SPICE-0028: Add support for multi-line string line continuations (#1507)

SPICE: https://github.com/apple/pkl-evolution/pull/31

docs/modules/language-reference/pages/index.adoc

@@ -255,8 +255,7 @@ String literals are enclosed in double quotes:
 "Hello, World!"
 ----
 
-TIP: Except for a few minor differences footnote:[Pkl's string literals have fewer character escape sequences,
-have stricter rules for line indentation in multiline strings, and do not have a line continuation character.],
+TIP: Except for a few minor differences footnote:[Pkl's string literals have fewer character escape sequences and stricter rules for line indentation in multiline strings.],
 String literals have the same syntax and semantics as in Swift 5. Learn one of them, know both of them!
 
 Inside a string literal, the following character escape sequences have special meaning:
@@ -362,6 +361,23 @@ str = """
   """
 ----
 
+To prevent line breaks from becoming part of the string's value, use a backslash (`\`) to end those lines.
+
+[source%tested,{pkl}]
+----
+str = """
+  Although the Dodo is extinct, \
+  the species will be remembered.
+  """
+----
+
+This multiline string is equivalent to the following single-line string:
+
+[source%parsed,{pkl-expr}]
+----
+"Although the Dodo is extinct, the species will be remembered."
+----
+
 [[custom-string-delimiters]]
 === Custom String Delimiters
 

pkl-core/src/main/java/org/pkl/core/util/SyntaxHighlighter.java

@@ -18,6 +18,7 @@ package org.pkl.core.util;
 import static org.pkl.parser.Token.FALSE;
 import static org.pkl.parser.Token.NULL;
 import static org.pkl.parser.Token.STRING_ESCAPE_BACKSLASH;
+import static org.pkl.parser.Token.STRING_ESCAPE_CONTINUATION;
 import static org.pkl.parser.Token.STRING_ESCAPE_NEWLINE;
 import static org.pkl.parser.Token.STRING_ESCAPE_QUOTE;
 import static org.pkl.parser.Token.STRING_ESCAPE_RETURN;
@@ -41,7 +42,8 @@ public final class SyntaxHighlighter {
           STRING_ESCAPE_RETURN,
           STRING_ESCAPE_QUOTE,
           STRING_ESCAPE_BACKSLASH,
-          STRING_ESCAPE_UNICODE);
+          STRING_ESCAPE_UNICODE,
+          STRING_ESCAPE_CONTINUATION);
 
   private static final EnumSet<Token> constant = EnumSet.of(TRUE, FALSE, NULL);
 

pkl-core/src/test/files/LanguageSnippetTests/input/basic/stringMultilineContinuation.pkl

@@ -0,0 +1,21 @@
+amends "../snippetTest.pkl"
+
+examples {
+  ["string continuation"] {
+    """
+    hello \
+    world
+    """
+
+    #"""
+    hello \#
+    world
+    """#
+  
+    """
+    hello \
+    \
+    world
+    """
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidCharacterEscape2.pkl

@@ -0,0 +1 @@
+res1 = "xxx\ xxx"

pkl-core/src/test/files/LanguageSnippetTests/input/errors/parser19.pkl

@@ -0,0 +1,5 @@
+foo = """
+  hello \
+\
+  world
+  """

pkl-core/src/test/files/LanguageSnippetTests/input/errors/parser20.pkl

@@ -0,0 +1,2 @@
+foo = "hello \
+world"

pkl-core/src/test/files/LanguageSnippetTests/input/errors/parser21.pkl

@@ -0,0 +1,5 @@
+foo =
+  """
+  hello \   
+  world
+  """

pkl-core/src/test/files/LanguageSnippetTests/output/basic/stringMultilineContinuation.pcf

@@ -0,0 +1,7 @@
+examples {
+  ["string continuation"] {
+    "hello world"
+    "hello world"
+    "hello world"
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidCharacterEscape.err

@@ -1,7 +1,7 @@
 –– Pkl Error ––
 Invalid character escape sequence `\a`.
 
-Valid character escape sequences are:  \n  \r  \t  \"  \\
+Valid character escape sequences are:  \n  \r  \t  \"  \\ \<newline>
 
 x | res1 = "xxx\axxx"
                ^^

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidCharacterEscape2.err

@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Invalid character escape sequence `\ `.
+
+Valid character escape sequences are:  \n  \r  \t  \"  \\ \<newline>
+
+x | res1 = "xxx\ xxx"
+               ^^
+at invalidCharacterEscape2 (file:///$snippetsDir/input/errors/invalidCharacterEscape2.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/parser19.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Line must match or exceed indentation of the String's last line.
+
+x | \
+    ^
+at parser19 (file:///$snippetsDir/input/errors/parser19.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/parser20.err

@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Invalid line continuation escape sequence.
+
+Line continuations are only allowed in multi-line strings.
+
+x | foo = "hello \
+                 ^^
+at parser20 (file:///$snippetsDir/input/errors/parser20.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/parser21.err

@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Invalid line continuation escape sequence.
+
+Whitespace between the continuation escape and following newline is not allowed.
+
+x | hello \
+          ^^^^^
+at parser21 (file:///$snippetsDir/input/errors/parser21.pkl)

pkl-formatter/src/main/java/org/pkl/formatter/Builder.java

@@ -75,6 +75,11 @@ final class Builder {
           OPERATOR ->
           new Text(node.text(source));
       case STRING_NEWLINE -> mustForceLine();
+      case STRING_CONTINUATION -> {
+        var escape = node.text(source);
+        yield new Nodes(
+            List.of(new Text(escape.substring(0, escape.length() - 1)), mustForceLine()));
+      }
       case MODULE_DECLARATION -> formatModuleDeclaration(node);
       case MODULE_DEFINITION -> formatModuleDefinition(node);
       case SINGLE_LINE_STRING_LITERAL_EXPR -> formatSingleLineString(node);
@@ -922,7 +927,9 @@ final class Builder {
       if (elem.type == NodeType.TERMINAL && text(elem).endsWith("(")) {
         isInStringInterpolation = true;
       }
-      result.add(format(elem));
+      var formatted = format(elem);
+      if (formatted instanceof Nodes formattedNodes) result.addAll(formattedNodes.nodes());
+      else result.add(formatted);
       prev = elem;
     }
     return result;

pkl-formatter/src/test/files/FormatterSnippetTests/input/multi-line-strings.pkl

@@ -48,6 +48,26 @@ quux {
 		"""
 }
 
+// line continuations
+corge {
+
+  """
+  hello \
+  world
+  """
+
+#"""
+hello \#
+world
+"""#
+
+"""
+hello \
+\
+world
+"""
+}
+
 obj {
     data {
       ["bar"] = """

pkl-formatter/src/test/files/FormatterSnippetTests/output/multi-line-strings.pkl

@@ -52,6 +52,25 @@ quux {
   """
 }
 
+// line continuations
+corge {
+  """
+  hello \
+  world
+  """
+
+  #"""
+  hello \#
+  world
+  """#
+
+  """
+  hello \
+  \
+  world
+  """
+}
+
 obj {
   data {
     ["bar"] =

pkl-parser/src/main/java/org/pkl/parser/GenericParserImpl.java

@@ -978,6 +978,8 @@ class GenericParserImpl {
             STRING_ESCAPE_RETURN,
             STRING_ESCAPE_UNICODE ->
             children.add(make(NodeType.STRING_ESCAPE, next().span));
+        case STRING_ESCAPE_CONTINUATION ->
+            throw parserError("invalidLineContinuationEscapeSequence");
         case INTERPOLATION_START -> {
           children.add(makeTerminal(next()));
           ff(children);
@@ -1011,6 +1013,8 @@ class GenericParserImpl {
           }
         }
         case STRING_NEWLINE -> children.add(make(NodeType.STRING_NEWLINE, next().span));
+        case STRING_ESCAPE_CONTINUATION ->
+            children.add(make(NodeType.STRING_CONTINUATION, next().span));
         case STRING_ESCAPE_NEWLINE,
             STRING_ESCAPE_TAB,
             STRING_ESCAPE_QUOTE,
@@ -1060,7 +1064,8 @@ class GenericParserImpl {
           throw parserError(ErrorMessages.create("stringIndentationMustMatchLastLine"), child.span);
         }
       }
-      previousNewline = child.type == NodeType.STRING_NEWLINE;
+      previousNewline =
+          child.type == NodeType.STRING_NEWLINE || child.type == NodeType.STRING_CONTINUATION;
     }
   }
 

pkl-parser/src/main/java/org/pkl/parser/Lexer.java

@@ -460,6 +460,22 @@ public final class Lexer {
         yield Token.INTERPOLATION_START;
       }
       case 'u' -> lexUnicodeEscape();
+      case '\n' -> Token.STRING_ESCAPE_CONTINUATION;
+      case ' ', '\t' -> {
+        var c = cursor;
+        var next = nextChar();
+        while (next == ' ' || next == '\t') next = nextChar();
+        if (next == '\n')
+          throw lexError(
+              ErrorMessages.create("invalidLineContinuationEscapeSequenceWhitespace"),
+              c - 2,
+              cursor - c + 2);
+
+        throw lexError(
+            ErrorMessages.create("invalidCharacterEscapeSequence", "\\" + (char) ch, "\\"),
+            c - 2,
+            2);
+      }
       default ->
           throw lexError(
               ErrorMessages.create("invalidCharacterEscapeSequence", "\\" + (char) ch, "\\"),

pkl-parser/src/main/java/org/pkl/parser/ParserImpl.java

@@ -1131,6 +1131,8 @@ final class ParserImpl {
           end = tk.span;
           builder.append(parseUnicodeEscape(tk));
         }
+        case STRING_ESCAPE_CONTINUATION ->
+            throw parserError("invalidLineContinuationEscapeSequence");
         case INTERPOLATION_START -> {
           var istart = next().span;
           if (!builder.isEmpty()) {
@@ -1167,7 +1169,8 @@ final class ParserImpl {
             STRING_ESCAPE_QUOTE,
             STRING_ESCAPE_BACKSLASH,
             STRING_ESCAPE_RETURN,
-            STRING_ESCAPE_UNICODE ->
+            STRING_ESCAPE_UNICODE,
+            STRING_ESCAPE_CONTINUATION ->
             stringTokens.add(new TempNode(next(), null));
         case INTERPOLATION_START -> {
           var istart = next();
@@ -1236,6 +1239,7 @@ final class ParserImpl {
             builder.append('\n');
             isNewLine = true;
           }
+          case STRING_ESCAPE_CONTINUATION -> isNewLine = true;
           case STRING_PART -> {
             var text = token.text(lexer);
             if (isNewLine) {
@@ -1642,6 +1646,7 @@ final class ParserImpl {
       case STRING_ESCAPE_BACKSLASH -> "\\";
       case STRING_ESCAPE_TAB -> "\t";
       case STRING_ESCAPE_RETURN -> "\r";
+      case STRING_ESCAPE_CONTINUATION -> "";
       case STRING_ESCAPE_UNICODE -> parseUnicodeEscape(tk);
       default -> throw new RuntimeException("Unreacheable code");
     };

pkl-parser/src/main/java/org/pkl/parser/Token.java

@@ -129,6 +129,7 @@ public enum Token {
   STRING_ESCAPE_QUOTE,
   STRING_ESCAPE_BACKSLASH,
   STRING_ESCAPE_UNICODE,
+  STRING_ESCAPE_CONTINUATION,
   STRING_END,
   STRING_PART;
 

pkl-parser/src/main/java/org/pkl/parser/syntax/generic/NodeType.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -70,6 +70,7 @@ public enum NodeType {
   STRING_CHARS,
   OPERATOR,
   STRING_NEWLINE,
+  STRING_CONTINUATION,
   STRING_ESCAPE,
 
   // members

pkl-parser/src/main/resources/org/pkl/parser/errorMessages.properties

@@ -44,13 +44,23 @@ The separator character (`_`) cannot follow `0x`, `0b`, `.`, `e`, or 'E' in a nu
 invalidCharacterEscapeSequence=\
 Invalid character escape sequence `{0}`.\n\
 \n\
-Valid character escape sequences are:  {1}n  {1}r  {1}t  {1}"  {1}\\
+Valid character escape sequences are:  {1}n  {1}r  {1}t  {1}"  {1}\\ {1}<newline>
 
 invalidUnicodeEscapeSequence=\
 Invalid Unicode escape sequence `{0}`.\n\
 \n\
 Valid Unicode escape sequences are {1}'{'0'}' to {1}'{'10FFFF'}' (1-6 hexadecimal characters).
 
+invalidLineContinuationEscapeSequence=\
+Invalid line continuation escape sequence.\n\
+\n\
+Line continuations are only allowed in multi-line strings.
+
+invalidLineContinuationEscapeSequenceWhitespace=\
+Invalid line continuation escape sequence.\n\
+\n\
+Whitespace between the continuation escape and following newline is not allowed.
+
 missingDelimiter=\
 Missing `{0}` delimiter.
 

pkl-parser/src/test/kotlin/org/pkl/parser/GenericSexpRenderer.kt

@@ -197,6 +197,7 @@ class GenericSexpRenderer(code: String) {
         NodeType.TERMINAL,
         NodeType.OPERATOR,
         NodeType.STRING_NEWLINE,
+        NodeType.STRING_CONTINUATION,
       )
 
     private val UNPACK_CHILDREN =

pkl-parser/src/test/kotlin/org/pkl/parser/ParserComparisonTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -97,6 +97,7 @@ class ParserComparisonTest {
         "errors/parser18.pkl",
         "errors/nested1.pkl",
         "errors/invalidCharacterEscape.pkl",
+        "errors/invalidCharacterEscape2.pkl",
         "errors/invalidUnicodeEscape.pkl",
         "errors/unterminatedUnicodeEscape.pkl",
         "errors/keywordNotAllowedHere1.pkl",