allow slurping in any position

NEWS.md

@@ -5,6 +5,8 @@ New language features
 ---------------------
 
 * It is now possible to assign to bindings in another module using `setproperty!(::Module, ::Symbol, x)`. ([#44137])
+* Slurping in assignments is now also allowed in non-final position. This is
+  handled via `Base.split_rest`. ([#42902])
 
 Language changes
 ----------------

base/bitarray.jl

@@ -1913,3 +1913,10 @@ function read!(s::IO, B::BitArray)
 end
 
 sizeof(B::BitArray) = sizeof(B.chunks)
+
+function _split_rest(a::Union{Vector, BitVector}, n::Int)
+    _check_length_split_rest(length(a), n)
+    last_n = a[end-n+1:end]
+    resize!(a, length(a) - n)
+    return a, last_n
+end

base/namedtuple.jl

@@ -415,3 +415,9 @@ macro NamedTuple(ex)
     types = [esc(e isa Symbol ? :Any : e.args[2]) for e in decls]
     return :(NamedTuple{($(vars...),), Tuple{$(types...)}})
 end
+
+function split_rest(t::NamedTuple{names}, n::Int, st...) where {names}
+    _check_length_split_rest(length(t), n)
+    names_front, names_last_n = split_rest(names, n, st...)
+    return NamedTuple{names_front}(t), NamedTuple{names_last_n}(t)
+end

base/strings/basic.jl

@@ -780,3 +780,16 @@ julia> codeunits("Juλia")
 ```
 """
 codeunits(s::AbstractString) = CodeUnits(s)
+
+function _split_rest(s::AbstractString, n::Int)
+    lastind = lastindex(s)
+    i = try
+        prevind(s, lastind, n)
+    catch e
+        e isa BoundsError || rethrow()
+        _check_length_split_rest(length(s), n)
+    end
+    last_n = SubString(s, nextind(s, i), lastind)
+    front = s[begin:i]
+    return front, last_n
+end

base/tuple.jl

@@ -108,12 +108,12 @@ if `collection` is an `AbstractString`, and an arbitrary iterator, falling back
 `Iterators.rest(collection[, itr_state])`, otherwise.
 
 Can be overloaded for user-defined collection types to customize the behavior of [slurping
-in assignments](@ref destructuring-assignment), like `a, b... = collection`.
+in assignments](@ref destructuring-assignment) in final position, like `a, b... = collection`.
 
 !!! compat "Julia 1.6"
     `Base.rest` requires at least Julia 1.6.
 
-See also: [`first`](@ref first), [`Iterators.rest`](@ref).
+See also: [`first`](@ref first), [`Iterators.rest`](@ref), [`Base.split_rest`](@ref).
 
 # Examples
 ```jldoctest
@@ -136,6 +136,58 @@ rest(a::Array, i::Int=1) = a[i:end]
 rest(a::Core.SimpleVector, i::Int=1) = a[i:end]
 rest(itr, state...) = Iterators.rest(itr, state...)
 
+"""
+    Base.split_rest(collection, n::Int[, itr_state]) -> (rest_but_n, last_n)
+
+Generic function for splitting the tail of `collection`, starting from a specific iteration
+state `itr_state`. Returns a tuple of two new collections. The first one contains all
+elements of the tail but the `n` last ones, which make up the second collection.
+
+The type of the first collection generally follows that of [`Base.rest`](@ref), except that
+the fallback case is not lazy, but is collected eagerly into a vector.
+
+Can be overloaded for user-defined collection types to customize the behavior of [slurping
+in assignments](@ref destructuring-assignment) in non-final position, like `a, b..., c = collection`.
+
+!!! compat "Julia 1.9"
+    `Base.split_rest` requires at least Julia 1.9.
+
+See also: [`Base.rest`](@ref).
+
+# Examples
+```jldoctest
+julia> a = [1 2; 3 4]
+2×2 Matrix{Int64}:
+ 1  2
+ 3  4
+
+julia> first, state = iterate(a)
+(1, 2)
+
+julia> first, Base.split_rest(a, 1, state)
+(1, ([3, 2], [4]))
+```
+"""
+function split_rest end
+function split_rest(itr, n::Int, state...)
+    if IteratorSize(itr) == IsInfinite()
+        throw(ArgumentError("Cannot split an infinite iterator in the middle."))
+    end
+    return _split_rest(rest(itr, state...), n)
+end
+_split_rest(itr, n::Int) = _split_rest(collect(itr), n)
+function _check_length_split_rest(len, n)
+    len < n && throw(ArgumentError(
+        "The iterator only contains $len elements, but at least $n were requested."
+    ))
+end
+function _split_rest(a::Union{AbstractArray, Core.SimpleVector}, n::Int)
+    _check_length_split_rest(length(a), n)
+    return a[begin:end-n], a[end-n+1:end]
+end
+
+split_rest(t::Tuple, n::Int, i=1) = t[i:end-n], t[end-n+1:end]
+
 # Use dispatch to avoid a branch in first
 first(::Tuple{}) = throw(ArgumentError("tuple must be non-empty"))
 first(t::Tuple) = t[1]

doc/src/base/collections.md

@@ -140,6 +140,7 @@ Base.replace(::Any, ::Pair...)
 Base.replace(::Base.Callable, ::Any)
 Base.replace!
 Base.rest
+Base.split_rest
 ```
 
 ## Indexable Collections

doc/src/manual/functions.md

@@ -475,6 +475,57 @@ Base.Iterators.Rest{Base.Generator{UnitRange{Int64}, typeof(abs2)}, Int64}(Base.
 
 See [`Base.rest`](@ref) for details on the precise handling and customization for specific iterators.
 
+!!! compat "Julia 1.9"
+    `...` in non-final position of an assignment requires Julia 1.9
+
+Slurping in assignments can also occur in any other position. As opposed to slurping the end
+of a collection however, this will always be eager.
+
+```jldoctest
+julia> a, b..., c = 1:5
+1:5
+
+julia> a
+1
+
+julia> b
+3-element Vector{Int64}:
+ 2
+ 3
+ 4
+
+julia> c
+5
+
+julia> front..., tail = "Hi!"
+"Hi!"
+
+julia> front
+"Hi"
+
+julia> tail
+'!': ASCII/Unicode U+0021 (category Po: Punctuation, other)
+```
+
+This is implemented in terms of the function [`Base.split_rest`](@ref).
+
+Note that for variadic function definitions, slurping is still only allowed in final position.
+This does not apply to [single argument destructuring](@ref man-argument-destructuring) though,
+as that does not affect method dispatch:
+
+```jldoctest
+julia> f(x..., y) = x
+ERROR: syntax: invalid "..." on non-final argument
+Stacktrace:
+[...]
+
+julia> f((x..., y)) = x
+f (generic function with 1 method)
+
+julia> f((1, 2, 3))
+(1, 2)
+```
+
 ## Property destructuring
 
 Instead of destructuring based on iteration, the right side of assignments can also be destructured using property names.
@@ -492,7 +543,7 @@ julia> b
 2
 ```
 
-## Argument destructuring
+## [Argument destructuring](@id man-argument-destructuring)
 
 The destructuring feature can also be used within a function argument.
 If a function argument name is written as a tuple (e.g. `(x, y)`) instead of just

src/julia-syntax.scm

@@ -1506,15 +1506,59 @@
                        after
                        (cons R elts)))
                 ((vararg? L)
+                 (if (any vararg? (cdr lhss))
+                     (error "multiple \"...\" on lhs of assignment"))
                  (if (null? (cdr lhss))
                      (let ((temp (if (eventually-call? (cadr L)) (gensy) (make-ssavalue))))
                        `(block ,@(reverse stmts)
                                (= ,temp (tuple ,@rhss))
                                ,@(reverse after)
                                (= ,(cadr L) ,temp)
                                (unnecessary (tuple ,@(reverse elts) (... ,temp)))))
-                     (error (string "invalid \"...\" on non-final assignment location \""
-                                    (cadr L) "\""))))
+                     (let ((lhss- (reverse lhss))
+                           (rhss- (reverse rhss))
+                           (lhs-tail '())
+                           (rhs-tail '()))
+                       (define (extract-tail)
+                         (if (not (or (null? lhss-) (null? rhss-)
+                                      (vararg? (car lhss-)) (vararg? (car rhss-))))
+                             (begin
+                               (set! lhs-tail (cons (car lhss-) lhs-tail))
+                               (set! rhs-tail (cons (car rhss-) rhs-tail))
+                               (set! lhss- (cdr lhss-))
+                               (set! rhss- (cdr rhss-))
+                               (extract-tail))))
+                       (extract-tail)
+                       (let* ((temp (if (any (lambda (x)
+                                               (or (eventually-call? x)
+                                                   (and (vararg? x) (eventually-call? (cadr x)))))
+                                             lhss-)
+                                        (gensy)
+                                        (make-ssavalue)))
+                              (assigns (make-assignment temp `(tuple ,@(reverse rhss-))))
+                              (assigns (if (symbol? temp)
+                                          `((local-def ,temp) ,assigns)
+                                          (list assigns)))
+                              (n (length lhss-))
+                              (st (gensy))
+                              (end (list after))
+                              (assigns (if (and (length= lhss- 1) (vararg? (car lhss-)))
+                                           (begin
+                                             (set-car! end
+                                                       (cons `(= ,(cadar lhss-) ,temp) (car end)))
+                                             assigns)
+                                           (append (if (> n 0)
+                                                       `(,@assigns (local ,st))
+                                                       assigns)
+                                                   (destructure- 1 (reverse lhss-) temp
+                                                                 n st end)))))
+                         (loop lhs-tail
+                               (append (map (lambda (x) (if (vararg? x) (cadr x) x)) lhss-) assigned)
+                               rhs-tail
+                               (append (reverse assigns) stmts)
+                               (car end)
+                               (cons `(... ,temp) elts))))))
+
                 ((vararg? R)
                  (let ((temp (make-ssavalue)))
                    `(block ,@(reverse stmts)
@@ -2187,6 +2231,49 @@
            lhss)
        (unnecessary ,xx))))
 
+(define (destructure- i lhss xx n st end)
+  (if (null? lhss)
+      '()
+      (let* ((lhs  (car lhss))
+             (lhs- (cond ((or (symbol? lhs) (ssavalue? lhs))
+                          lhs)
+                         ((vararg? lhs)
+                          (let ((lhs- (cadr lhs)))
+                            (if (or (symbol? lhs-) (ssavalue? lhs-))
+                                lhs
+                                `(|...| ,(if (eventually-call? lhs-)
+                                             (gensy)
+                                             (make-ssavalue))))))
+                         ;; can't use ssavalues if it's a function definition
+                         ((eventually-call? lhs) (gensy))
+                         (else (make-ssavalue)))))
+        (if (and (vararg? lhs) (any vararg? (cdr lhss)))
+            (error "multiple \"...\" on lhs of assignment"))
+        (if (not (eq? lhs lhs-))
+            (if (vararg? lhs)
+                (set-car! end (cons (expand-forms `(= ,(cadr lhs) ,(cadr lhs-))) (car end)))
+                (set-car! end (cons (expand-forms `(= ,lhs ,lhs-)) (car end)))))
+        (if (vararg? lhs-)
+            (if (= i n)
+                (if (underscore-symbol? (cadr lhs-))
+                    '()
+                    (list (expand-forms
+                            `(= ,(cadr lhs-) (call (top rest) ,xx ,@(if (eq? i 1) '() `(,st)))))))
+                (let ((tail (if (eventually-call? lhs) (gensy) (make-ssavalue))))
+                  (cons (expand-forms
+                          (lower-tuple-assignment
+                            (list (cadr lhs-) tail)
+                            `(call (top split_rest) ,xx ,(- n i) ,@(if (eq? i 1) '() `(,st)))))
+                        (destructure- 1 (cdr lhss) tail (- n i) st end))))
+            (cons (expand-forms
+                    (lower-tuple-assignment
+                      (if (= i n)
+                          (list lhs-)
+                          (list lhs- st))
+                      `(call (top indexed_iterate)
+                             ,xx ,i ,@(if (eq? i 1) '() `(,st)))))
+                  (destructure- (+ i 1) (cdr lhss) xx n st end))))))
+
 (define (expand-tuple-destruct lhss x)
   (define (sides-match? l r)
     ;; l and r either have equal lengths, or r has a trailing ...
@@ -2203,64 +2290,26 @@
        (tuple-to-assignments lhss x))
       ;; (a, b, ...) = other
       (begin
-        ;; like memq, but if last element of lhss is (... sym),
-        ;; check against sym instead
+        ;; like memq, but if lhs is (... sym), check against sym instead
         (define (in-lhs? x lhss)
           (if (null? lhss)
               #f
               (let ((l (car lhss)))
                 (cond ((and (pair? l) (eq? (car l) '|...|))
-                       (if (null? (cdr lhss))
-                           (eq? (cadr l) x)
-                           (error (string "invalid \"...\" on non-final assignment location \""
-                                          (cadr l) "\""))))
+                       (eq? (cadr l) x))
                       ((eq? l x) #t)
                       (else (in-lhs? x (cdr lhss)))))))
         ;; in-lhs? also checks for invalid syntax, so always call it first
         (let* ((xx  (maybe-ssavalue lhss x in-lhs?))
                (ini (if (eq? x xx) '() (list (sink-assignment xx (expand-forms x)))))
                (n   (length lhss))
-               ;; skip last assignment if it is an all-underscore vararg
-               (n   (if (> n 0)
-                        (let ((l (last lhss)))
-                          (if (and (vararg? l) (underscore-symbol? (cadr l)))
-                              (- n 1)
-                              n))
-                        n))
                (st  (gensy))
-               (end '()))
+               (end (list (list))))
           `(block
             ,@(if (> n 0) `((local ,st)) '())
             ,@ini
-            ,@(map (lambda (i lhs)
-                     (let ((lhs- (cond ((or (symbol? lhs) (ssavalue? lhs))
-                                        lhs)
-                                       ((vararg? lhs)
-                                        (let ((lhs- (cadr lhs)))
-                                          (if (or (symbol? lhs-) (ssavalue? lhs-))
-                                              lhs
-                                              `(|...| ,(if (eventually-call? lhs-)
-                                                           (gensy)
-                                                           (make-ssavalue))))))
-                                       ;; can't use ssavalues if it's a function definition
-                                       ((eventually-call? lhs) (gensy))
-                                       (else (make-ssavalue)))))
-                       (if (not (eq? lhs lhs-))
-                           (if (vararg? lhs)
-                               (set! end (cons (expand-forms `(= ,(cadr lhs) ,(cadr lhs-))) end))
-                               (set! end (cons (expand-forms `(= ,lhs ,lhs-)) end))))
-                       (expand-forms
-                         (if (vararg? lhs-)
-                             `(= ,(cadr lhs-) (call (top rest) ,xx ,@(if (eq? i 0) '() `(,st))))
-                             (lower-tuple-assignment
-                               (if (= i (- n 1))
-                                   (list lhs-)
-                                   (list lhs- st))
-                               `(call (top indexed_iterate)
-                                      ,xx ,(+ i 1) ,@(if (eq? i 0) '() `(,st))))))))
-                   (iota n)
-                   lhss)
-            ,@(reverse end)
+            ,@(destructure- 1 lhss xx n st end)
+            ,@(reverse (car end))
             (unnecessary ,xx))))))
 
 ;; move an assignment into the last statement of a block to keep more statements at top level