (kw<"as"> identifier)? "."
+  } |
   Assertion {
     kw<"default">? (NotOp | UnknownOp)? RelationName
     ParameterList<AssertionArgument>
@@ -170,7 +173,7 @@ VariableName[@dynamicPrecedence=10] { QualifiedName ~name }
 
 NodeName { QualifiedName }
 
-QualifiedName[implicitCompletion=true] { identifier ("::" identifier)* }
+QualifiedName[implicitCompletion=true] { "::"? identifier (QualifiedNameSeparator "::" identifier)* }
 
 kw<term> { @specialize[@name={term},implicitCompletion=true]<identifier, term> }
 
@@ -184,6 +187,10 @@ sep1<separator, content> { content (separator content)* }
 
 @skip { LineComment | BlockComment | whitespace }
 
+@external tokens qualifiedNameSeparator from "./tokens" {
+  QualifiedNameSeparator
+}
+
 @tokens {
   whitespace { std.whitespace+ }
 
diff --git a/subprojects/frontend/src/language/problemLanguageSupport.ts b/subprojects/frontend/src/language/problemLanguageSupport.ts
index 2483c2e3..aebccfa4 100644
--- a/subprojects/frontend/src/language/problemLanguageSupport.ts
+++ b/subprojects/frontend/src/language/problemLanguageSupport.ts
@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2021-2023 The Refinery Authors <https://refinery.tools/>
+ * SPDX-FileCopyrightText: 2021-2024 The Refinery Authors <https://refinery.tools/>
  *
  * SPDX-License-Identifier: EPL-2.0
  */
@@ -28,7 +28,7 @@ const parserWithMetadata = parser.configure({
       LineComment: t.lineComment,
       BlockComment: t.blockComment,
       'module problem class enum pred fn scope': t.definitionKeyword,
-      'declare atom multi': t.definitionKeyword,
+      'import as declare atom multi': t.definitionKeyword,
       'abstract extends refers contains container opposite': t.modifier,
       'default error contained containment': t.modifier,
       'true false unknown error': t.keyword,
diff --git a/subprojects/frontend/src/language/tokens.ts b/subprojects/frontend/src/language/tokens.ts
new file mode 100644
index 00000000..b5d022f1
--- /dev/null
+++ b/subprojects/frontend/src/language/tokens.ts
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2018-2024 by Marijn Haverbeke <marijnh@gmail.com> and others
+ * Copyright (C) 2024 The Refinery Authors <https://refinery.tools/>
+ *
+ * SPDX-License-Identifier: MIT AND EPL-2.0
+ *
+ * Based on the CSS tokenizer at
+ * https://github.com/lezer-parser/css/blob/790568c968a660a94bf0fbd97a86c66da1c529e5/src/tokens.js
+ */
+
+import { ExternalTokenizer } from '@lezer/lr';
+
+/* eslint-disable-next-line import/no-unresolved --
+  Synthetic import from `@lezer/generator/rollup` cannot be found by ESLint.
+*/
+import { QualifiedNameSeparator } from './problem.grammar.terms';
+
+const colon = 58;
+const underscore = 95;
+
+function isAlpha(ch: number) {
+  return (ch >= 65 && ch <= 90) || (ch >= 97 && ch <= 122) || ch >= 161;
+}
+
+function isDigit(ch: number) {
+  return ch >= 48 && ch <= 57;
+}
+
+function isIdentifier(ch: number) {
+  return isAlpha(ch) || isDigit(ch) || ch === underscore;
+}
+
+/* eslint-disable-next-line import/prefer-default-export --
+  Lezer requires a named export.
+*/
+export const qualifiedNameSeparator = new ExternalTokenizer((input) => {
+  if (input.peek(0) === colon && input.peek(1) === colon) {
+    const previous = input.peek(-1);
+    if (isIdentifier(previous)) {
+      // Inject an extra 0-length token into the token stream to let Lezer
+      // consume the `::` on its own. Explicitly consuming 2 characters here
+      // leads to inconsistent highlighting of qualified names.
+      input.acceptToken(QualifiedNameSeparator);
+    }
+  }
+});