From a29e523febfe8b6ecbebfb0f651fd757e8a1a8eb Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Tue, 5 Oct 2021 19:53:09 +0200
Subject: [PATCH 1/7] Make `view(::AbstractWeights, ...)` return an
 `AbstractWeights`

This is necessary to preserve the information regarding the type of weights.
---
 src/weights.jl  |  6 ++++++
 test/weights.jl | 13 ++++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/weights.jl b/src/weights.jl
index 34fe4cd77..111f19a55 100644
--- a/src/weights.jl
+++ b/src/weights.jl
@@ -37,6 +37,12 @@ end
 
 Base.getindex(wv::W, ::Colon) where {W <: AbstractWeights} = W(copy(wv.values), sum(wv))
 
+@propagate_inbounds function Base.view(wv::W, inds...) where {W <: AbstractWeights}
+    @boundscheck checkbounds(wv, inds...)
+    @inbounds v = view(wv.values, inds...)
+    W(v, sum(v))
+end
+
 @propagate_inbounds function Base.setindex!(wv::AbstractWeights, v::Real, i::Int)
     s = v - wv[i]
     wv.values[i] = v
diff --git a/test/weights.jl b/test/weights.jl
index 7735e04f7..b84ff2c33 100644
--- a/test/weights.jl
+++ b/test/weights.jl
@@ -37,7 +37,7 @@ weight_funcs = (weights, aweights, fweights, pweights)
     @test sum(sa, wv) === 7.0
 end
 
-@testset "$f, setindex!" for f in weight_funcs
+@testset "$f, getindex, view, setindex!" for f in weight_funcs
     w = [1., 2., 3.]
     wv = f(w)
 
@@ -46,6 +46,17 @@ end
     @test sum(wv) === 6.
     @test wv == w
 
+    @test wv[[1, 3]] == w[[1, 3]]
+    @test typeof(wv[[1, 3]]) === typeof(wv)
+    @test sum( wv[[1, 3]]) === sum(w[[1, 3]])
+
+    # Check view
+    @test view(wv, [1, 3]) == view(wv, [true, false, true]) == w[[1, 3]]
+    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) === typeof(wv)
+    @test sum(view(wv, [1, 3])) === sum(view(wv, [true, false, true])) === sum(w[[1, 3]])
+    @test_throws BoundsError view(wv, [1, 5])
+    @test_throws BoundsError view(wv, [true, false, true, true])
+
     # Test setindex! success
     @test (wv[1] = 4) === 4             # setindex! returns original val
     @test wv[1] === 4.                  # value correctly converted and set

From 04dd2385fc09c6b5c98fbc998837a367b9a24fa5 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Sun, 7 Nov 2021 14:21:33 +0100
Subject: [PATCH 2/7] Make `view` return a `SubArray` of `AbstractWeights`

---
 docs/src/weights.md |  1 +
 src/weights.jl      | 14 +++++++++++---
 test/pairwise.jl    | 10 ++++++++--
 test/weights.jl     |  7 ++++++-
 4 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/docs/src/weights.md b/docs/src/weights.md
index 50f6c1bcc..7a2a4ef52 100644
--- a/docs/src/weights.md
+++ b/docs/src/weights.md
@@ -8,6 +8,7 @@ In statistical applications, it is not uncommon to assign weights to samples. To
 !!! note
     - The weight vector is a light-weight wrapper of the input vector. The input vector is NOT copied during construction.
     - The weight vector maintains the sum of weights, which is computed upon construction. If the value of the sum is pre-computed, one can supply it as the second argument to the constructor and save the time of computing the sum again.
+    - Views of weight vectors are also `AbstractWeights`, but their sum has to be recomputed by each call to `sum` as weights in the parent vector may have been mutated.
 
 
 ## Implementations
diff --git a/src/weights.jl b/src/weights.jl
index 111f19a55..838f2b552 100644
--- a/src/weights.jl
+++ b/src/weights.jl
@@ -14,6 +14,7 @@ macro weights(name)
             sum::S
         end
         $(esc(name))(values::AbstractVector{<:Real}) = $(esc(name))(values, sum(values))
+        $(esc(:weightstype))(::Type{<:$(esc(name))}) = $(esc(name))
     end
 end
 
@@ -37,12 +38,19 @@ end
 
 Base.getindex(wv::W, ::Colon) where {W <: AbstractWeights} = W(copy(wv.values), sum(wv))
 
-@propagate_inbounds function Base.view(wv::W, inds...) where {W <: AbstractWeights}
+@propagate_inbounds function Base.view(wv::W, inds...) where {S <: Real, W <: AbstractWeights{S}}
     @boundscheck checkbounds(wv, inds...)
-    @inbounds v = view(wv.values, inds...)
-    W(v, sum(v))
+    @inbounds v = invoke(view, Tuple{AbstractArray, Vararg{Any}},
+                         wv, inds...)
+    # Sum is not actually used but compute the right type for clarity
+    weightstype(W)(v, zero(S))
 end
 
+# Always recompute the sum for views of AbstractWeights, as we cannot know whether
+# the parent array has been mutated
+Base.sum(wv::AbstractWeights{S, T, <:SubArray{T, <:Any, <:AbstractWeights}}) where
+    {S<:Real, T<:Real} = sum(wv.values)
+
 @propagate_inbounds function Base.setindex!(wv::AbstractWeights, v::Real, i::Int)
     s = v - wv[i]
     wv.values[i] = v
diff --git a/test/pairwise.jl b/test/pairwise.jl
index 09699b276..fae07b55c 100644
--- a/test/pairwise.jl
+++ b/test/pairwise.jl
@@ -39,8 +39,14 @@ arbitrary_fun(x, y) = cor(x, y)
 
         @inferred pairwise(f, x, y)
 
-        @test_throws Union{ArgumentError,MethodError} pairwise(f, [Int[]], [Int[]])
-        @test_throws Union{ArgumentError,MethodError} pairwise!(f, zeros(1, 1), [Int[]], [Int[]])
+        if f === cov
+            @test pairwise(f, [Int[]], [Int[]]) ==
+                pairwise!(f, zeros(1, 1), [Int[]], [Int[]]) ==
+                reshape([0.0], 1, 1)
+        else
+            @test_throws Union{ArgumentError,MethodError} pairwise(f, [Int[]], [Int[]])
+            @test_throws Union{ArgumentError,MethodError} pairwise!(f, zeros(1, 1), [Int[]], [Int[]])
+        end
 
         res = pairwise(f, [], [])
         @test size(res) == (0, 0)
diff --git a/test/weights.jl b/test/weights.jl
index b84ff2c33..2030a1003 100644
--- a/test/weights.jl
+++ b/test/weights.jl
@@ -52,10 +52,15 @@ end
 
     # Check view
     @test view(wv, [1, 3]) == view(wv, [true, false, true]) == w[[1, 3]]
-    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) === typeof(wv)
+    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) <: StatsBase.weightstype(typeof(wv))
     @test sum(view(wv, [1, 3])) === sum(view(wv, [true, false, true])) === sum(w[[1, 3]])
     @test_throws BoundsError view(wv, [1, 5])
     @test_throws BoundsError view(wv, [true, false, true, true])
+    v = view(wv, [1, 3])
+    wv[1] += 1
+    @test sum(v) == sum(view(wv, [1, 3]))
+    v[1] -= 1
+    @test wv[1] == 1
 
     # Test setindex! success
     @test (wv[1] = 4) === 4             # setindex! returns original val

From 192ccca63ce15843ba060e036df4f41c5d085022 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Sun, 7 Nov 2021 15:49:33 +0100
Subject: [PATCH 3/7] Make `copy` preserve weights type

---
 src/weights.jl  | 17 ++++++++++++++---
 test/weights.jl | 11 ++++++++++-
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/src/weights.jl b/src/weights.jl
index 838f2b552..a7c75febe 100644
--- a/src/weights.jl
+++ b/src/weights.jl
@@ -36,7 +36,10 @@ end
     W(v, sum(v))
 end
 
-Base.getindex(wv::W, ::Colon) where {W <: AbstractWeights} = W(copy(wv.values), sum(wv))
+Base.getindex(wv::AbstractWeights, ::Colon) = copy(wv)
+
+Base.copy(wv::W) where {W <: AbstractWeights} =
+    weightstype(W)(copy(wv.values), sum(wv))
 
 @propagate_inbounds function Base.view(wv::W, inds...) where {S <: Real, W <: AbstractWeights{S}}
     @boundscheck checkbounds(wv, inds...)
@@ -51,6 +54,10 @@ end
 Base.sum(wv::AbstractWeights{S, T, <:SubArray{T, <:Any, <:AbstractWeights}}) where
     {S<:Real, T<:Real} = sum(wv.values)
 
+Base.copy(wv::W) where
+    {S<:Real, T<:Real, W<:AbstractWeights{S, T, <:SubArray{T, <:Any, <:AbstractWeights}}} =
+    weightstype(W)(copy(view(parent(wv.values).values, parentindices(wv.values)...)), wv.sum)
+
 @propagate_inbounds function Base.setindex!(wv::AbstractWeights, v::Real, i::Int)
     s = v - wv[i]
     wv.values[i] = v
@@ -351,8 +358,12 @@ end
 
 for w in (AnalyticWeights, FrequencyWeights, ProbabilityWeights, Weights)
     @eval begin
-        Base.isequal(x::$w, y::$w) = isequal(x.sum, y.sum) && isequal(x.values, y.values)
-        Base.:(==)(x::$w, y::$w)   = (x.sum == y.sum) && (x.values == y.values)
+        Base.isequal(x::$w, y::$w) =
+            (x.values isa SubArray || y.values isa SubArray || isequal(x.values, y.values)) &&
+                isequal(x.values, y.values)
+        Base.:(==)(x::$w, y::$w)   =
+            (x.values isa SubArray || y.values isa SubArray || (x.sum == y.sum)) &&
+                (x.values == y.values)
     end
 end
 
diff --git a/test/weights.jl b/test/weights.jl
index 2030a1003..35c14eff0 100644
--- a/test/weights.jl
+++ b/test/weights.jl
@@ -50,9 +50,16 @@ end
     @test typeof(wv[[1, 3]]) === typeof(wv)
     @test sum( wv[[1, 3]]) === sum(w[[1, 3]])
 
+    # Check copy
+    @test copy(wv) == wv
+    @test typeof(copy(wv)) === typeof(wv)
+
     # Check view
+    @test view(wv, :) == wv
+    @test typeof(view(wv, :)) <: StatsBase.weightstype(typeof(wv))
     @test view(wv, [1, 3]) == view(wv, [true, false, true]) == w[[1, 3]]
-    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) <: StatsBase.weightstype(typeof(wv))
+    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) <:
+        StatsBase.weightstype(typeof(wv))
     @test sum(view(wv, [1, 3])) === sum(view(wv, [true, false, true])) === sum(w[[1, 3]])
     @test_throws BoundsError view(wv, [1, 5])
     @test_throws BoundsError view(wv, [true, false, true, true])
@@ -61,6 +68,8 @@ end
     @test sum(v) == sum(view(wv, [1, 3]))
     v[1] -= 1
     @test wv[1] == 1
+    @test copy(v) == v[:] == v
+    @test typeof(copy(v)) === typeof(v[:]) == typeof(wv)
 
     # Test setindex! success
     @test (wv[1] = 4) === 4             # setindex! returns original val

From 91208f46aced1eede5ebda881b123e1f93ca7365 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Sun, 7 Nov 2021 15:59:15 +0100
Subject: [PATCH 4/7] Revert unrelated changes

---
 test/pairwise.jl | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/test/pairwise.jl b/test/pairwise.jl
index fae07b55c..09699b276 100644
--- a/test/pairwise.jl
+++ b/test/pairwise.jl
@@ -39,14 +39,8 @@ arbitrary_fun(x, y) = cor(x, y)
 
         @inferred pairwise(f, x, y)
 
-        if f === cov
-            @test pairwise(f, [Int[]], [Int[]]) ==
-                pairwise!(f, zeros(1, 1), [Int[]], [Int[]]) ==
-                reshape([0.0], 1, 1)
-        else
-            @test_throws Union{ArgumentError,MethodError} pairwise(f, [Int[]], [Int[]])
-            @test_throws Union{ArgumentError,MethodError} pairwise!(f, zeros(1, 1), [Int[]], [Int[]])
-        end
+        @test_throws Union{ArgumentError,MethodError} pairwise(f, [Int[]], [Int[]])
+        @test_throws Union{ArgumentError,MethodError} pairwise!(f, zeros(1, 1), [Int[]], [Int[]])
 
         res = pairwise(f, [], [])
         @test size(res) == (0, 0)

From efa81b6cd454ae6f59bb801071b9b88641236104 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Sun, 7 Nov 2021 16:19:50 +0100
Subject: [PATCH 5/7] Fixes

---
 src/weights.jl  | 11 ++++++++---
 test/weights.jl |  4 ++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/weights.jl b/src/weights.jl
index a7c75febe..b6f41362e 100644
--- a/src/weights.jl
+++ b/src/weights.jl
@@ -43,12 +43,17 @@ Base.copy(wv::W) where {W <: AbstractWeights} =
 
 @propagate_inbounds function Base.view(wv::W, inds...) where {S <: Real, W <: AbstractWeights{S}}
     @boundscheck checkbounds(wv, inds...)
-    @inbounds v = invoke(view, Tuple{AbstractArray, Vararg{Any}},
-                         wv, inds...)
+    @inbounds v = invoke(view, Tuple{AbstractArray, Vararg{Any}}, wv, inds...)
     # Sum is not actually used but compute the right type for clarity
     weightstype(W)(v, zero(S))
 end
 
+# This method is implemented for backward compatibility
+@propagate_inbounds function Base.view(wv::W, inds::Integer) where {S <: Real, W <: AbstractWeights{S}}
+    @boundscheck checkbounds(wv, inds)
+    @inbounds invoke(view, Tuple{AbstractArray, Vararg{Any}}, wv, inds...)
+end
+
 # Always recompute the sum for views of AbstractWeights, as we cannot know whether
 # the parent array has been mutated
 Base.sum(wv::AbstractWeights{S, T, <:SubArray{T, <:Any, <:AbstractWeights}}) where
@@ -362,7 +367,7 @@ for w in (AnalyticWeights, FrequencyWeights, ProbabilityWeights, Weights)
             (x.values isa SubArray || y.values isa SubArray || isequal(x.values, y.values)) &&
                 isequal(x.values, y.values)
         Base.:(==)(x::$w, y::$w)   =
-            (x.values isa SubArray || y.values isa SubArray || (x.sum == y.sum)) &&
+            (x.values isa SubArray || y.values isa SubArray || x.sum == y.sum) &&
                 (x.values == y.values)
     end
 end
diff --git a/test/weights.jl b/test/weights.jl
index 35c14eff0..87870b8e8 100644
--- a/test/weights.jl
+++ b/test/weights.jl
@@ -70,6 +70,10 @@ end
     @test wv[1] == 1
     @test copy(v) == v[:] == v
     @test typeof(copy(v)) === typeof(v[:]) == typeof(wv)
+    @test view(wv, 1) == fill(wv[1])
+    @test view(wv, 1) isa SubArray
+    @test view(wv, CartesianIndex(1, 1)) == fill(wv[CartesianIndex(1, 1)])
+    @test view(wv, CartesianIndex(1, 1)) isa SubArray
 
     # Test setindex! success
     @test (wv[1] = 4) === 4             # setindex! returns original val

From 95b3650f3891f1b1f870780c4ee9c8c0b3214697 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Sun, 7 Nov 2021 16:29:50 +0100
Subject: [PATCH 6/7] More fixes

---
 src/weights.jl  | 5 +++--
 test/weights.jl | 6 ++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/weights.jl b/src/weights.jl
index b6f41362e..7720f809d 100644
--- a/src/weights.jl
+++ b/src/weights.jl
@@ -49,8 +49,9 @@ Base.copy(wv::W) where {W <: AbstractWeights} =
 end
 
 # This method is implemented for backward compatibility
-@propagate_inbounds function Base.view(wv::W, inds::Integer) where {S <: Real, W <: AbstractWeights{S}}
-    @boundscheck checkbounds(wv, inds)
+@propagate_inbounds function Base.view(wv::W, inds::Union{Integer, CartesianIndex}...) where
+    {S <: Real, W <: AbstractWeights{S}}
+    @boundscheck checkbounds(wv, inds...)
     @inbounds invoke(view, Tuple{AbstractArray, Vararg{Any}}, wv, inds...)
 end
 
diff --git a/test/weights.jl b/test/weights.jl
index 87870b8e8..33c3611d1 100644
--- a/test/weights.jl
+++ b/test/weights.jl
@@ -70,10 +70,16 @@ end
     @test wv[1] == 1
     @test copy(v) == v[:] == v
     @test typeof(copy(v)) === typeof(v[:]) == typeof(wv)
+    @test view(wv, [1, 2], 1) == wv[[1, 2]]
+    @test typeof(view(wv, [1, 2], 1)) <: StatsBase.weightstype(typeof(wv))
     @test view(wv, 1) == fill(wv[1])
     @test view(wv, 1) isa SubArray
+    @test view(wv, 1, 1) == fill(wv[1])
+    @test view(wv, 1, 1) isa SubArray
     @test view(wv, CartesianIndex(1, 1)) == fill(wv[CartesianIndex(1, 1)])
     @test view(wv, CartesianIndex(1, 1)) isa SubArray
+    @test view(wv, CartesianIndex(1, 1), 1) == fill(wv[CartesianIndex(1, 1), 1])
+    @test view(wv, CartesianIndex(1, 1), 1) isa SubArray
 
     # Test setindex! success
     @test (wv[1] = 4) === 4             # setindex! returns original val

From f9db85a287e66f78229b88c8bb96a5899c7d3669 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Thu, 11 Nov 2021 17:30:45 +0100
Subject: [PATCH 7/7] Safer approach, more tests, review fixes

---
 src/weights.jl  |  34 +++++----
 test/cov.jl     |  16 ++--
 test/moments.jl |  39 ++++++----
 test/weights.jl | 189 ++++++++++++++++++++++++++++--------------------
 4 files changed, 166 insertions(+), 112 deletions(-)

diff --git a/src/weights.jl b/src/weights.jl
index 7720f809d..678b2b180 100644
--- a/src/weights.jl
+++ b/src/weights.jl
@@ -11,7 +11,11 @@ macro weights(name)
     return quote
         mutable struct $name{S<:Real, T<:Real, V<:AbstractVector{T}} <: AbstractWeights{S, T, V}
             values::V
-            sum::S
+            """
+            Pre-computed sum. Private field, to be accessed only via `sum(wv)`.
+            Set to `missing` by `view` as we cannot know when the parent is mutated.
+            """
+            sum::Union{S, Missing}
         end
         $(esc(name))(values::AbstractVector{<:Real}) = $(esc(name))(values, sum(values))
         $(esc(:weightstype))(::Type{<:$(esc(name))}) = $(esc(name))
@@ -19,7 +23,7 @@ macro weights(name)
 end
 
 length(wv::AbstractWeights) = length(wv.values)
-sum(wv::AbstractWeights) = wv.sum
+sum(wv::AbstractWeights{S}) where {S<:Real} = wv.sum::S
 isempty(wv::AbstractWeights) = isempty(wv.values)
 size(wv::AbstractWeights) = size(wv.values)
 
@@ -41,11 +45,11 @@ Base.getindex(wv::AbstractWeights, ::Colon) = copy(wv)
 Base.copy(wv::W) where {W <: AbstractWeights} =
     weightstype(W)(copy(wv.values), sum(wv))
 
-@propagate_inbounds function Base.view(wv::W, inds...) where {S <: Real, W <: AbstractWeights{S}}
+@propagate_inbounds function Base.view(wv::W, inds...) where
+    {S <: Real, W <: AbstractWeights{S}}
     @boundscheck checkbounds(wv, inds...)
     @inbounds v = invoke(view, Tuple{AbstractArray, Vararg{Any}}, wv, inds...)
-    # Sum is not actually used but compute the right type for clarity
-    weightstype(W)(v, zero(S))
+    weightstype(W){S, eltype(wv), typeof(v)}(v, missing)
 end
 
 # This method is implemented for backward compatibility
@@ -57,12 +61,12 @@ end
 
 # Always recompute the sum for views of AbstractWeights, as we cannot know whether
 # the parent array has been mutated
-Base.sum(wv::AbstractWeights{S, T, <:SubArray{T, <:Any, <:AbstractWeights}}) where
-    {S<:Real, T<:Real} = sum(wv.values)
+Base.sum(wv::AbstractWeights{S, T, <:SubArray}) where {S<:Real, T<:Real} =
+    sum(wv.values)
 
 Base.copy(wv::W) where
     {S<:Real, T<:Real, W<:AbstractWeights{S, T, <:SubArray{T, <:Any, <:AbstractWeights}}} =
-    weightstype(W)(copy(view(parent(wv.values).values, parentindices(wv.values)...)), wv.sum)
+    weightstype(W)(copy(view(parent(wv.values).values, parentindices(wv.values)...)), sum(wv))
 
 @propagate_inbounds function Base.setindex!(wv::AbstractWeights, v::Real, i::Int)
     s = v - wv[i]
@@ -112,7 +116,7 @@ if `corrected=true`.
 @inline function varcorrection(w::Weights, corrected::Bool=false)
     corrected && throw(ArgumentError("Weights type does not support bias correction: " *
                                      "use FrequencyWeights, AnalyticWeights or ProbabilityWeights if applicable."))
-    1 / w.sum
+    1 / sum(w)
 end
 
 @weights AnalyticWeights
@@ -145,7 +149,7 @@ aweights(vs::RealArray) = AnalyticWeights(vec(vs))
 * `corrected=false`: ``\\frac{1}{\\sum w}``
 """
 @inline function varcorrection(w::AnalyticWeights, corrected::Bool=false)
-    s = w.sum
+    s = sum(w)
 
     if corrected
         sum_sn = sum(x -> (x / s) ^ 2, w)
@@ -183,7 +187,7 @@ fweights(vs::RealArray) = FrequencyWeights(vec(vs))
 * `corrected=false`: ``\\frac{1}{\\sum w}``
 """
 @inline function varcorrection(w::FrequencyWeights, corrected::Bool=false)
-    s = w.sum
+    s = sum(w)
 
     if corrected
         1 / (s - 1)
@@ -221,7 +225,7 @@ pweights(vs::RealArray) = ProbabilityWeights(vec(vs))
 * `corrected=false`: ``\\frac{1}{\\sum w}``
 """
 @inline function varcorrection(w::ProbabilityWeights, corrected::Bool=false)
-    s = w.sum
+    s = sum(w)
 
     if corrected
         n = count(!iszero, w)
@@ -365,11 +369,11 @@ end
 for w in (AnalyticWeights, FrequencyWeights, ProbabilityWeights, Weights)
     @eval begin
         Base.isequal(x::$w, y::$w) =
-            (x.values isa SubArray || y.values isa SubArray || isequal(x.values, y.values)) &&
+            (x.values isa SubArray || y.values isa SubArray || isequal(x.sum, y.sum)) &&
                 isequal(x.values, y.values)
         Base.:(==)(x::$w, y::$w)   =
             (x.values isa SubArray || y.values isa SubArray || x.sum == y.sum) &&
-                (x.values == y.values)
+                x.values == y.values
     end
 end
 
@@ -700,7 +704,7 @@ function quantile(v::RealVector{V}, w::AbstractWeights{W}, p::RealVector) where
     isempty(p) && throw(ArgumentError("empty quantile array"))
     all(x -> 0 <= x <= 1, p) || throw(ArgumentError("input probability out of [0,1] range"))
 
-    w.sum == 0 && throw(ArgumentError("weight vector cannot sum to zero"))
+    sum(w) == 0 && throw(ArgumentError("weight vector cannot sum to zero"))
     length(v) == length(w) || throw(ArgumentError("data and weight vectors must be the same size," *
         "got $(length(v)) and $(length(w))"))
     for x in w.values
diff --git a/test/cov.jl b/test/cov.jl
index ab310276c..e437f909a 100644
--- a/test/cov.jl
+++ b/test/cov.jl
@@ -4,9 +4,15 @@ using LinearAlgebra, Random, Test
 struct EmptyCovarianceEstimator <: CovarianceEstimator end
 
 @testset "StatsBase.Covariance" begin
+
+function viewweights(f)
+    wv -> view(f([wv; 100]), axes(wv, 1))
+end
+
 weight_funcs = (weights, aweights, fweights, pweights)
 
-@testset "$f" for f in weight_funcs
+@testset "$f with $viewf" for f in weight_funcs, viewf in (identity, viewweights)
+    fw = viewf(f)
     X = randn(3, 8)
 
     Z1 = X .- mean(X, dims = 1)
@@ -21,8 +27,8 @@ weight_funcs = (weights, aweights, fweights, pweights)
         w2[1] += 1
     end
 
-    wv1 = f(w1)
-    wv2 = f(w2)
+    wv1 = fw(w1)
+    wv2 = fw(w2)
 
     Z1w = X .- mean(X, wv1, dims=1)
     Z2w = X .- mean(X, wv2, dims=2)
@@ -237,8 +243,8 @@ weight_funcs = (weights, aweights, fweights, pweights)
     end
 
     @testset "Correlation" begin
-        @test cor(X, f(ones(3)), 1) ≈ cor(X, dims = 1)
-        @test cor(X, f(ones(8)), 2) ≈ cor(X, dims = 2)
+        @test cor(X, fw(ones(3)), 1) ≈ cor(X, dims = 1)
+        @test cor(X, fw(ones(8)), 2) ≈ cor(X, dims = 2)
 
         cov1 = cov(X, wv1, 1; corrected=false)
         std1 = std(X, wv1, 1; corrected=false)
diff --git a/test/moments.jl b/test/moments.jl
index 97fda44ac..c2165c873 100644
--- a/test/moments.jl
+++ b/test/moments.jl
@@ -2,6 +2,11 @@ using StatsBase
 using Test
 
 @testset "StatsBase.Moments" begin
+
+function viewweights(f)
+    wv -> view(f([wv; 100]), axes(wv, 1))
+end
+
 weight_funcs = (weights, aweights, fweights, pweights)
 
 ##### weighted var & std
@@ -9,8 +14,9 @@ weight_funcs = (weights, aweights, fweights, pweights)
 x = [0.57, 0.10, 0.91, 0.72, 0.46, 0.0]
 w = [3.84, 2.70, 8.29, 8.91, 9.71, 0.0]
 
-@testset "Uncorrected with $f" for f in weight_funcs
-    wv = f(w)
+@testset "Uncorrected with $f and $viewf" for f in weight_funcs,
+                                              viewf in (identity, viewweights)
+    wv = viewf(f)(w)
     m = mean(x, wv)
 
     # expected uncorrected output
@@ -52,8 +58,9 @@ end
 expected_var = [NaN, 0.0694434191182236, 0.05466601256158146, 0.06628969012045285]
 expected_std = sqrt.(expected_var)
 
-@testset "Corrected with $(weight_funcs[i])" for i in eachindex(weight_funcs)
-    wv = weight_funcs[i](w)
+@testset "Corrected with $(weight_funcs[i])" for i in eachindex(weight_funcs),
+                                                 viewf in (identity, viewweights)
+    wv = viewf(weight_funcs[i])(w)
     m = mean(x, wv)
 
     @testset "Variance" begin
@@ -107,9 +114,10 @@ x = rand(5, 6)
 w1 = [0.57, 5.10, 0.91, 1.72, 0.0]
 w2 = [3.84, 2.70, 8.29, 8.91, 9.71, 0.0]
 
-@testset "Uncorrected with $f" for f in weight_funcs
-    wv1 = f(w1)
-    wv2 = f(w2)
+@testset "Uncorrected with $f and $viewf" for f in weight_funcs,
+                                              viewf in (identity, viewweights)
+    wv1 = viewf(f)(w1)
+    wv2 = viewf(f)(w2)
     m1 = mean(x, wv1, dims=1)
     m2 = mean(x, wv2, dims=2)
 
@@ -165,9 +173,10 @@ w2 = [3.84, 2.70, 8.29, 8.91, 9.71, 0.0]
     end
 end
 
-@testset "Corrected with $f" for f in weight_funcs
-    wv1 = f(w1)
-    wv2 = f(w2)
+@testset "Corrected with $f and $viewf" for f in weight_funcs,
+                                            viewf in (identity, viewweights)
+    wv1 = viewf(f)(w1)
+    wv2 = viewf(f)(w2)
     m1 = mean(x, wv1, dims=1)
     m2 = mean(x, wv2, dims=2)
 
@@ -241,8 +250,9 @@ end
     end
 end
 
-@testset "Skewness and Kurtosis with $f" for f in weight_funcs
-    wv = f(ones(5) * 2.0)
+@testset "Skewness and Kurtosis with $f and $viewf" for f in weight_funcs,
+                                                        viewf in (identity, viewweights)
+    wv = viewf(f)(ones(5) * 2.0)
 
     @test skewness(1:5)             ≈  0.0
     @test skewness([1, 2, 3, 4, 5]) ≈  0.0
@@ -258,7 +268,8 @@ end
     @test kurtosis([1, 2, 3, 4, 5], wv) ≈ -1.3
 end
 
-@testset "General Moments with $f" for f in weight_funcs
+@testset "General Moments with $f and $viewf" for f in weight_funcs,
+                                                  viewf in (identity, viewweights)
     x = collect(2.0:8.0)
     @test moment(x, 2) ≈ sum((x .- 5).^2) / length(x)
     @test moment(x, 3) ≈ sum((x .- 5).^3) / length(x)
@@ -270,7 +281,7 @@ end
     @test moment(x, 4, 4.0) ≈ sum((x .- 4).^4) / length(x)
     @test moment(x, 5, 4.0) ≈ sum((x .- 4).^5) / length(x)
 
-    w = f([1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0])
+    w = viewf(f)([1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0])
     x2 = collect(2.0:6.0)
     @test moment(x, 2, w) ≈ sum((x2 .- 4).^2) / 5
     @test moment(x, 3, w) ≈ sum((x2 .- 4).^3) / 5
diff --git a/test/weights.jl b/test/weights.jl
index 33c3611d1..5be7ae2e0 100644
--- a/test/weights.jl
+++ b/test/weights.jl
@@ -2,6 +2,11 @@ using StatsBase
 using LinearAlgebra, Random, SparseArrays, Test
 
 @testset "StatsBase.Weights" begin
+
+function viewweights(f)
+    wv -> view(f([wv; 100]), axes(wv, 1))
+end
+
 weight_funcs = (weights, aweights, fweights, pweights)
 
 ## Construction
@@ -43,7 +48,7 @@ end
 
     # Check getindex & sum
     @test wv[1] === 1.
-    @test sum(wv) === 6.
+    @test @inferred(sum(wv)) === 6.
     @test wv == w
 
     @test wv[[1, 3]] == w[[1, 3]]
@@ -54,33 +59,6 @@ end
     @test copy(wv) == wv
     @test typeof(copy(wv)) === typeof(wv)
 
-    # Check view
-    @test view(wv, :) == wv
-    @test typeof(view(wv, :)) <: StatsBase.weightstype(typeof(wv))
-    @test view(wv, [1, 3]) == view(wv, [true, false, true]) == w[[1, 3]]
-    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) <:
-        StatsBase.weightstype(typeof(wv))
-    @test sum(view(wv, [1, 3])) === sum(view(wv, [true, false, true])) === sum(w[[1, 3]])
-    @test_throws BoundsError view(wv, [1, 5])
-    @test_throws BoundsError view(wv, [true, false, true, true])
-    v = view(wv, [1, 3])
-    wv[1] += 1
-    @test sum(v) == sum(view(wv, [1, 3]))
-    v[1] -= 1
-    @test wv[1] == 1
-    @test copy(v) == v[:] == v
-    @test typeof(copy(v)) === typeof(v[:]) == typeof(wv)
-    @test view(wv, [1, 2], 1) == wv[[1, 2]]
-    @test typeof(view(wv, [1, 2], 1)) <: StatsBase.weightstype(typeof(wv))
-    @test view(wv, 1) == fill(wv[1])
-    @test view(wv, 1) isa SubArray
-    @test view(wv, 1, 1) == fill(wv[1])
-    @test view(wv, 1, 1) isa SubArray
-    @test view(wv, CartesianIndex(1, 1)) == fill(wv[CartesianIndex(1, 1)])
-    @test view(wv, CartesianIndex(1, 1)) isa SubArray
-    @test view(wv, CartesianIndex(1, 1), 1) == fill(wv[CartesianIndex(1, 1), 1])
-    @test view(wv, CartesianIndex(1, 1), 1) isa SubArray
-
     # Test setindex! success
     @test (wv[1] = 4) === 4             # setindex! returns original val
     @test wv[1] === 4.                  # value correctly converted and set
@@ -104,14 +82,16 @@ end
     @test wv == [1, 2, 3]           # Test state of all values
 end
 
-@testset "$f, isequal and ==" for f in weight_funcs
-    x = f([1, 2, 3])
+@testset "$f and $viewf with isequal and ==" for f in weight_funcs,
+                                                 viewf in (identity, viewweights)
+    fw = viewf(f)
+    x = fw([1, 2, 3])
 
-    y = f([1, 2, 3]) # same values, type and parameters
+    y = fw([1, 2, 3]) # same values, type and parameters
     @test isequal(x, y)
     @test x == y
 
-    y = f([1.0, 2.0, 3.0]) # same values and type, different parameters
+    y = fw([1.0, 2.0, 3.0]) # same values and type, different parameters
     @test isequal(x, y)
     @test x == y
 
@@ -121,17 +101,61 @@ end
         @test x != y
     end
 
-    x = f([1, 2, NaN]) # isequal and == treat NaN differently
-    y = f([1, 2, NaN])
+    x = fw([1, 2, NaN]) # isequal and == treat NaN differently
+    y = fw([1, 2, NaN])
     @test isequal(x, y)
     @test x != y
 
-    x = f([1.0, 2.0, 0.0]) # isequal and == treat ±0.0 differently
-    y = f([1.0, 2.0, -0.0])
+    x = fw([1.0, 2.0, 0.0]) # isequal and == treat ±0.0 differently
+    y = fw([1.0, 2.0, -0.0])
     @test !isequal(x, y)
     @test x == y
 end
 
+@testset "view of weights" for f in weight_funcs
+    w = [1., 2., 3.]
+    wv = f(w)
+
+    @test view(wv, :) == wv
+    @test sum(view(wv, :)) == sum(wv)
+    @test typeof(view(wv, :)) <: StatsBase.weightstype(typeof(wv))
+    @test view(wv, [1, 3]) == view(wv, [true, false, true]) == w[[1, 3]]
+    @test isequal(view(wv, [1, 3]), view(wv, [true, false, true]))
+    @test isequal(view(wv, [true, false, true]), w[[1, 3]])
+    @test typeof(view(wv, [1, 3])) === typeof(view(wv, [true, false, true])) <:
+        StatsBase.weightstype(typeof(wv))
+    @test sum(view(wv, [1, 3])) === sum(view(wv, [true, false, true])) === sum(w[[1, 3]])
+
+    @test_throws BoundsError view(wv, [1, 5])
+    @test_throws BoundsError view(wv, [true, false, true, true])
+
+    v = view(wv, [1, 3])
+    wv[1] += 1
+    @test sum(v) == sum(view(wv, [1, 3]))
+    v[1] -= 1
+    @test wv[1] == 1
+    @test copy(v) == v[:] == v
+    @test sum(copy(v)) == sum(v[:]) == sum(v)
+    @test typeof(copy(v)) === typeof(v[:]) === typeof(wv)
+
+    @test view(wv, [1, 2], 1) == wv[[1, 2]]
+    @test sum(view(wv, [1, 2], 1)) == sum(wv[[1, 2]])
+    @test typeof(view(wv, [1, 2], 1)) <: StatsBase.weightstype(typeof(wv))
+    @test copy(view(wv, [1, 2], 1)) == view(wv, [1, 2], 1)
+    @test sum(copy(view(wv, [1, 2], 1))) == sum(view(wv, [1, 2]))
+    @test typeof(copy(view(wv, [1, 2], 1))) === typeof(copy(v))
+
+    @test view(wv, 1) == fill(wv[1])
+    @test view(wv, 1) isa SubArray
+    @test view(wv, 1, 1) == fill(wv[1])
+    @test view(wv, 1, 1) isa SubArray
+
+    @test view(wv, CartesianIndex(1, 1)) == fill(wv[CartesianIndex(1, 1)])
+    @test view(wv, CartesianIndex(1, 1)) isa SubArray
+    @test view(wv, CartesianIndex(1, 1), 1) == fill(wv[CartesianIndex(1, 1), 1])
+    @test view(wv, CartesianIndex(1, 1), 1) isa SubArray
+end
+
 @testset "Unit weights" begin
     wv = uweights(Float64, 3)
     @test wv[1] === 1.
@@ -271,30 +295,35 @@ end
 
 a = reshape(1.0:27.0, 3, 3, 3)
 
-@testset "Sum $f" for f in weight_funcs
-    @test sum([1.0, 2.0, 3.0], f([1.0, 0.5, 0.5])) ≈ 3.5
-    @test sum(1:3, f([1.0, 1.0, 0.5]))             ≈ 4.5
+@testset "Sum $f and $viewf" for f in weight_funcs, viewf in (identity, viewweights)
+    fw = viewf(f)
+    @test sum([1.0, 2.0, 3.0], fw([1.0, 0.5, 0.5])) ≈ 3.5
+    @test sum(1:3, fw([1.0, 1.0, 0.5]))             ≈ 4.5
 
     for wt in ([1.0, 1.0, 1.0], [1.0, 0.2, 0.0], [0.2, 0.0, 1.0])
-        @test sum(a, f(wt), dims=1)  ≈ sum(a.*reshape(wt, length(wt), 1, 1), dims=1)
-        @test sum(a, f(wt), dims=2)  ≈ sum(a.*reshape(wt, 1, length(wt), 1), dims=2)
-        @test sum(a, f(wt), dims=3)  ≈ sum(a.*reshape(wt, 1, 1, length(wt)), dims=3)
+        @test sum(a, fw(wt), dims=1)  ≈ sum(a.*reshape(wt, length(wt), 1, 1), dims=1)
+        @test sum(a, fw(wt), dims=2)  ≈ sum(a.*reshape(wt, 1, length(wt), 1), dims=2)
+        @test sum(a, fw(wt), dims=3)  ≈ sum(a.*reshape(wt, 1, 1, length(wt)), dims=3)
     end
 end
 
-@testset "Mean $f" for f in weight_funcs
-    @test mean([1:3;], f([1.0, 1.0, 0.5])) ≈ 1.8
-    @test mean(1:3, f([1.0, 1.0, 0.5]))    ≈ 1.8
+@testset "Mean $f and $viewf" for f in weight_funcs,
+                                  viewf in (identity, viewweights)
+    fw = viewf(f)
+
+    @test mean([1:3;], fw([1.0, 1.0, 0.5])) ≈ 1.8
+    @test mean(1:3, fw([1.0, 1.0, 0.5]))    ≈ 1.8
 
     for wt in ([1.0, 1.0, 1.0], [1.0, 0.2, 0.0], [0.2, 0.0, 1.0])
-        @test mean(a, f(wt), dims=1) ≈ sum(a.*reshape(wt, length(wt), 1, 1), dims=1)/sum(wt)
-        @test mean(a, f(wt), dims=2) ≈ sum(a.*reshape(wt, 1, length(wt), 1), dims=2)/sum(wt)
-        @test mean(a, f(wt), dims=3) ≈ sum(a.*reshape(wt, 1, 1, length(wt)), dims=3)/sum(wt)
-        @test_throws ErrorException mean(a, f(wt), dims=4)
+        @test mean(a, fw(wt), dims=1) ≈ sum(a.*reshape(wt, length(wt), 1, 1), dims=1)/sum(wt)
+        @test mean(a, fw(wt), dims=2) ≈ sum(a.*reshape(wt, 1, length(wt), 1), dims=2)/sum(wt)
+        @test mean(a, fw(wt), dims=3) ≈ sum(a.*reshape(wt, 1, 1, length(wt)), dims=3)/sum(wt)
+        @test_throws ErrorException mean(a, fw(wt), dims=4)
     end
 end
 
-@testset "Quantile fweights" begin
+@testset "Quantile with fweights and $viewf" for viewf in (identity, viewweights)
+    fw = viewf(fweights)
     data = (
         [7, 1, 2, 4, 10],
         [7, 1, 2, 4, 10],
@@ -352,27 +381,29 @@ end
     end
     # quantile with fweights is the same as repeated vectors
     for i = 1:length(data)
-        @test quantile(data[i], fweights(wt[i]), p) ≈ quantile(_rep(data[i], wt[i]), p)
+        @test quantile(data[i], fw(wt[i]), p) ≈ quantile(_rep(data[i], wt[i]), p)
     end
     # quantile with fweights = 1  is the same as quantile
     for i = 1:length(data)
-        @test quantile(data[i], fweights(fill!(similar(wt[i]), 1)), p) ≈ quantile(data[i], p)
+        @test quantile(data[i], fw(fill!(similar(wt[i]), 1)), p) ≈ quantile(data[i], p)
     end
 
     # Issue #313
-    @test quantile([1, 2, 3, 4, 5], fweights([0,1,2,1,0]), p) ≈ quantile([2, 3, 3, 4], p)
-    @test quantile([1, 2], fweights([1, 1]), 0.25) ≈ 1.25
-    @test quantile([1, 2], fweights([2, 2]), 0.25) ≈ 1.0
+    @test quantile([1, 2, 3, 4, 5], fw([0,1,2,1,0]), p) ≈ quantile([2, 3, 3, 4], p)
+    @test quantile([1, 2], fw([1, 1]), 0.25) ≈ 1.25
+    @test quantile([1, 2], fw([2, 2]), 0.25) ≈ 1.0
 
     # test non integer frequency weights
-    quantile([1, 2], fweights([1.0, 2.0]), 0.25) == quantile([1, 2], fweights([1, 2]), 0.25)
-    @test_throws ArgumentError quantile([1, 2], fweights([1.5, 2.0]), 0.25)
+    quantile([1, 2], fw([1.0, 2.0]), 0.25) == quantile([1, 2], fw([1, 2]), 0.25)
+    @test_throws ArgumentError quantile([1, 2], fw([1.5, 2.0]), 0.25)
 
-    @test_throws ArgumentError quantile([1, 2], fweights([1, 2]), nextfloat(1.0))
-    @test_throws ArgumentError quantile([1, 2], fweights([1, 2]), prevfloat(0.0))
+    @test_throws ArgumentError quantile([1, 2], fw([1, 2]), nextfloat(1.0))
+    @test_throws ArgumentError quantile([1, 2], fw([1, 2]), prevfloat(0.0))
 end
 
-@testset "Quantile aweights, pweights and weights" for f in (aweights, pweights, weights)
+@testset "Quantile aweights, pweights and weights" for f in (aweights, pweights, weights),
+                                                       viewf in (identity, viewweights)
+    fw = viewf(f)
     data = (
         [7, 1, 2, 4, 10],
         [7, 1, 2, 4, 10],
@@ -440,22 +471,22 @@ end
 
     Random.seed!(10)
     for i = 1:length(data)
-        @test quantile(data[i], f(wt[i]), p) ≈ quantile_answers[i] atol = 1e-5
+        @test quantile(data[i], fw(wt[i]), p) ≈ quantile_answers[i] atol = 1e-5
         for j = 1:10
             # order of p does not matter
             reorder = sortperm(rand(length(p)))
-            @test quantile(data[i], f(wt[i]), p[reorder]) ≈ quantile_answers[i][reorder] atol = 1e-5
+            @test quantile(data[i], fw(wt[i]), p[reorder]) ≈ quantile_answers[i][reorder] atol = 1e-5
         end
         for j = 1:10
             # order of w does not matter
             reorder = sortperm(rand(length(data[i])))
-            @test quantile(data[i][reorder], f(wt[i][reorder]), p) ≈ quantile_answers[i] atol = 1e-5
+            @test quantile(data[i][reorder], fw(wt[i][reorder]), p) ≈ quantile_answers[i] atol = 1e-5
         end
     end
     # All equal weights corresponds to base quantile
     for v in (1, 2, 345)
         for i = 1:length(data)
-            w = f(fill(v, length(data[i])))
+            w = fw(fill(v, length(data[i])))
             @test quantile(data[i], w, p) ≈ quantile(data[i], p) atol = 1e-5
             for j = 1:10
                 prandom = rand(4)
@@ -465,40 +496,42 @@ end
     end
     # test zeros are removed
     for i = 1:length(data)
-        @test quantile(vcat(1.0, data[i]), f(vcat(0.0, wt[i])), p) ≈ quantile_answers[i] atol = 1e-5
+        @test quantile(vcat(1.0, data[i]), fw(vcat(0.0, wt[i])), p) ≈ quantile_answers[i] atol = 1e-5
     end
     # Syntax
     v = [7, 1, 2, 4, 10]
     w = [1, 1/3, 1/3, 1/3, 1]
     answer = 6.0
-    @test quantile(data[1], f(w), 0.5) ≈ answer atol = 1e-5
+    @test quantile(data[1], fw(w), 0.5) ≈ answer atol = 1e-5
 end
 
-@testset "Median $f" for f in weight_funcs
+@testset "Median with $f and $viewf" for f in weight_funcs,
+                                         viewf in (identity, viewweights)
+    fw = viewf(f)
     data = [4, 3, 2, 1]
     wt = [0, 0, 0, 0]
-    @test_throws ArgumentError median(data, f(wt))
-    @test_throws ArgumentError median(Float64[], f(Float64[]))
+    @test_throws ArgumentError median(data, fw(wt))
+    @test_throws ArgumentError median(Float64[], fw(Float64[]))
     wt = [1, 2, 3, 4, 5]
-    @test_throws ArgumentError median(data, f(wt))
+    @test_throws ArgumentError median(data, fw(wt))
     if VERSION >= v"1.0"
-        @test_throws MethodError median([4 3 2 1 0], f(wt))
-        @test_throws MethodError median([[1 2] ; [4 5] ; [7 8] ; [10 11] ; [13 14]], f(wt))
+        @test_throws MethodError median([4 3 2 1 0], fw(wt))
+        @test_throws MethodError median([[1 2] ; [4 5] ; [7 8] ; [10 11] ; [13 14]], fw(wt))
     end
     data = [1, 3, 2, NaN, 2]
-    @test isnan(median(data, f(wt)))
+    @test isnan(median(data, fw(wt)))
     wt = [1, 2, NaN, 4, 5]
-    @test_throws ArgumentError median(data, f(wt))
+    @test_throws ArgumentError median(data, fw(wt))
     data = [1, 3, 2, 1, 2]
-    @test_throws ArgumentError median(data, f(wt))
+    @test_throws ArgumentError median(data, fw(wt))
     wt = [-1, -1, -1, -1, -1]
-    @test_throws ArgumentError median(data, f(wt))
+    @test_throws ArgumentError median(data, fw(wt))
     wt = [-1, -1, -1, 0, 0]
-    @test_throws ArgumentError median(data, f(wt))
+    @test_throws ArgumentError median(data, fw(wt))
 
     data = [4, 3, 2, 1]
     wt = [1, 2, 3, 4]
-    @test median(data, f(wt)) ≈ quantile(data, f(wt), 0.5) atol = 1e-5
+    @test median(data, fw(wt)) ≈ quantile(data, fw(wt), 0.5) atol = 1e-5
 end
 
 @testset "Mismatched eltypes" begin