Support init keyword in sum/prod/maximum/minimum (#36188)

Co-authored-by: Tim Holy <tim.holy@gmail.com>
Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>

NEWS.md

@@ -47,6 +47,7 @@ Standard library changes
 * The function `isapprox(x,y)` now accepts the `norm` keyword argument also for numeric (i.e., non-array) arguments `x` and `y` ([#35883]).
 * `view`, `@view`, and `@views` now work on `AbstractString`s, returning a `SubString` when appropriate ([#35879]).
 * All `AbstractUnitRange{<:Integer}`s now work with `SubString`, `view`, `@view` and `@views` on strings ([#35879]).
+* `sum`, `prod`, `maximum`, and `minimum` now support `init` keyword argument ([#36188], [#35839]).
 
 #### LinearAlgebra
 * New method `LinearAlgebra.issuccess(::CholeskyPivoted)` for checking whether pivoted Cholesky factorization was successful ([#36002]).

base/reduce.jl

@@ -45,7 +45,7 @@ function mapfoldl_impl(f::F, op::OP, nt, itr) where {F,OP}
 end
 
 function foldl_impl(op::OP, nt, itr) where {OP}
-    v = _foldl_impl(op, get(nt, :init, _InitialValue()), itr)
+    v = _foldl_impl(op, nt, itr)
     v isa _InitialValue && return reduce_empty_iter(op, itr)
     return v
 end
@@ -157,7 +157,7 @@ Like [`mapreduce`](@ref), but with guaranteed left associativity, as in [`foldl`
 If provided, the keyword argument `init` will be used exactly once. In general, it will be
 necessary to provide `init` to work with empty collections.
 """
-mapfoldl(f, op, itr; kw...) = mapfoldl_impl(f, op, kw.data, itr)
+mapfoldl(f, op, itr; init=_InitialValue()) = mapfoldl_impl(f, op, init, itr)
 
 """
     foldl(op, itr; [init])
@@ -200,7 +200,7 @@ Like [`mapreduce`](@ref), but with guaranteed right associativity, as in [`foldr
 provided, the keyword argument `init` will be used exactly once. In general, it will be
 necessary to provide `init` to work with empty collections.
 """
-mapfoldr(f, op, itr; kw...) = mapfoldr_impl(f, op, kw.data, itr)
+mapfoldr(f, op, itr; init=_InitialValue()) = mapfoldr_impl(f, op, init, itr)
 
 
 """
@@ -462,14 +462,21 @@ reduce(op, a::Number) = a  # Do we want this?
 ## sum
 
 """
-    sum(f, itr)
+    sum(f, itr; [init])
 
 Sum the results of calling function `f` on each element of `itr`.
 
 The return type is `Int` for signed integers of less than system word size, and
 `UInt` for unsigned integers of less than system word size.  For all other
 arguments, a common return type is found to which all arguments are promoted.
 
+The value returned for empty `itr` can be specified by `init`. It must be
+the additive identity (i.e. zero) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> sum(abs2, [2; 3; 4])
@@ -491,60 +498,88 @@ In the former case, the integers are widened to system word size and therefore
 the result is 128. In the latter case, no such widening happens and integer
 overflow results in -128.
 """
-sum(f, a) = mapreduce(f, add_sum, a)
+sum(f, a; kw...) = mapreduce(f, add_sum, a; kw...)
 
 """
-    sum(itr)
+    sum(itr; [init])
 
 Returns the sum of all elements in a collection.
 
 The return type is `Int` for signed integers of less than system word size, and
 `UInt` for unsigned integers of less than system word size.  For all other
 arguments, a common return type is found to which all arguments are promoted.
 
+The value returned for empty `itr` can be specified by `init`. It must be
+the additive identity (i.e. zero) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> sum(1:20)
 210
+
+julia> sum(1:20; init = 0.0)
+210.0
 ```
 """
-sum(a) = sum(identity, a)
-sum(a::AbstractArray{Bool}) = count(a)
+sum(a; kw...) = sum(identity, a; kw...)
+sum(a::AbstractArray{Bool}; kw...) =
+    kw.data === NamedTuple() ? count(a) : reduce(add_sum, a; kw...)
 
 ## prod
 """
-    prod(f, itr)
+    prod(f, itr; [init])
 
 Returns the product of `f` applied to each element of `itr`.
 
 The return type is `Int` for signed integers of less than system word size, and
 `UInt` for unsigned integers of less than system word size.  For all other
 arguments, a common return type is found to which all arguments are promoted.
 
+The value returned for empty `itr` can be specified by `init`. It must be the
+multiplicative identity (i.e. one) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> prod(abs2, [2; 3; 4])
 576
 ```
 """
-prod(f, a) = mapreduce(f, mul_prod, a)
+prod(f, a; kw...) = mapreduce(f, mul_prod, a; kw...)
 
 """
-    prod(itr)
+    prod(itr; [init])
 
 Returns the product of all elements of a collection.
 
 The return type is `Int` for signed integers of less than system word size, and
 `UInt` for unsigned integers of less than system word size.  For all other
 arguments, a common return type is found to which all arguments are promoted.
 
+The value returned for empty `itr` can be specified by `init`. It must be the
+multiplicative identity (i.e. one) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
-julia> prod(1:20)
-2432902008176640000
+julia> prod(1:5)
+120
+
+julia> prod(1:5; init = 1.0)
+120.0
 ```
 """
-prod(a) = mapreduce(identity, mul_prod, a)
+prod(a; kw...) = mapreduce(identity, mul_prod, a; kw...)
 
 ## maximum & minimum
 _fast(::typeof(min),x,y) = min(x,y)
@@ -610,62 +645,122 @@ function mapreduce_impl(f, op::Union{typeof(max), typeof(min)},
 end
 
 """
-    maximum(f, itr)
+    maximum(f, itr; [init])
 
 Returns the largest result of calling function `f` on each element of `itr`.
 
+The value returned for empty `itr` can be specified by `init`. It must be
+a neutral element for `max` (i.e. which is less than or equal to any
+other element) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> maximum(length, ["Julion", "Julia", "Jule"])
 6
+
+julia> maximum(length, []; init=-1)
+-1
+
+julia> maximum(sin, Real[]; init=-1.0)  # good, since output of sin is >= -1
+-1.0
 ```
 """
-maximum(f, a) = mapreduce(f, max, a)
+maximum(f, a; kw...) = mapreduce(f, max, a; kw...)
 
 """
-    minimum(f, itr)
+    minimum(f, itr; [init])
 
 Returns the smallest result of calling function `f` on each element of `itr`.
 
+The value returned for empty `itr` can be specified by `init`. It must be
+a neutral element for `min` (i.e. which is greater than or equal to any
+other element) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> minimum(length, ["Julion", "Julia", "Jule"])
 4
+
+julia> minimum(length, []; init=typemax(Int64))
+9223372036854775807
+
+julia> minimum(sin, Real[]; init=1.0)  # good, since output of sin is <= 1
+1.0
 ```
 """
-minimum(f, a) = mapreduce(f, min, a)
+minimum(f, a; kw...) = mapreduce(f, min, a; kw...)
 
 """
-    maximum(itr)
+    maximum(itr; [init])
 
 Returns the largest element in a collection.
 
+The value returned for empty `itr` can be specified by `init`. It must be
+a neutral element for `max` (i.e. which is less than or equal to any
+other element) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> maximum(-20.5:10)
 9.5
 
 julia> maximum([1,2,3])
 3
+
+julia> maximum(())
+ERROR: ArgumentError: reducing over an empty collection is not allowed
+Stacktrace:
+[...]
+
+julia> maximum((); init=-Inf)
+-Inf
 ```
 """
-maximum(a) = mapreduce(identity, max, a)
+maximum(a; kw...) = mapreduce(identity, max, a; kw...)
 
 """
-    minimum(itr)
+    minimum(itr; [init])
 
 Returns the smallest element in a collection.
 
+The value returned for empty `itr` can be specified by `init`. It must be
+a neutral element for `min` (i.e. which is greater than or equal to any
+other element) as it is unspecified whether `init` is used
+for non-empty collections.
+
+!!! compat "Julia 1.6"
+    Keyword argument `init` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> minimum(-20.5:10)
 -20.5
 
 julia> minimum([1,2,3])
 1
+
+julia> minimum([])
+ERROR: ArgumentError: reducing over an empty collection is not allowed
+Stacktrace:
+[...]
+
+julia> minimum([]; init=Inf)
+Inf
 ```
 """
-minimum(a) = mapreduce(identity, min, a)
+minimum(a; kw...) = mapreduce(identity, min, a; kw...)
 
 ## all & any
 

base/reducedim.jl

@@ -307,21 +307,21 @@ julia> mapreduce(isodd, |, a, dims=1)
  1  1  1  1
 ```
 """
-mapreduce(f, op, A::AbstractArrayOrBroadcasted; dims=:, kw...) =
-    _mapreduce_dim(f, op, kw.data, A, dims)
+mapreduce(f, op, A::AbstractArrayOrBroadcasted; dims=:, init=_InitialValue()) =
+    _mapreduce_dim(f, op, init, A, dims)
 mapreduce(f, op, A::AbstractArrayOrBroadcasted...; kw...) =
     reduce(op, map(f, A...); kw...)
 
-_mapreduce_dim(f, op, nt::NamedTuple{(:init,)}, A::AbstractArrayOrBroadcasted, ::Colon) =
-    mapfoldl(f, op, A; nt...)
+_mapreduce_dim(f, op, nt, A::AbstractArrayOrBroadcasted, ::Colon) =
+    mapfoldl_impl(f, op, nt, A)
 
-_mapreduce_dim(f, op, ::NamedTuple{()}, A::AbstractArrayOrBroadcasted, ::Colon) =
+_mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, ::Colon) =
     _mapreduce(f, op, IndexStyle(A), A)
 
-_mapreduce_dim(f, op, nt::NamedTuple{(:init,)}, A::AbstractArrayOrBroadcasted, dims) =
-    mapreducedim!(f, op, reducedim_initarray(A, dims, nt.init), A)
+_mapreduce_dim(f, op, nt, A::AbstractArrayOrBroadcasted, dims) =
+    mapreducedim!(f, op, reducedim_initarray(A, dims, nt), A)
 
-_mapreduce_dim(f, op, ::NamedTuple{()}, A::AbstractArrayOrBroadcasted, dims) =
+_mapreduce_dim(f, op, ::_InitialValue, A::AbstractArrayOrBroadcasted, dims) =
     mapreducedim!(f, op, reducedim_init(f, op, A, dims), A)
 
 """
@@ -717,12 +717,12 @@ for (fname, _fname, op) in [(:sum,     :_sum,     :add_sum), (:prod,    :_prod,
                             (:maximum, :_maximum, :max),     (:minimum, :_minimum, :min)]
     @eval begin
         # User-facing methods with keyword arguments
-        @inline ($fname)(a::AbstractArray; dims=:) = ($_fname)(a, dims)
-        @inline ($fname)(f, a::AbstractArray; dims=:) = ($_fname)(f, a, dims)
+        @inline ($fname)(a::AbstractArray; dims=:, kw...) = ($_fname)(a, dims; kw...)
+        @inline ($fname)(f, a::AbstractArray; dims=:, kw...) = ($_fname)(f, a, dims; kw...)
 
         # Underlying implementations using dispatch
-        ($_fname)(a, ::Colon) = ($_fname)(identity, a, :)
-        ($_fname)(f, a, ::Colon) = mapreduce(f, $op, a)
+        ($_fname)(a, ::Colon; kw...) = ($_fname)(identity, a, :; kw...)
+        ($_fname)(f, a, ::Colon; kw...) = mapreduce(f, $op, a; kw...)
     end
 end
 
@@ -743,8 +743,8 @@ for (fname, op) in [(:sum, :add_sum), (:prod, :mul_prod),
             mapreducedim!(f, $(op), initarray!(r, $(op), init, A), A)
         $(fname!)(r::AbstractArray, A::AbstractArray; init::Bool=true) = $(fname!)(identity, r, A; init=init)
 
-        $(_fname)(A, dims)    = $(_fname)(identity, A, dims)
-        $(_fname)(f, A, dims) = mapreduce(f, $(op), A, dims=dims)
+        $(_fname)(A, dims; kw...)    = $(_fname)(identity, A, dims; kw...)
+        $(_fname)(f, A, dims; kw...) = mapreduce(f, $(op), A; dims=dims, kw...)
     end
 end
 

base/reflection.jl

@@ -1221,10 +1221,12 @@ See also [`applicable`](@ref).
 julia> hasmethod(length, Tuple{Array})
 true
 
-julia> hasmethod(sum, Tuple{Function, Array}, (:dims,))
+julia> f(; oranges=0) = oranges;
+
+julia> hasmethod(f, Tuple{}, (:oranges,))
 true
 
-julia> hasmethod(sum, Tuple{Function, Array}, (:apples, :bananas))
+julia> hasmethod(f, Tuple{}, (:apples, :bananas))
 false
 
 julia> g(; xs...) = 4;