RFC: add <- as an assignment-like operator

NEWS.md

@@ -4,6 +4,9 @@ Julia v0.6.0 Release Notes
 New language features
 ---------------------
 
+  * `<-` is now a valid operator that can be defined for custom purposes.
+    It has assignment precedence. ([#19765])
+
 Language changes
 ----------------
 

src/julia-parser.scm

@@ -8,7 +8,7 @@
 ;; be an operator.
 (define prec-assignment
   (append! (add-dots '(= += -= *= /= //= |\\=| ^= ÷= %= <<= >>= >>>= |\|=| &= ⊻=))
-           '(:= => ~ $=)))
+           '(:= => ~ $= <-)))
 (define prec-conditional '(?))
 (define prec-arrow       (append!
                           '(-- -->)
@@ -729,7 +729,9 @@
                        (let ((args (parse-chain s down '~)))
                          `(macrocall @~ ,ex ,@(butlast args)
                                      ,(loop (last args) (peek-token s)))))
-                   (list t ex (parse-assignment s down)))))))
+                   (if (syntactic-op? t)
+                       (list t ex (parse-assignment s down))
+                       (list 'call t ex (parse-assignment s down))))))))
 
 (define (parse-eq s)
   (let ((lno (input-port-line (ts:port s))))

test/parse.jl

@@ -384,6 +384,12 @@ end
 @test parse("x<:y<:z").head === :comparison
 @test parse("x>:y<:z").head === :comparison
 
+# PR #19765
+let <-(x,y) = 2x + y
+    @test (3<-10) == 16
+end
+@test parse("x<-y<-z") == Expr(:call, :(<-), :x, Expr(:call, :(<-), :y, :z))
+
 # issue #11169
 uncalled(x) = @test false
 fret() = uncalled(return true)