source: greatly improve location reporting for multi-line source extracts

compiler/source.py

@@ -1,8 +1,11 @@
 from __future__ import annotations
 
+import math
+from dataclasses import dataclass
+
 from beartype import beartype
 from beartype.typing import Optional
-from dataclasses import dataclass
+
 
 @beartype
 @dataclass
@@ -44,21 +47,35 @@ class SourceLocation:
             source_lines[-1] = source_lines[-1][:self.end.character]
         return "\n".join(source_lines)
 
-    def show_in_source(self) -> str:
-        source = self.source.splitlines(keepends=False)
-        source_line = source[self.begin.line]
-        result = [source_line]
-        if self.begin.line != self.end.line:
-            return "\n".join(result)
-
-        line = " " * self.begin.character
-        line += "^" + "-" * max(0, (self.end.character - self.begin.character - 1))
-        line += " " * (len(source_line) - len(line))
+    @staticmethod
+    def underline(string: str, begin: int, end: int) -> str:
+        assert begin <= end
+        result = [string]
+        line = " " * begin
+        line += "^" + "~" * max(0, (end - begin - 1))
+        line += " " * (len(string) - len(line))
 
         result += [line]
 
         return "\n".join(result)
 
+    def show_in_source(self) -> str:
+        source = self.source.splitlines(keepends=False)
+        line_number_maxlen = int(math.log10(len(source)) + 1)
+        lines = source[self.begin.line:self.end.line + 1]
+        result = []
+        begin_char = self.begin.character
+        while len(lines) > 0:
+            line = lines.pop(0)
+            end_char = self.end.character if len(lines) == 0 else len(line)
+            result += [SourceLocation.underline(line, begin_char, end_char)]
+            begin_char = 0
 
+        for i, source_line in enumerate(result):
+            line_prefix = f"Line {self.begin.line + i:0>{line_number_maxlen}}: "
+            lines = source_line.splitlines(keepends=False)
+            lines[0] = line_prefix + lines[0]
+            lines[1] = " " * len(line_prefix) + lines[1]
+            result[i] = "\n".join(lines)
 
-
+        return "\n".join(result)