Remove special syntax for : in indexing []

Add news and a minor documentation update.

NEWS.md

@@ -114,6 +114,11 @@ Language changes
     macro. Instead, the string is first unindented and then `x_str` is invoked,
     as if the string had been single-quoted ([#10228]).
 
+  * Colons (`:`) within indexing expressions are no longer lowered to the range
+    `1:end`. Instead, the `:` identifier is passed directly. Custom array types
+    that implement `getindex` or `setindex!` methods must also extend those
+    methods to support arguments of type `Colon` ([#10331]).
+
 Command line option changes
 ---------------------------
 
@@ -1422,6 +1427,7 @@ Too numerous to mention.
 [#10180]: https://github.com/JuliaLang/julia/issues/10180
 [#10228]: https://github.com/JuliaLang/julia/issues/10228
 [#10328]: https://github.com/JuliaLang/julia/issues/10328
+[#10331]: https://github.com/JuliaLang/julia/issues/10331
 [#10332]: https://github.com/JuliaLang/julia/issues/10332
 [#10333]: https://github.com/JuliaLang/julia/issues/10333
 [#10380]: https://github.com/JuliaLang/julia/issues/10380

doc/manual/arrays.rst

@@ -222,9 +222,10 @@ The general syntax for indexing into an n-dimensional array A is::
 where each ``I_k`` may be:
 
 1. A scalar integer
-2. A ``Range`` of the form ``:``, ``a:b``, or ``a:b:c``
-3. An arbitrary integer vector, including the empty vector ``[]``
-4. A boolean vector
+2. A ``Range`` of the form ``a:b``, or ``a:b:c``
+3. A ``:`` or ``Colon()`` to select entire dimensions
+4. An arbitrary integer vector, including the empty vector ``[]``
+5. A boolean vector
 
 The result ``X`` generally has dimensions
 ``(length(I_1), length(I_2), ..., length(I_n))``, with location
@@ -286,10 +287,11 @@ The general syntax for assigning values in an n-dimensional array A is::
 
 where each ``I_k`` may be:
 
-1. A scalar value
-2. A ``Range`` of the form ``:``, ``a:b``, or ``a:b:c``
-3. An arbitrary integer vector, including the empty vector ``[]``
-4. A boolean vector
+1. A scalar integer
+2. A ``Range`` of the form ``a:b``, or ``a:b:c``
+3. A ``:`` or ``Colon()`` to select entire dimensions
+4. An arbitrary integer vector, including the empty vector ``[]``
+5. A boolean vector
 
 If ``X`` is an array, its size must be ``(length(I_1), length(I_2), ..., length(I_n))``,
 and the value in location ``i_1, i_2, ..., i_n`` of ``A`` is overwritten with

src/julia-syntax.scm

@@ -301,18 +301,17 @@
 (define (expand-compare-chain e)
   (car (expand-vector-compare e)))
 
-;; last = is this last index?
+;; return the appropriate computation for an `end` symbol for indexing
+;; the array `a` in the `n`th index.
+;; `tuples` are a list of the splatted arguments that precede index `n`
+;; `last` = is this last index?
+;; returns a call to endof(a), trailingsize(a,n), or size(a,n)
 (define (end-val a n tuples last)
   (if (null? tuples)
       (if last
           (if (= n 1)
               `(call (top endof) ,a)
               `(call (top trailingsize) ,a ,n))
-              #;`(call (top div)
-                     (call (top length) ,a)
-                     (call (top *)
-                           ,@(map (lambda (d) `(call (top size) ,a ,(1+ d)))
-                                  (iota (- n 1)))))
           `(call (top size) ,a ,n))
       (let ((dimno `(call (top +) ,(- n (length tuples))
                           ,.(map (lambda (t) `(call (top length) ,t))
@@ -321,8 +320,7 @@
             `(call (top trailingsize) ,a ,dimno)
             `(call (top size) ,a ,dimno)))))
 
-; replace end inside ex with (call (top size) a n)
-; affects only the closest ref expression, so doesn't go inside nested refs
+; replace `end` for the closest ref expression, so doesn't go inside nested refs
 (define (replace-end ex a n tuples last)
   (cond ((eq? ex 'end)                (end-val a n tuples last))
         ((or (atom? ex) (quoted? ex)) ex)
@@ -335,29 +333,7 @@
                (map (lambda (x) (replace-end x a n tuples last))
                     (cdr ex))))))
 
-; translate index x from colons to ranges
-(define (expand-index-colon x)
-  (cond ((eq? x ':) `(call colon 1 end))
-        ((and (pair? x)
-              (eq? (car x) ':))
-         (cond ((length= x 3)
-                (if (eq? (caddr x) ':)
-                    ;; (: a :) a:
-                    `(call colon ,(cadr x) end)
-                    ;; (: a b)
-                    `(call colon ,(cadr x) ,(caddr x))))
-               ((length= x 4)
-                (if (eq? (cadddr x) ':)
-                    ;; (: a b :) a:b:
-                    `(call colon ,(cadr x) ,(caddr x) end)
-                    ;; (: a b c)
-                    `(call colon ,@(cdr x))))
-               (else x)))
-        (else x)))
-
-;; : inside indexing means 1:end
-;; expand end to size(a,n),
-;;     or div(length(a), prod(size(a)[1:(n-1)])) for the last index
+;; go through indices and replace the `end` symbol
 ;; a = array being indexed, i = list of indexes
 ;; returns (values index-list stmts) where stmts are statements that need
 ;; to execute first.
@@ -386,8 +362,7 @@
                           (cons `(... ,g) ret))))
               (loop (cdr lst) (+ n 1)
                     stmts tuples
-                    (cons (replace-end (expand-index-colon idx) a n tuples last)
-                          ret)))))))
+                    (cons (replace-end idx a n tuples last) ret)))))))
 
 (define (make-decl n t) `(|::| ,n ,t))