From b697f477670bc85e97ace52fc746f38a6b397712 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Sat, 18 Jul 2020 23:00:25 -0400
Subject: [PATCH] add operators `<--` and `<-->`. fixes #36666

---
 NEWS.md              |  2 ++
 src/julia-parser.scm | 11 +++++++++--
 test/syntax.jl       |  4 ++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index 861210f4df5af..c9e23ca39c934 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -6,6 +6,8 @@ New language features
 
 * Types written with `where` syntax can now be used to define constructors, e.g.
   `(Foo{T} where T)(x) = ...`.
+* `<--` and `<-->` are now available as infix operators, with the same precedence
+  and associativity as other arrow-like operators ([#36666]).
 
 Language changes
 ----------------
diff --git a/src/julia-parser.scm b/src/julia-parser.scm
index dfce9770a2fbe..7c5ae157f5252 100644
--- a/src/julia-parser.scm
+++ b/src/julia-parser.scm
@@ -15,7 +15,7 @@
 (define prec-conditional '(?))
 (define prec-arrow       (append!
                           '(-- -->)
-                          (add-dots '(← → ↔ ↚ ↛ ↞ ↠ ↢ ↣ ↦ ↤ ↮ ⇎ ⇍ ⇏ ⇐ ⇒ ⇔ ⇴ ⇶ ⇷ ⇸ ⇹ ⇺ ⇻ ⇼ ⇽ ⇾ ⇿ ⟵ ⟶ ⟷ ⟹ ⟺ ⟻ ⟼ ⟽ ⟾ ⟿ ⤀ ⤁ ⤂ ⤃ ⤄ ⤅ ⤆ ⤇ ⤌ ⤍ ⤎ ⤏ ⤐ ⤑ ⤔ ⤕ ⤖ ⤗ ⤘ ⤝ ⤞ ⤟ ⤠ ⥄ ⥅ ⥆ ⥇ ⥈ ⥊ ⥋ ⥎ ⥐ ⥒ ⥓ ⥖ ⥗ ⥚ ⥛ ⥞ ⥟ ⥢ ⥤ ⥦ ⥧ ⥨ ⥩ ⥪ ⥫ ⥬ ⥭ ⥰ ⧴ ⬱ ⬰ ⬲ ⬳ ⬴ ⬵ ⬶ ⬷ ⬸ ⬹ ⬺ ⬻ ⬼ ⬽ ⬾ ⬿ ⭀ ⭁ ⭂ ⭃ ⭄ ⭇ ⭈ ⭉ ⭊ ⭋ ⭌ ￩ ￫ ⇜ ⇝ ↜ ↝ ↩ ↪ ↫ ↬ ↼ ↽ ⇀ ⇁ ⇄ ⇆ ⇇ ⇉ ⇋ ⇌ ⇚ ⇛ ⇠ ⇢ ↷ ↶ ↺ ↻))))
+                          (add-dots '(← → ↔ ↚ ↛ ↞ ↠ ↢ ↣ ↦ ↤ ↮ ⇎ ⇍ ⇏ ⇐ ⇒ ⇔ ⇴ ⇶ ⇷ ⇸ ⇹ ⇺ ⇻ ⇼ ⇽ ⇾ ⇿ ⟵ ⟶ ⟷ ⟹ ⟺ ⟻ ⟼ ⟽ ⟾ ⟿ ⤀ ⤁ ⤂ ⤃ ⤄ ⤅ ⤆ ⤇ ⤌ ⤍ ⤎ ⤏ ⤐ ⤑ ⤔ ⤕ ⤖ ⤗ ⤘ ⤝ ⤞ ⤟ ⤠ ⥄ ⥅ ⥆ ⥇ ⥈ ⥊ ⥋ ⥎ ⥐ ⥒ ⥓ ⥖ ⥗ ⥚ ⥛ ⥞ ⥟ ⥢ ⥤ ⥦ ⥧ ⥨ ⥩ ⥪ ⥫ ⥬ ⥭ ⥰ ⧴ ⬱ ⬰ ⬲ ⬳ ⬴ ⬵ ⬶ ⬷ ⬸ ⬹ ⬺ ⬻ ⬼ ⬽ ⬾ ⬿ ⭀ ⭁ ⭂ ⭃ ⭄ ⭇ ⭈ ⭉ ⭊ ⭋ ⭌ ￩ ￫ ⇜ ⇝ ↜ ↝ ↩ ↪ ↫ ↬ ↼ ↽ ⇀ ⇁ ⇄ ⇆ ⇇ ⇉ ⇋ ⇌ ⇚ ⇛ ⇠ ⇢ ↷ ↶ ↺ ↻ <-- <-->))))
 (define prec-lazy-or     '(|\|\||))
 (define prec-lazy-and    '(&&))
 (define prec-comparison
@@ -249,7 +249,14 @@
                                  (or sufchar? (opchar? c)))
                              (let* ((newop (string str c))
                                     (opsym (string->symbol newop)))
-                               (if (operator? opsym)
+                               (if (or (operator? opsym)
+                                       (and (eq? opsym '<---)
+                                            (error (string "invalid operator \"" newop "\"")))
+                                       ;; <- is not an operator but <-- and <--> are
+                                       (and (eq? opsym '<-)
+                                            (read-char port)
+                                            (begin0 (eqv? (peek-char port) #\-)
+                                                    (io.ungetc port #\-))))
                                    (begin (read-char port)
                                           (loop newop (peek-char port) sufchar?))
                                    str))
diff --git a/test/syntax.jl b/test/syntax.jl
index 9feb7bfa709a3..992b17de0f29f 100644
--- a/test/syntax.jl
+++ b/test/syntax.jl
@@ -2292,3 +2292,7 @@ end
 
     @test (Int .<: [Integer] .<: [Real]) == [true]
 end
+
+@test :(a <-- b <-- c) == Expr(:call, :<--, :a, Expr(:call, :<--, :b, :c))
+@test :(a<-->b<-->c) == Expr(:call, :<-->, :a, Expr(:call, :<-->, :b, :c))
+@test_throws ParseError("invalid operator \"<---\"") Meta.parse("1<---2")