support a[begin] for a[firstindex(a)]

NEWS.md

@@ -11,6 +11,9 @@ New language features
 * Function composition now supports multiple functions: `∘(f, g, h) = f ∘ g ∘ h`
 and splatting `∘(fs...)` for composing an iterable collection of functions ([#33568]).
 
+* `a[begin]` can now be used to address the first element of an integer-indexed collection `a`.
+  The index is computed by `firstindex(a)` ([#33946]).
+
 Language changes
 ----------------
 

base/show.jl

@@ -1113,7 +1113,7 @@ function show_unquoted_quote_expr(io::IO, @nospecialize(value), indent::Int, pre
         end
     else
         if isa(value,Expr) && value.head === :block
-            show_block(io, "quote", value, indent, quote_level)
+            show_block(IOContext(io, beginsym=>false), "quote", value, indent, quote_level)
             print(io, "end")
         else
             print(io, ":(")
@@ -1188,6 +1188,10 @@ function is_core_macro(arg, macro_name::AbstractString)
     arg === GlobalRef(Core, Symbol(macro_name))
 end
 
+# symbol for IOContext flag signaling whether "begin" is treated
+# as an ordinary symbol, which is true in indexing expressions.
+const beginsym = gensym(:beginsym)
+
 # TODO: implement interpolated strings
 function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::Int = 0)
     head, args, nargs = ex.head, ex.args, length(ex.args)
@@ -1322,7 +1326,7 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
     # other call-like expressions ("A[1,2]", "T{X,Y}", "f.(X,Y)")
     elseif haskey(expr_calls, head) && nargs >= 1  # :ref/:curly/:calldecl/:(.)
         funcargslike = head === :(.) ? args[2].args : args[2:end]
-        show_call(io, head, args[1], funcargslike, indent, quote_level)
+        show_call(head == :ref ? IOContext(io, beginsym=>true) : io, head, args[1], funcargslike, indent, quote_level)
 
     # comprehensions
     elseif head === :typed_comprehension && nargs == 2
@@ -1358,50 +1362,52 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
     # function calls need to transform the function from :call to :calldecl
     # so that operators are printed correctly
     elseif head === :function && nargs==2 && is_expr(args[1], :call)
-        show_block(io, head, Expr(:calldecl, args[1].args...), args[2], indent, quote_level)
+        show_block(IOContext(io, beginsym=>false), head, Expr(:calldecl, args[1].args...), args[2], indent, quote_level)
         print(io, "end")
 
     elseif (head === :function || head === :macro) && nargs == 1
         print(io, head, ' ')
-        show_unquoted(io, args[1])
+        show_unquoted(IOContext(io, beginsym=>false), args[1])
         print(io, " end")
 
     elseif head === :do && nargs == 2
-        show_unquoted(io, args[1], indent, -1, quote_level)
+        iob = IOContext(io, beginsym=>false)
+        show_unquoted(iob, args[1], indent, -1, quote_level)
         print(io, " do ")
-        show_list(io, args[2].args[1].args, ", ", 0, 0, quote_level)
+        show_list(iob, args[2].args[1].args, ", ", 0, 0, quote_level)
         for stmt in args[2].args[2].args
             print(io, '\n', " "^(indent + indent_width))
-            show_unquoted(io, stmt, indent + indent_width, -1, quote_level)
+            show_unquoted(iob, stmt, indent + indent_width, -1, quote_level)
         end
         print(io, '\n', " "^indent)
         print(io, "end")
 
     # block with argument
     elseif head in (:for,:while,:function,:macro,:if,:elseif,:let) && nargs==2
         if Meta.isexpr(args[2], :block)
-            show_block(io, head, args[1], args[2], indent, quote_level)
+            show_block(IOContext(io, beginsym=>false), head, args[1], args[2], indent, quote_level)
         else
-            show_block(io, head, args[1], Expr(:block, args[2]), indent, quote_level)
+            show_block(IOContext(io, beginsym=>false), head, args[1], Expr(:block, args[2]), indent, quote_level)
         end
         print(io, "end")
 
     elseif (head === :if || head === :elseif) && nargs == 3
-        show_block(io, head, args[1], args[2], indent, quote_level)
+        iob = IOContext(io, beginsym=>false)
+        show_block(iob, head, args[1], args[2], indent, quote_level)
         if isa(args[3],Expr) && args[3].head === :elseif
-            show_unquoted(io, args[3], indent, prec, quote_level)
+            show_unquoted(iob, args[3], indent, prec, quote_level)
         else
-            show_block(io, "else", args[3], indent, quote_level)
+            show_block(iob, "else", args[3], indent, quote_level)
             print(io, "end")
         end
 
     elseif head === :module && nargs==3 && isa(args[1],Bool)
-        show_block(io, args[1] ? :module : :baremodule, args[2], args[3], indent, quote_level)
+        show_block(IOContext(io, beginsym=>false), args[1] ? :module : :baremodule, args[2], args[3], indent, quote_level)
         print(io, "end")
 
     # type declaration
     elseif head === :struct && nargs==3
-        show_block(io, args[1] ? Symbol("mutable struct") : Symbol("struct"), args[2], args[3], indent, quote_level)
+        show_block(IOContext(io, beginsym=>false), args[1] ? Symbol("mutable struct") : Symbol("struct"), args[2], args[3], indent, quote_level)
         print(io, "end")
 
     elseif head === :primitive && nargs == 2
@@ -1411,7 +1417,7 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
 
     elseif head === :abstract && nargs == 1
         print(io, "abstract type ")
-        show_list(io, args, ' ', indent, 0, quote_level)
+        show_list(IOContext(io, beginsym=>false), args, ' ', indent, 0, quote_level)
         print(io, " end")
 
     # empty return (i.e. "function f() return end")
@@ -1510,31 +1516,47 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
         show_linenumber(io, args...)
 
     elseif head === :try && 3 <= nargs <= 4
-        show_block(io, "try", args[1], indent, quote_level)
+        iob = IOContext(io, beginsym=>false)
+        show_block(iob, "try", args[1], indent, quote_level)
         if is_expr(args[3], :block)
-            show_block(io, "catch", args[2] === false ? Any[] : args[2], args[3], indent, quote_level)
+            show_block(iob, "catch", args[2] === false ? Any[] : args[2], args[3], indent, quote_level)
         end
         if nargs >= 4 && is_expr(args[4], :block)
-            show_block(io, "finally", Any[], args[4], indent, quote_level)
+            show_block(iob, "finally", Any[], args[4], indent, quote_level)
         end
         print(io, "end")
 
     elseif head === :block
-        show_block(io, "begin", ex, indent, quote_level)
-        print(io, "end")
+        # print as (...; ...; ...;) inside indexing expression
+        if get(io, beginsym, false)
+            print(io, '(')
+            ind = indent + indent_width
+            for i = 1:length(ex.args)
+                i > 1 && print(io, ";\n", ' '^ind)
+                show_unquoted(io, ex.args[i], ind, -1, quote_level)
+            end
+            if length(ex.args) < 2
+                print(isempty(ex.args) ? "nothing;)" : ";)")
+            else
+                print(io, ')')
+            end
+        else
+            show_block(io, "begin", ex, indent, quote_level)
+            print(io, "end")
+        end
 
     elseif head === :quote && nargs == 1 && isa(args[1], Symbol)
-        show_unquoted_quote_expr(io, args[1]::Symbol, indent, 0, quote_level+1)
+        show_unquoted_quote_expr(IOContext(io, beginsym=>false), args[1]::Symbol, indent, 0, quote_level+1)
     elseif head === :quote && nargs == 1 && Meta.isexpr(args[1], :block)
-        show_block(io, "quote", Expr(:quote, args[1].args...), indent,
+        show_block(IOContext(io, beginsym=>false), "quote", Expr(:quote, args[1].args...), indent,
                    quote_level+1)
         print(io, "end")
     elseif head === :quote && nargs == 1
         print(io, ":(")
-        show_unquoted(io, args[1], indent+2, 0, quote_level+1)
+        show_unquoted(IOContext(io, beginsym=>false), args[1], indent+2, 0, quote_level+1)
         print(io, ")")
     elseif head === :quote
-        show_block(io, "quote", ex, indent, quote_level+1)
+        show_block(IOContext(io, beginsym=>false), "quote", ex, indent, quote_level+1)
         print(io, "end")
 
     elseif head === :gotoifnot && nargs == 2 && isa(args[2], Int)

doc/src/manual/functions.md

@@ -838,8 +838,8 @@ the results (see [Pre-allocating outputs](@ref)). A convenient syntax for this i
 is equivalent to `broadcast!(identity, X, ...)` except that, as above, the `broadcast!` loop is
 fused with any nested "dot" calls. For example, `X .= sin.(Y)` is equivalent to `broadcast!(sin, X, Y)`,
 overwriting `X` with `sin.(Y)` in-place. If the left-hand side is an array-indexing expression,
-e.g. `X[2:end] .= sin.(Y)`, then it translates to `broadcast!` on a `view`, e.g.
-`broadcast!(sin, view(X, 2:lastindex(X)), Y)`,
+e.g. `X[begin+1:end] .= sin.(Y)`, then it translates to `broadcast!` on a `view`, e.g.
+`broadcast!(sin, view(X, firstindex(X)+1:lastindex(X)), Y)`,
 so that the left-hand side is updated in-place.
 
 Since adding dots to many operations and function calls in an expression

doc/src/manual/interfaces.md

@@ -164,8 +164,8 @@ julia> collect(Iterators.reverse(Squares(4)))
 |:-------------------- |:-------------------------------- |
 | `getindex(X, i)`     | `X[i]`, indexed element access   |
 | `setindex!(X, v, i)` | `X[i] = v`, indexed assignment   |
-| `firstindex(X)`      | The first index                  |
-| `lastindex(X)`        | The last index, used in `X[end]` |
+| `firstindex(X)`         | The first index, used in `X[begin]` |
+| `lastindex(X)`           | The last index, used in `X[end]` |
 
 For the `Squares` iterable above, we can easily compute the `i`th element of the sequence by squaring
 it.  We can expose this as an indexing expression `S[i]`. To opt into this behavior, `Squares`
@@ -181,8 +181,8 @@ julia> Squares(100)[23]
 529
 ```
 
-Additionally, to support the syntax `S[end]`, we must define [`lastindex`](@ref) to specify the last
-valid index. It is recommended to also define [`firstindex`](@ref) to specify the first valid index:
+Additionally, to support the syntax `S[begin]` and `S[end]`, we must define [`firstindex`](@ref) and
+[`lastindex`](@ref) to specify the first and last valid indices, respectively:
 
 ```jldoctest squaretype
 julia> Base.firstindex(S::Squares) = 1

doc/src/manual/strings.md

@@ -169,6 +169,9 @@ julia> """Contains "quote" characters"""
 If you want to extract a character from a string, you index into it:
 
 ```jldoctest helloworldstring
+julia> str[begin]
+'H': ASCII/Unicode U+0048 (category Lu: Letter, uppercase)
+
 julia> str[1]
 'H': ASCII/Unicode U+0048 (category Lu: Letter, uppercase)
 
@@ -181,8 +184,8 @@ julia> str[end]
 
 Many Julia objects, including strings, can be indexed with integers. The index of the first
 element (the first character of a string) is returned by [`firstindex(str)`](@ref), and the index of the last element (character)
-with [`lastindex(str)`](@ref). The keyword `end` can be used inside an indexing
-operation as shorthand for the last index along the given dimension.
+with [`lastindex(str)`](@ref). The keywords `begin` and `end` can be used inside an indexing
+operation as shorthand for the first and last indices, respectively, along the given dimension.
 String indexing, like most indexing in Julia, is 1-based: `firstindex` always returns `1` for any `AbstractString`.
 As we will see below, however, `lastindex(str)` is *not* in general the same as `length(str)` for a string,
 because some Unicode characters can occupy multiple "code units".
@@ -198,10 +201,10 @@ julia> str[end÷2]
 ' ': ASCII/Unicode U+0020 (category Zs: Separator, space)
 ```
 
-Using an index less than 1 or greater than `end` raises an error:
+Using an index less than `begin` (`1`) or greater than `end` raises an error:
 
 ```jldoctest helloworldstring
-julia> str[0]
+julia> str[begin-1]
 ERROR: BoundsError: attempt to access String
   at index [0]
 [...]

src/julia-parser.scm

@@ -160,7 +160,10 @@
                          struct
                          module baremodule using import export))
 
-(define initial-reserved-word? (Set initial-reserved-words))
+(define initial-reserved-word?
+  (let ((reserved? (Set initial-reserved-words)))
+    (lambda (s) (and (reserved? s)
+                     (not (and (eq? s 'begin) end-symbol)))))) ; begin == firstindex inside [...]
 
 (define reserved-words (append initial-reserved-words '(end else elseif catch finally true false))) ;; todo: make this more complete
 
@@ -1319,8 +1322,6 @@
 
 ;; parse expressions or blocks introduced by syntactic reserved words
 (define (parse-resword s word)
-  (if (and (eq? word 'begin) end-symbol)
-      (parser-depwarn s "\"begin\" inside indexing expression" ""))
   (with-bindings
    ((expect-end-current-line (input-port-line (ts:port s))))
    (with-normal-context

src/julia-syntax.scm

@@ -86,7 +86,7 @@
 (define (expand-compare-chain e)
   (car (expand-vector-compare e)))
 
-;; return the appropriate computation for an `end` symbol for indexing
+;; return the appropriate computation for a `begin` or `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?
@@ -101,20 +101,31 @@
                                  tuples))))
             `(call (top lastindex) ,a ,dimno))))
 
-;; replace `end` for the closest ref expression, so doesn't go inside nested refs
-(define (replace-end ex a n tuples last)
+(define (begin-val a n tuples last)
+  (if (null? tuples)
+      (if (and last (= n 1))
+          `(call (top firstindex) ,a)
+          `(call (top first) (call (top axes) ,a ,n)))
+      (let ((dimno `(call (top +) ,(- n (length tuples))
+                          ,.(map (lambda (t) `(call (top length) ,t))
+                                 tuples))))
+            `(call (top first) (call (top axes) ,a ,dimno)))))
+
+;; replace `begin` and `end` for the closest ref expression, so doesn't go inside nested refs
+(define (replace-beginend ex a n tuples last)
   (cond ((eq? ex 'end)                (end-val a n tuples last))
+        ((eq? ex 'begin)              (begin-val a n tuples last))
         ((or (atom? ex) (quoted? ex)) ex)
         ((eq? (car ex) 'ref)
          ;; inside ref only replace within the first argument
-         (list* 'ref (replace-end (cadr ex) a n tuples last)
+         (list* 'ref (replace-beginend (cadr ex) a n tuples last)
                 (cddr ex)))
         (else
          (cons (car ex)
-               (map (lambda (x) (replace-end x a n tuples last))
+               (map (lambda (x) (replace-beginend x a n tuples last))
                     (cdr ex))))))
 
-;; go through indices and replace the `end` symbol
+;; go through indices and replace the `begin` or `end` symbol
 ;; a = array being indexed, i = list of indices
 ;; returns (values index-list stmts) where stmts are statements that need
 ;; to execute first.
@@ -133,17 +144,17 @@
                   (loop (cdr lst) (+ n 1)
                         stmts
                         (cons (cadr idx) tuples)
-                        (cons `(... ,(replace-end (cadr idx) a n tuples last))
+                        (cons `(... ,(replace-beginend (cadr idx) a n tuples last))
                               ret))
                   (let ((g (make-ssavalue)))
                     (loop (cdr lst) (+ n 1)
-                          (cons `(= ,g ,(replace-end (cadr idx) a n tuples last))
+                          (cons `(= ,g ,(replace-beginend (cadr idx) a n tuples last))
                                 stmts)
                           (cons g tuples)
                           (cons `(... ,g) ret))))
               (loop (cdr lst) (+ n 1)
                     stmts tuples
-                    (cons (replace-end idx a n tuples last) ret)))))))
+                    (cons (replace-beginend idx a n tuples last) ret)))))))
 
 ;; GF method does not need to keep decl expressions on lambda args
 ;; except for rest arg
@@ -1476,7 +1487,7 @@
   (let ((a    (cadr e))
         (idxs (cddr e)))
     (let* ((reuse (and (pair? a)
-                       (contains (lambda (x) (eq? x 'end))
+                       (contains (lambda (x) (or (eq? x 'begin) (eq? x 'end)))
                                  idxs)))
            (arr   (if reuse (make-ssavalue) a))
            (stmts (if reuse `((= ,arr ,a)) '())))
@@ -1488,7 +1499,7 @@
 
 (define (expand-update-operator op op= lhs rhs . declT)
   (cond ((and (pair? lhs) (eq? (car lhs) 'ref))
-         ;; expand indexing inside op= first, to remove "end" and ":"
+         ;; expand indexing inside op= first, to remove "begin", "end", and ":"
          (let* ((ex (partially-expand-ref lhs))
                 (stmts (butlast (cdr ex)))
                 (refex (last    (cdr ex)))

test/abstractarray.jl

@@ -484,7 +484,7 @@ function test_primitives(::Type{T}, shape, ::Type{TestAbstractArray}) where T
     @test lastindex(B, 2) == lastindex(A, 2) == last(axes(B, 2))
 
     # first(a)
-    @test first(B) == B[firstindex(B)] == B[1] == A[1] # TODO: use B[begin] once parser transforms it
+    @test first(B) == B[firstindex(B)] == B[begin] == B[1] == A[1] == A[begin]
     @test firstindex(B) == firstindex(A) == first(LinearIndices(B))
     @test firstindex(B, 1) == firstindex(A, 1) == first(axes(B, 1))
     @test firstindex(B, 2) == firstindex(A, 2) == first(axes(B, 2))

test/offsetarray.jl

@@ -116,6 +116,12 @@ end
 @test A[OffsetArray([true true;  false true], A.offsets)] == [1,3,4]
 @test_throws BoundsError A[[true true;  false true]]
 
+# begin, end
+a0 = rand(2,3,4,2)
+a = OffsetArray(a0, (-2,-3,4,5))
+@test a[begin,end,end,begin] == a0[begin,end,end,begin] ==
+      a0[1,3,4,1] == a0[end-1,begin+2,begin+3,end-1]
+
 # view
 S = view(A, :, 3)
 @test S == OffsetArray([1,2], (A.offsets[1],))
@@ -344,6 +350,7 @@ v2 = copy(v)
 @test push!(v2, 1) === v2
 @test v2[axes(v, 1)] == v
 @test v2[end] == 1
+@test v2[begin] == v[begin] == v[-2]
 v2 = copy(v)
 @test push!(v2, 2, 1) === v2
 @test v2[axes(v, 1)] == v