deprecate (;) (#30115) (#34039)

Author: JeffBezanson

NEWS.md

@@ -18,6 +18,9 @@ Language changes
 
 * Calling `show` or `repr` on a 0-dimensional `AbstractArray` now shows valid code for creating an equivalent 0-dimensional array, instead of only showing the contained value. ([#33206])
 
+* The syntax `(;)`, which used to parse as an empty block expression, is deprecated.
+  In the future it will indicate an empty named tuple ([#30115]).
+
 Multi-threading changes
 -----------------------
 

doc/src/base/punctuation.md

@@ -26,7 +26,7 @@ Extended documentation for mathematical symbols & functions is [here](@ref math-
 | `()`        | parentheses with no arguments constructs an empty [`Tuple`](@ref)                           |
 | `(a,...)`   | parentheses with comma-separated arguments constructs a tuple containing its arguments      |
 | `(a=1,...)` | parentheses with comma-separated assignments constructs a [`NamedTuple`](@ref)              |
-| `(;)`       | parentheses can also be used to group one or more semicolon separated expressions           |
+| `(x;y)`     | parentheses can also be used to group one or more semicolon separated expressions           |
 | `a[]`       | [array indexing](@ref man-array-indexing) (calling [`getindex`](@ref) or [`setindex!`](@ref)) |
 | `[,]`       | [vector literal constructor](@ref man-array-literals) (calling [`vect`](@ref Base.vect))    |
 | `[;]`       | [vertical concatenation](@ref man-array-concatenation) (calling [`vcat`](@ref) or [`hvcat`](@ref)) |

doc/src/manual/control-flow.md

@@ -2,7 +2,7 @@
 
 Julia provides a variety of control flow constructs:
 
-  * [Compound Expressions](@ref man-compound-expressions): `begin` and `(;)`.
+  * [Compound Expressions](@ref man-compound-expressions): `begin` and `;`.
   * [Conditional Evaluation](@ref man-conditional-evaluation): `if`-`elseif`-`else` and `?:` (ternary operator).
   * [Short-Circuit Evaluation](@ref): `&&`, `||` and chained comparisons.
   * [Repeated Evaluation: Loops](@ref man-loops): `while` and `for`.
@@ -19,7 +19,7 @@ direct usage of tasks, but certain problems can be solved much more easily by us
 
 Sometimes it is convenient to have a single expression which evaluates several subexpressions
 in order, returning the value of the last subexpression as its value. There are two Julia constructs
-that accomplish this: `begin` blocks and `(;)` chains. The value of both compound expression constructs
+that accomplish this: `begin` blocks and `;` chains. The value of both compound expression constructs
 is that of the last subexpression. Here's an example of a `begin` block:
 
 ```jldoctest
@@ -32,7 +32,7 @@ julia> z = begin
 ```
 
 Since these are fairly small, simple expressions, they could easily be placed onto a single line,
-which is where the `(;)` chain syntax comes in handy:
+which is where the `;` chain syntax comes in handy:
 
 ```jldoctest
 julia> z = (x = 1; y = 2; x + y)
@@ -41,7 +41,7 @@ julia> z = (x = 1; y = 2; x + y)
 
 This syntax is particularly useful with the terse single-line function definition form introduced
 in [Functions](@ref). Although it is typical, there is no requirement that `begin` blocks be multiline
-or that `(;)` chains be single-line:
+or that `;` chains be single-line:
 
 ```jldoctest
 julia> begin x = 1; y = 2; x + y end

src/julia-parser.scm

@@ -1910,17 +1910,22 @@
 ;; convert an arglist to a tuple or block expr
 ;; leading-semi? means we saw (; ...)
 ;; comma? means there was a comma after the first expression
-(define (arglist-to-tuple leading-semi? comma? args . first)
+(define (arglist-to-tuple s leading-semi? comma? args . first)
   (if (and (pair? first) (null? args) (not leading-semi?) (not comma?))
       `(block ,@first)  ;; this case is (x;)
       (or (and (not comma?) (length= args 1) (pair? (car args)) (eq? (caar args) 'parameters)
                (let ((blk (parameters-to-block (car args))))
                  (and blk (or (and (not leading-semi?)
                                    `(block ,@first ,@blk))
                               (and (null? first) (null? blk)
-                                   `(block))))))  ;; all semicolons inside ()
+                                   ;; all semicolons inside ()
+                                   (if (length= (car args) 1)
+                                       (begin (parser-depwarn s "(;)" "begin end")
+                                              ;; should eventually be (tuple (parameters))
+                                              `(block))
+                                       `(block)))))))
           (and (null? first) (null? args) (not comma?)
-               `(block))  ;; this case is (;)
+               (error "unreachable"))
           (rm-linenums
            (if (and (pair? args) (pair? (car args)) (eq? (caar args) 'parameters))
                `(tuple ,(car args) ,@first ,@(map kw-to-= (cdr args)))
@@ -1974,7 +1979,7 @@
        (take-token s)  ;; take #\)
        '(|::| . #f))
       ((eqv? nxt #\;)
-       (let ((ex (arglist-to-tuple #t #f (parse-arglist s #\) ))))
+       (let ((ex (arglist-to-tuple s #t #f (parse-arglist s #\) ))))
          (cons ex (eq? (car ex) 'tuple))))
       (else
        ;; here we parse the first subexpression separately, so
@@ -1991,10 +1996,11 @@
                ((eqv? t #\,)
                 ;; tuple (x,) (x,y) etc.
                 (take-token s)
-                (cons (arglist-to-tuple #f #t (parse-arglist s #\) ) ex)
+                (cons (arglist-to-tuple s #f #t (parse-arglist s #\) ) ex)
                       #t))
                ((eqv? t #\;)
                 (cons (arglist-to-tuple
+                       s
                        #f
                        ;; consider `(x...; ` the start of an arglist, since it's not useful as a block
                        (vararg? ex)

src/julia-syntax.scm

@@ -1768,7 +1768,9 @@
              (current-vals  '())
              (expr          #f))
     (if (null? L)
-        (merge expr (to-nt current-names current-vals))
+        (or (merge expr (to-nt current-names current-vals))
+            ;; if that result is #f the named tuple is empty
+            '(call (core NamedTuple)))
         (let ((el (car L)))
           (cond ((or (assignment? el) (kwarg? el))
                  (if (not (symbol? (cadr el)))

test/namedtuple.jl

@@ -133,7 +133,7 @@ end
 @test Meta.lower(Main, Meta.parse("(; f(x))")) == Expr(:error, "invalid named tuple element \"f(x)\"")
 @test Meta.lower(Main, Meta.parse("(;1=0)")) == Expr(:error, "invalid named tuple field name \"1\"")
 
-@test Meta.parse("(;)") == quote end
+@test eval(Expr(:tuple, Expr(:parameters))) === NamedTuple()
 @test Meta.lower(Main, Meta.parse("(1,;2)")) == Expr(:error, "unexpected semicolon in tuple")
 
 # splatting

test/syntax.jl

@@ -171,7 +171,7 @@ macro test999_str(args...); args; end
 
 # blocks vs. tuples
 @test Meta.parse("()") == Expr(:tuple)
-@test Meta.parse("(;)") == Expr(:block)
+@test_skip Meta.parse("(;)") == Expr(:tuple, Expr(:parameters))
 @test Meta.parse("(;;;;)") == Expr(:block)
 @test_throws ParseError Meta.parse("(,)")
 @test_throws ParseError Meta.parse("(;,)")
@@ -1322,7 +1322,7 @@ end
 @test Meta.parse("-(x)^2")     == Expr(:call, :-, Expr(:call, :^, :x, 2))
 @test Meta.parse("-(a=1)^2")   == Expr(:call, :-, Expr(:call, :^, Expr(:(=), :a, 1), 2))
 @test Meta.parse("-(x;y)^2")   == Expr(:call, :-, Expr(:call, :^, Expr(:block, :x, LineNumberNode(1,:none), :y), 2))
-@test Meta.parse("-(;)^2")     == Expr(:call, :-, Expr(:call, :^, Expr(:block), 2))
+@test_skip Meta.parse("-(;)^2")     == Expr(:call, :-, Expr(:call, :^, Expr(:tuple, Expr(:parameters)), 2))
 @test Meta.parse("-(;;;;)^2")  == Expr(:call, :-, Expr(:call, :^, Expr(:block), 2))
 @test Meta.parse("-(x;;;)^2")  == Expr(:call, :-, Expr(:call, :^, Expr(:block, :x), 2))
 @test Meta.parse("+((1,2))")   == Expr(:call, :+, Expr(:tuple, 1, 2))