allow escaping newlines with \ inside strings (#40753)

This allows the use of `\` in front of newlines inside
non-raw/non-custom string or command literals as a line continuation
character, so the following newline is ignored. This way, long strings
without any newlines in them don't have to be written in a single line
or be broken up. I think we might also want to use this to improve the
printing of long strings in the REPL by printing them as multiline
strings, making use of `\` for long lines if necessary, but that can be
discussed separately.

The command literal part is technically breaking, but the current
behavior is probably unintuitive enough that this can be considered a
minor change. For string literals, this should be entirely non-breaking
since a single `\` before a newline currently throws a parsing error.

closes #37728

NEWS.md

@@ -16,6 +16,9 @@ New language features
   in `[A; B]` has always described concatenating along the first dimension (vertically), now two
   semicolons `[A;; B]` do so in the second dimension (horizontally), three semicolons `;;;` in the
   third, and so on. ([#33697])
+* A backslash (`\`) before a newline inside a string literal now removes the newline while also
+  respecting indentation. This can be used to split up long strings without newlines into multiple
+  lines of code. ([#40753])
 
 Language changes
 ----------------
@@ -114,6 +117,8 @@ Standard library changes
 * `@lock` is now exported from Base ([#39588]).
 * The experimental function `Base.catch_stack()` has been renamed to `current_exceptions()`, exported from Base and given a more specific return type ([#29901])
 * Some degree trigonometric functions, `sind`, `cosd`, `tand`, `asind`, `acosd`, `asecd`, `acscd`, `acotd`, `atand` now accept an square matrix ([#39758]).
+* A backslash before a newline in command literals now always removes the newline, similar to standard string
+  literals, whereas the result was not well-defined before. ([#40753])
 
 #### Package Manager
 

base/shell.jl

@@ -87,15 +87,18 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
             elseif !in_single_quotes && c == '"'
                 in_double_quotes = !in_double_quotes
                 i = consume_upto!(arg, s, i, j)
-            elseif c == '\\'
-                if in_double_quotes
+            elseif !in_single_quotes && c == '\\'
+                if !isempty(st) && peek(st)[2] == '\n'
+                    i = consume_upto!(arg, s, i, j) + 1
+                    _ = popfirst!(st)
+                elseif in_double_quotes
                     isempty(st) && error("unterminated double quote")
                     k, c′ = peek(st)
                     if c′ == '"' || c′ == '$' || c′ == '\\'
                         i = consume_upto!(arg, s, i, j)
                         _ = popfirst!(st)
                     end
-                elseif !in_single_quotes
+                else
                     isempty(st) && error("dangling backslash")
                     i = consume_upto!(arg, s, i, j)
                     _ = popfirst!(st)

doc/src/manual/strings.md

@@ -166,6 +166,14 @@ julia> """Contains "quote" characters"""
 "Contains \"quote\" characters"
 ```
 
+Long lines in strings can be broken up by preceding the newline with a backslash (`\`):
+
+```jldoctest
+julia> "This is a long \
+       line"
+"This is a long line"
+```
+
 If you want to extract a character from a string, you index into it:
 
 ```jldoctest helloworldstring
@@ -639,6 +647,15 @@ julia> """
 "Hello,\nworld."
 ```
 
+If the newline is removed using a backslash, dedentation will be respected as well:
+
+```jldoctest
+julia> """
+         Averylong\
+         word"""
+"Averylongword"
+```
+
 Trailing whitespace is left unaltered.
 
 Triple-quoted string literals can contain `"` characters without escaping.

src/julia-parser.scm

@@ -311,6 +311,9 @@
 (define (numchk n s)
   (or n (error (string "invalid numeric constant \"" s "\""))))
 
+(define (string-lastchar s)
+  (string.char s (string.dec s (length s))))
+
 (define (read-number port leadingdot neg)
   (let ((str  (open-output-string))
         (pred char-numeric?)
@@ -412,7 +415,7 @@
                    (string.sub s 1)
                    s)
                r is-float32-literal)))
-      (if (and (eqv? #\. (string.char s (string.dec s (length s))))
+      (if (and (eqv? #\. (string-lastchar s))
                (let ((nxt (peek-char port)))
                  (and (not (eof-object? nxt))
                       (or (identifier-start-char? nxt)
@@ -2182,16 +2185,35 @@
 (define (unescape-parsed-string-literal strs)
   (map-at even? unescape-string strs))
 
+;; remove `\` followed by a newline
+(define (strip-escaped-newline s)
+  (let ((in  (open-input-string s))
+        (out (open-output-string)))
+    (define (loop preceding-backslash?)
+          (let ((c (read-char in)))
+            (cond ((eof-object? c))
+                  (preceding-backslash?
+                   (if (not (eqv? c #\newline))
+                       (begin (write-char #\\ out) (write-char c out)))
+                   (loop #f))
+                  ((eqv? c #\\) (loop #t))
+                  (else (write-char c out) (loop #f)))))
+    (loop #f)
+    (io.tostring! out)))
+
 (define (parse-string-literal s delim raw)
-  (let ((p (ts:port s)))
-    ((if raw identity unescape-parsed-string-literal)
-     (if (eqv? (peek-char p) delim)
-         (if (eqv? (peek-char (take-char p)) delim)
-             (map-first strip-leading-newline
-                        (dedent-triplequoted-string
-                         (parse-string-literal- 2 (take-char p) s delim raw)))
-             (list ""))
-         (parse-string-literal- 0 p s delim raw)))))
+  (let* ((p (ts:port s))
+         (str (if (eqv? (peek-char p) delim)
+                  (if (eqv? (peek-char (take-char p)) delim)
+                      (map-first strip-leading-newline
+                                 (dedent-triplequoted-string
+                                   (parse-string-literal- 2 (take-char p) s delim raw)))
+                      (list ""))
+                  (parse-string-literal- 0 p s delim raw))))
+    (if raw str (unescape-parsed-string-literal
+                  (map (lambda (s)
+                         (if (string? s) (strip-escaped-newline s) s))
+                       str)))))
 
 (define (strip-leading-newline s)
   (let ((n (sizeof s)))

test/syntax.jl

@@ -2830,3 +2830,100 @@ end
     x[3], x[1:2]... = x
     @test x == [2, 3, 1]
 end
+
+@testset "escaping newlines inside strings" begin
+    c = "c"
+
+    @test "a\
+b" == "ab"
+    @test "a\
+    b" == "a    b"
+    @test raw"a\
+b" == "a\\\nb"
+    @test "a$c\
+b" == "acb"
+    @test "\\
+" == "\\\n"
+
+
+    @test """
+          a\
+          b""" == "ab"
+    @test """
+          a\
+            b""" == "a  b"
+    @test """
+            a\
+          b""" == "  ab"
+    @test raw"""
+          a\
+          b""" == "a\\\nb"
+    @test """
+          a$c\
+          b""" == "acb"
+
+    @test """
+          \
+          """ == ""
+    @test """
+          \\
+          """ == "\\\n"
+    @test """
+          \\\
+          """ == "\\"
+    @test """
+          \\\\
+          """ == "\\\\\n"
+    @test """
+          \\\\\
+          """ == "\\\\"
+    @test """
+          \
+          \
+          """ == ""
+    @test """
+          \\
+          \
+          """ == "\\\n"
+    @test """
+          \\\
+          \
+          """ == "\\"
+
+
+    @test `a\
+b` == `ab`
+    @test `a\
+    b` == `a    b`
+    @test `a$c\
+b` == `acb`
+    @test `"a\
+b"` == `ab`
+    @test `'a\
+b'` == `$("a\\\nb")`
+    @test `\\
+` == `'\'`
+
+
+    @test ```
+          a\
+          b``` == `ab`
+    @test ```
+          a\
+            b``` == `a  b`
+    @test ```
+            a\
+          b``` == `  ab`
+    @test ```
+          a$c\
+          b``` == `acb`
+    @test ```
+          "a\
+          b"``` == `ab`
+    @test ```
+          'a\
+          b'``` == `$("a\\\nb")`
+    @test ```
+          \\
+          ``` == `'\'`
+end