add syntax for empty n-dimensional arrays (JuliaLang#41618)

Author: simeonschaub

NEWS.md

@@ -19,6 +19,7 @@ New language features
 * Mutable struct fields may now be annotated as `const` to prevent changing
   them after construction, providing for greater clarity and optimization
   ability of these objects ([#43305]).
+* Empty n-dimensional arrays can now be created using multiple semicolons inside square brackets, i.e. `[;;;]` creates a 0×0×0 `Array`. ([#41618])
 
 Language changes
 ----------------

src/ast.scm

@@ -141,7 +141,7 @@
            ((typed_hcat)  (string (deparse (cadr e))
                                   (deparse (cons 'hcat (cddr e)))))
            ((ncat)        (string #\[ (deparse-arglist (cddr e) (string (deparse-semicolons (cadr e)) " "))
-                                      (if (= (length (cddr e)) 1)
+                                      (if (<= (length (cddr e)) 1)
                                           (deparse-semicolons (cadr e))
                                           "") #\]))
            ((typed_ncat)  (string (deparse (cadr e))

src/julia-parser.scm

@@ -2016,24 +2016,40 @@
                   (where-enabled #t)
                   (whitespace-newline #f)
                   (for-generator #t))
-    (if (eqv? (require-token s) closer)
-        (begin (take-token s)
-               '())
-        (let* ((first (parse-eq* s))
-               (t (peek-token s)))
-          (cond ((or (eqv? t #\,) (eqv? t closer))
-                 (parse-vect s first closer))
-                ((eq? t 'for)
-                 (expect-space-before s 'for)
-                 (take-token s)
-                 (parse-comprehension s first closer))
-                ((eqv? t #\newline)
+    (let ((t (require-token s)))
+      (cond ((eqv? t closer)
+             (take-token s)
+             '())
+            ((eqv? t #\;)
+             (take-token s)
+             (define (loop (n 1))
+               (let ((t (with-whitespace-newline (require-token s))))
                  (take-token s)
-                 (if (memv (peek-token s) (list #\, closer))
-                     (parse-vect s first closer)
-                     (parse-array s first closer #t last-end-symbol)))
-                (else
-                 (parse-array s first closer #f last-end-symbol)))))))
+                 (cond ((eqv? t #\;)
+                        (if (ts:space? s)
+                            (error (string "unexpected space inside "
+                                           (deparse `(ncat ,n)) " expression")))
+                        (loop (+ n 1)))
+                       ((eqv? t closer) `(ncat ,n))
+                       (else (error (string "unexpected \"" t "\" inside "
+                                            (deparse `(ncat ,n)) " expression"))))))
+             (loop))
+            (else
+             (let* ((first (parse-eq* s))
+                    (t (peek-token s)))
+               (cond ((or (eqv? t #\,) (eqv? t closer))
+                      (parse-vect s first closer))
+                     ((eq? t 'for)
+                      (expect-space-before s 'for)
+                      (take-token s)
+                      (parse-comprehension s first closer))
+                     ((eqv? t #\newline)
+                      (take-token s)
+                      (if (memv (peek-token s) (list #\, closer))
+                          (parse-vect s first closer)
+                          (parse-array s first closer #t last-end-symbol)))
+                     (else
+                      (parse-array s first closer #f last-end-symbol)))))))))
 
 (define (kw-to-= e) (if (kwarg? e) (cons '= (cdr e)) e))
 (define (=-to-kw e) (if (assignment? e) (cons 'kw (cdr e)) e))

test/syntax.jl

@@ -3088,3 +3088,37 @@ function checkUserAccess(u::User)
 	return false
 end
 """)
+
+@testset "empty nd arrays" begin
+    @test :([])    == Expr(:vect)
+    @test :([;])   == Expr(:ncat, 1)
+    @test :([;;])  == Expr(:ncat, 2)
+    @test :([;;;]) == Expr(:ncat, 3)
+
+    @test []    == Array{Any}(undef, 0)
+    @test [;]   == Array{Any}(undef, 0)
+    @test [;;]  == Array{Any}(undef, 0, 0)
+    @test [;;;] == Array{Any}(undef, 0, 0, 0)
+
+    @test :(T[])    == Expr(:ref, :T)
+    @test :(T[;])   == Expr(:typed_ncat, :T, 1)
+    @test :(T[;;])  == Expr(:typed_ncat, :T, 2)
+    @test :(T[;;;]) == Expr(:typed_ncat, :T, 3)
+
+    @test Int[]    == Array{Int}(undef, 0)
+    @test Int[;]   == Array{Int}(undef, 0)
+    @test Int[;;]  == Array{Int}(undef, 0, 0)
+    @test Int[;;;] == Array{Int}(undef, 0, 0, 0)
+
+    @test :([  ]) == Expr(:vect)
+    @test :([
+            ]) == Expr(:vect)
+    @test :([ ;; ]) == Expr(:ncat, 2)
+    @test :([
+             ;;
+            ]) == Expr(:ncat, 2)
+
+    @test_throws ParseError Meta.parse("[; ;]")
+    @test_throws ParseError Meta.parse("[;; ;]")
+    @test_throws ParseError Meta.parse("[;\n;]")
+end