unify reduce/reducedim empty case behaviors

Previously, Julia tried to guess at initial values for empty dimensional reductions in a slightly different way than whole-array reductions. This change unifies those behaviors, such that `mapreduce_empty` is called for empty dimensional reductions, just like it is called for empty whole-array reductions.

Beyond the unification (which is valuable in its own right), this change enables some downstream simplifications to dimensional reductions and is likely to be the "most breaking" public behavior in a refactoring targeting more correctness improvements.

Author: mbauman

NEWS.md

@@ -33,6 +33,10 @@ New library features
 Standard library changes
 ------------------------
 
+* Empty dimensional reductions (e.g., `reduce` and `mapreduce` with the `dims` keyword
+  selecting one or more dimensions) now behave like their whole-array (`dims=:`) counterparts,
+  only returning values in unambiguous cases and erroring otherwise.
+
 #### JuliaSyntaxHighlighting
 
 #### LinearAlgebra

base/reducedim.jl

@@ -337,8 +337,10 @@ _mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, ::Colon) =
 _mapreduce_dim(f, op, nt, A::AbstractArrayOrBroadcasted, dims) =
     mapreducedim!(f, op, reducedim_initarray(A, dims, nt), A)
 
-_mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, dims) =
+function _mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, dims)
+    isempty(A) && return fill(mapreduce_empty(f, op, eltype(A)), reduced_indices(A, dims))
     mapreducedim!(f, op, reducedim_init(f, op, A, dims), A)
+end
 
 """
     reduce(f, A::AbstractArray; dims=:, [init])
@@ -1124,10 +1126,7 @@ findmin(f, A::AbstractArray; dims=:) = _findmin(f, A, dims)
 function _findmin(f, A, region)
     ri = reduced_indices0(A, region)
     if isempty(A)
-        if prod(map(length, reduced_indices(A, region))) != 0
-            throw(ArgumentError("collection slices must be non-empty"))
-        end
-        similar(A, promote_op(f, eltype(A)), ri), zeros(eltype(keys(A)), ri)
+        _empty_reduce_error()
     else
         fA = f(first(A))
         findminmax!(f, isgreater, fill!(similar(A, _findminmax_inittype(f, A), ri), fA),
@@ -1197,10 +1196,7 @@ findmax(f, A::AbstractArray; dims=:) = _findmax(f, A, dims)
 function _findmax(f, A, region)
     ri = reduced_indices0(A, region)
     if isempty(A)
-        if prod(map(length, reduced_indices(A, region))) != 0
-            throw(ArgumentError("collection slices must be non-empty"))
-        end
-        similar(A, promote_op(f, eltype(A)), ri), zeros(eltype(keys(A)), ri)
+        _empty_reduce_error()
     else
         fA = f(first(A))
         findminmax!(f, isless, fill!(similar(A, _findminmax_inittype(f, A), ri), fA),

test/reducedim.jl

@@ -209,19 +209,19 @@ end
 
     for f in (minimum, maximum)
         @test_throws "reducing over an empty collection is not allowed" f(A, dims=1)
-        @test isequal(f(A, dims=2), zeros(Int, 0, 1))
+        @test_throws "reducing over an empty collection is not allowed" isequal(f(A, dims=2), zeros(Int, 0, 1))
         @test_throws "reducing over an empty collection is not allowed" f(A, dims=(1, 2))
-        @test isequal(f(A, dims=3), zeros(Int, 0, 1))
+        @test_throws "reducing over an empty collection is not allowed" isequal(f(A, dims=3), zeros(Int, 0, 1))
     end
     for f in (findmin, findmax)
-        @test_throws ArgumentError f(A, dims=1)
-        @test isequal(f(A, dims=2), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
-        @test_throws ArgumentError f(A, dims=(1, 2))
-        @test isequal(f(A, dims=3), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
-        @test_throws ArgumentError f(abs2, A, dims=1)
-        @test isequal(f(abs2, A, dims=2), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
-        @test_throws ArgumentError f(abs2, A, dims=(1, 2))
-        @test isequal(f(abs2, A, dims=3), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
+        @test_throws "reducing over an empty collection is not allowed" f(A, dims=1)
+        @test_throws "reducing over an empty collection is not allowed" isequal(f(A, dims=2), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
+        @test_throws "reducing over an empty collection is not allowed" f(A, dims=(1, 2))
+        @test_throws "reducing over an empty collection is not allowed" isequal(f(A, dims=3), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
+        @test_throws "reducing over an empty collection is not allowed" f(abs2, A, dims=1)
+        @test_throws "reducing over an empty collection is not allowed" isequal(f(abs2, A, dims=2), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
+        @test_throws "reducing over an empty collection is not allowed" f(abs2, A, dims=(1, 2))
+        @test_throws "reducing over an empty collection is not allowed" isequal(f(abs2, A, dims=3), (zeros(Int, 0, 1), zeros(Int, 0, 1)))
     end
 
 end