Add backslash line continuation support in parser preprocessor

Author: ad-si

src/lib.rs

@@ -1693,6 +1693,13 @@ pub fn insert_statement_separators(input: &str) -> String {
       continue;
     }
 
+    // Handle line continuation: backslash followed by newline
+    if ch == '\\' && i + 1 < len && chars[i + 1] == '\n' {
+      // Skip both the backslash and the newline — the next line continues this one
+      i += 2;
+      continue;
+    }
+
     // Track nesting depth (including <| |> for associations)
     if ch == '<' && i + 1 < len && chars[i + 1] == '|' {
       depth += 1;

tests/interpreter_tests/syntax.rs

@@ -3701,3 +3701,35 @@ mod tilde_infix {
     );
   }
 }
+
+mod line_continuation {
+  use super::*;
+
+  #[test]
+  fn backslash_newline_in_definition() {
+    // Backslash at end of line continues the expression on the next line
+    assert_eq!(interpret("f[x_] :=\\\n  x^2\nf[5]").unwrap(), "25");
+  }
+
+  #[test]
+  fn backslash_newline_in_expression() {
+    assert_eq!(interpret("1 +\\\n2 +\\\n3").unwrap(), "6");
+  }
+
+  #[test]
+  fn backslash_newline_preserves_function_def() {
+    assert_eq!(
+      interpret(
+        "ImaginaryQ[u_] :=\\\n  Head[u]===Complex && Re[u]===0\nImaginaryQ[3 I]"
+      )
+      .unwrap(),
+      "True"
+    );
+  }
+
+  #[test]
+  fn backslash_newline_not_in_strings() {
+    // Backslash inside strings should NOT be treated as line continuation
+    assert_eq!(interpret(r#""hello\nworld""#).unwrap(), r#"hello\nworld"#);
+  }
+}