allow slurping in lhs of assignment

NEWS.md

@@ -28,6 +28,11 @@ New language features
 * The postfix conjugate transpose operator `'` now accepts Unicode modifiers as
   suffixes, so e.g. `a'ᵀ` is parsed as `var"'ᵀ"(a)`, which can be defined by the
   user. `a'ᵀ` parsed as `a' * ᵀ` before, so this is a minor change ([#37247]).
+* It is now possible to use varargs on the left-hand side of assignments for taking any
+  number of items from the front of an iterable collection, while also collecting the rest,
+  like `a, b... = [1, 2, 3]`, for example. This syntax is implemented using `Base.rest`,
+  which can be overloaded to customize its behavior for different collection types
+  ([#37410]).
 
 Language changes
 ----------------
@@ -96,6 +101,8 @@ New library functions
   efficiently ([#35816]).
 * New function `addenv` for adding environment mappings into a `Cmd` object, returning the new `Cmd` object.
 * New function `insorted` for determining whether an element is in a sorted collection or not ([#37490]).
+* New function `Base.rest` for taking the rest of a collection, starting from a specific
+  iteration state, in a generic way ([#37410]).
 
 New library features
 --------------------

base/abstractarray.jl

@@ -2399,3 +2399,11 @@ function hash(A::AbstractArray, h::UInt)
 
     return h
 end
+
+# The semantics of `collect` are weird. Better to write our own
+function rest(a::AbstractArray{T}, state...) where {T}
+    v = Vector{T}(undef, 0)
+    # assume only very few items are taken from the front
+    sizehint!(v, length(a))
+    return foldl(push!, Iterators.rest(a, state...), init=v)
+end

base/tuple.jl

@@ -96,6 +96,38 @@ function indexed_iterate(I, i, state)
     x
 end
 
+"""
+    Base.rest(collection[, itr_state])
+
+Generic function for taking the tail of `collection`, starting from a specific iteration
+state `itr_state`. Return a `Tuple`, if `collection` itself is a `Tuple`, a `Vector`, if
+`collection` is an `AbstractArray` and `Iterators.rest(collection[, itr_state])` otherwise.
+Can be overloaded for user-defined collection types to customize the behavior of slurping
+in assignments, like `a, b... = collection`.
+
+!!! compat "Julia 1.6"
+    `Base.rest` requires at least Julia 1.6.
+
+# 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.rest(a, state)
+(1, [3, 2, 4])
+```
+"""
+function rest end
+rest(t::Tuple) = t
+rest(t::Tuple, i::Int) = ntuple(x -> getfield(t, x+i-1), length(t)-i+1)
+rest(a::Array, i::Int=1) = a[i:end]
+rest(itr, state...) = Iterators.rest(itr, state...)
+
 # 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

@@ -138,6 +138,7 @@ Base.filter!
 Base.replace(::Any, ::Pair...)
 Base.replace(::Base.Callable, ::Any)
 Base.replace!
+Base.rest
 ```
 
 ## Indexable Collections

src/julia-syntax.scm

@@ -1430,7 +1430,8 @@
                 ,@(reverse after)
                 (unnecessary (tuple ,@(reverse elts))))
         (let ((L (car lhss))
-              (R (car rhss)))
+              ;; rhss can be null iff L is a vararg
+              (R (if (null? rhss) '() (car rhss))))
           (cond ((and (symbol-like? L)
                       (or (not (pair? R)) (quoted? R) (equal? R '(null)))
                       ;; overwrite var immediately if it doesn't occur elsewhere
@@ -1442,6 +1443,16 @@
                        (cons (make-assignment L R) stmts)
                        after
                        (cons R elts)))
+                ((vararg? L)
+                 (if (null? (cdr lhss))
+                     (let ((temp (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) "\""))))
                 ((vararg? R)
                  (let ((temp (make-ssavalue)))
                    `(block ,@(reverse stmts)
@@ -2066,6 +2077,7 @@
             (define (sides-match? l r)
               ;; l and r either have equal lengths, or r has a trailing ...
               (cond ((null? l)          (null? r))
+                    ((vararg? (car l))  #t)
                     ((null? r)          #f)
                     ((vararg? (car r))  (null? (cdr r)))
                     (else               (sides-match? (cdr l) (cdr r)))))
@@ -2075,26 +2087,43 @@
                 (expand-forms
                  (tuple-to-assignments lhss x))
                 ;; (a, b, ...) = other
-                (let* ((xx  (if (or (and (symbol? x) (not (memq x lhss)))
-                                    (ssavalue? x))
-                                x (make-ssavalue)))
-                       (ini (if (eq? x xx) '() (list (sink-assignment xx (expand-forms x)))))
-                       (n   (length lhss))
-                       (st  (gensy)))
-                  `(block
-                    (local ,st)
-                    ,@ini
-                    ,.(map (lambda (i lhs)
-                             (expand-forms
-                              (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)
-                    (unnecessary ,xx))))))
+                (begin
+                  ;; like memq, but if last element of lhss 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? l x) #t)
+                                (else (in-lhs? x (cdr lhss)))))))
+                  ;; in-lhs? also checks for invalid syntax, so always call it first
+                  (let* ((xx  (if (or (and (not (in-lhs? x lhss)) (symbol? x))
+                                      (ssavalue? x))
+                                  x (make-ssavalue)))
+                         (ini (if (eq? x xx) '() (list (sink-assignment xx (expand-forms x)))))
+                         (n   (length lhss))
+                         (st  (gensy)))
+                    `(block
+                      (local ,st)
+                      ,@ini
+                      ,.(map (lambda (i lhs)
+                               (expand-forms
+                                 (if (and (pair? lhs) (eq? (car 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)
+                      (unnecessary ,xx)))))))
          ((typed_hcat)
           (error "invalid spacing in left side of indexed assignment"))
          ((typed_vcat)

test/abstractarray.jl

@@ -1193,3 +1193,10 @@ end
     @test last(itr, 1) == [itr[end]]
     @test_throws ArgumentError last(itr, -6)
 end
+
+@testset "Base.rest" begin
+    a = reshape(1:4, 2, 2)'
+    @test Base.rest(a) == a[:]
+    _, st = iterate(a)
+    @test Base.rest(a, st) == [3, 2, 4]
+end

test/syntax.jl

@@ -2493,3 +2493,54 @@ end
 
 # PR #37973
 @test Meta.parse("1¦2⌿3") == Expr(:call, :¦, 1, Expr(:call, :⌿, 2, 3))
+
+@testset "slurp in assignments" begin
+    res = begin x, y, z... = 1:7 end
+    @test res == 1:7
+    @test x == 1 && y == 2
+    @test z == Vector(3:7)
+
+    res = begin x, y, z... = [1, 2] end
+    @test res == [1, 2]
+    @test x == 1 && y == 2
+    @test z == Int[]
+
+    x = 1
+    res = begin x..., = x end
+    @test res == 1
+    @test x == 1
+
+    x, y, z... = 1:7
+    res = begin y, z, x... = z..., x, y end
+    @test res == ((3:7)..., 1, 2)
+    @test y == 3
+    @test z == 4
+    @test x == ((5:7)..., 1, 2)
+
+    res = begin x, _, y... = 1, 2 end
+    @test res == (1, 2)
+    @test x == 1
+    @test y == ()
+
+    res = begin x, y... = 1 end
+    @test res == 1
+    @test x == 1
+    @test y == Iterators.rest(1, nothing)
+
+    res = begin x, y, z... = 1, 2, 3:5 end
+    @test res == (1, 2, 3:5)
+    @test x == 1 && y == 2
+    @test z == (3:5,)
+
+    @test Meta.isexpr(Meta.@lower(begin a, b..., c = 1:3 end), :error)
+    @test Meta.isexpr(Meta.@lower(begin a, b..., c = 1, 2, 3 end), :error)
+    @test Meta.isexpr(Meta.@lower(begin a, b..., c... = 1, 2, 3 end), :error)
+
+    @test_throws BoundsError begin x, y, z... = 1:1 end
+    @test_throws BoundsError begin x, y, _, z... = 1, 2 end
+
+    car((a, d...)) = a
+    cdr((a, d...)) = d
+    @test car(1:3) == 1
+    @test cdr(1:3) == [2, 3]
+end

test/tuple.jl

@@ -575,3 +575,21 @@ end
     @test_throws BoundsError (1,2.0)[0:1]
     @test_throws BoundsError (1,2.0)[0:0]
 end
+
+@testset "Base.rest" begin
+    t = (1, 2.0, 0x03, 4f0)
+    @test Base.rest(t) === t
+    @test Base.rest(t, 2) === (2.0, 0x03, 4f0)
+
+    a = [1 2; 3 4]
+    @test Base.rest(a) == a[:]
+    @test pointer(Base.rest(a)) != pointer(a)
+    @test Base.rest(a, 3) == [2, 4]
+
+    itr = (-i for i in a)
+    @test Base.rest(itr) == itr
+    _, st = iterate(itr)
+    r = Base.rest(itr, st)
+    @test r isa Iterators.Rest
+    @test collect(r) == -[3, 2, 4]
+end