add Returns

NEWS.md

@@ -33,6 +33,7 @@ New library functions
 * Two argument methods `findmax(f, domain)`, `argmax(f, domain)` and the corresponding `min` versions ([#27613]).
 * `isunordered(x)` returns true if `x` is value that is normally unordered, such as `NaN` or `missing`.
 * New macro `Base.@invokelatest f(args...; kwargs...)` provides a convenient way to call `Base.invokelatest(f, args...; kwargs...)` ([#37971])
+* New function/type `Returns(value)` that creates a function, which returns `value` for any arguments ([ #39794])
 * New macro `Base.@invoke f(arg1::T1, arg2::T2; kwargs...)` provides an easier syntax to call `invoke(f, Tuple{T1,T2}, arg1, arg2; kwargs...)` ([#38438])
 
 New library features

base/abstractarray.jl

@@ -1604,7 +1604,7 @@ _cat_size_shape(dims, shape) = shape
 
 _cshp(ndim::Int, ::Tuple{}, ::Tuple{}, ::Tuple{}) = ()
 _cshp(ndim::Int, ::Tuple{}, ::Tuple{}, nshape) = nshape
-_cshp(ndim::Int, dims, ::Tuple{}, ::Tuple{}) = ntuple(b -> 1, Val(length(dims)))
+_cshp(ndim::Int, dims, ::Tuple{}, ::Tuple{}) = ntuple(Returns(1), Val(length(dims)))
 @inline _cshp(ndim::Int, dims, shape, ::Tuple{}) =
     (shape[1] + dims[1], _cshp(ndim + 1, tail(dims), tail(shape), ())...)
 @inline _cshp(ndim::Int, dims, ::Tuple{}, nshape) =
@@ -1810,7 +1810,7 @@ function hvcat(nbc::Integer, as...)
     mod(n,nbc) != 0 &&
         throw(ArgumentError("number of arrays $n is not a multiple of the requested number of block columns $nbc"))
     nbr = div(n,nbc)
-    hvcat(ntuple(i->nbc, nbr), as...)
+    hvcat(ntuple(Returns(nbc), nbr), as...)
 end
 
 """
@@ -2238,9 +2238,9 @@ function mapslices(f, A::AbstractArray; dims)
     end
     nextra = max(0, length(dims)-ndims(r1))
     if eltype(Rsize) == Int
-        Rsize[dims] = [size(r1)..., ntuple(d->1, nextra)...]
+        Rsize[dims] = [size(r1)..., ntuple(Returns(1), nextra)...]
     else
-        Rsize[dims] = [axes(r1)..., ntuple(d->OneTo(1), nextra)...]
+        Rsize[dims] = [axes(r1)..., ntuple(Returns(OneTo(1)), nextra)...]
     end
     R = similar(r1, tuple(Rsize...,))
 

base/abstractarraymath.jl

@@ -123,7 +123,7 @@ julia> selectdim(A, 2, 3)
 @noinline function _selectdim(A, d, i, idxs)
     d >= 1 || throw(ArgumentError("dimension must be ≥ 1, got $d"))
     nd = ndims(A)
-    d > nd && (i == 1 || throw(BoundsError(A, (ntuple(k->Colon(),d-1)..., i))))
+    d > nd && (i == 1 || throw(BoundsError(A, (ntuple(Returns(Colon()),d-1)..., i))))
     return view(A, idxs...)
 end
 
@@ -225,7 +225,7 @@ function repeat(A::AbstractArray, counts...)
 end
 
 """
-    repeat(A::AbstractArray; inner=ntuple(x->1, ndims(A)), outer=ntuple(x->1, ndims(A)))
+    repeat(A::AbstractArray; inner=ntuple(Returns(1), ndims(A)), outer=ntuple(Returns(1), ndims(A)))
 
 Construct an array by repeating the entries of `A`. The i-th element of `inner` specifies
 the number of times that the individual entries of the i-th dimension of `A` should be
@@ -491,7 +491,7 @@ julia> collect(eachslice(M, dims=2))
     length(dims) == 1 || throw(ArgumentError("only single dimensions are supported"))
     dim = first(dims)
     dim <= ndims(A) || throw(DimensionMismatch("A doesn't have $dim dimensions"))
-    inds_before = ntuple(d->(:), dim-1)
-    inds_after = ntuple(d->(:), ndims(A)-dim)
+    inds_before = ntuple(Returns(:), dim-1)
+    inds_after = ntuple(Returns(:), ndims(A)-dim)
     return (view(A, inds_before..., i, inds_after...) for i in axes(A, dim))
 end

base/array.jl

@@ -1734,7 +1734,7 @@ function vcat(arrays::Vector{T}...) where T
     return arr
 end
 
-_cat(n::Integer, x::Integer...) = reshape([x...], (ntuple(x->1, n-1)..., length(x)))
+_cat(n::Integer, x::Integer...) = reshape([x...], (ntuple(Returns(1), n-1)..., length(x)))
 
 ## find ##
 

base/dict.jl

@@ -797,7 +797,7 @@ function iterate(d::ImmutableDict{K,V}, t=d) where {K, V}
     !isdefined(t, :parent) && return nothing
     (Pair{K,V}(t.key, t.value), t.parent)
 end
-length(t::ImmutableDict) = count(x->true, t)
+length(t::ImmutableDict) = count(Returns(true), t)
 isempty(t::ImmutableDict) = !isdefined(t, :parent)
 empty(::ImmutableDict, ::Type{K}, ::Type{V}) where {K, V} = ImmutableDict{K,V}()
 

base/env.jl

@@ -77,7 +77,7 @@ variable may result in an uppercase `ENV` key.)
 const ENV = EnvDict()
 
 getindex(::EnvDict, k::AbstractString) = access_env(k->throw(KeyError(k)), k)
-get(::EnvDict, k::AbstractString, def) = access_env(k->def, k)
+get(::EnvDict, k::AbstractString, def) = access_env(Returns(def), k)
 get(f::Callable, ::EnvDict, k::AbstractString) = access_env(k->f(), k)
 in(k::AbstractString, ::KeySet{String, EnvDict}) = _hasenv(k)
 pop!(::EnvDict, k::AbstractString) = (v = ENV[k]; _unsetenv(k); v)

base/exports.jl

@@ -70,6 +70,7 @@ export
     Rational,
     Regex,
     RegexMatch,
+    Returns,
     RoundFromZero,
     RoundDown,
     RoundingMode,

base/multidimensional.jl

@@ -104,9 +104,9 @@ module IteratorsMD
 
     # zeros and ones
     zero(::CartesianIndex{N}) where {N} = zero(CartesianIndex{N})
-    zero(::Type{CartesianIndex{N}}) where {N} = CartesianIndex(ntuple(x -> 0, Val(N)))
+    zero(::Type{CartesianIndex{N}}) where {N} = CartesianIndex(ntuple(Returns(0), Val(N)))
     oneunit(::CartesianIndex{N}) where {N} = oneunit(CartesianIndex{N})
-    oneunit(::Type{CartesianIndex{N}}) where {N} = CartesianIndex(ntuple(x -> 1, Val(N)))
+    oneunit(::Type{CartesianIndex{N}}) where {N} = CartesianIndex(ntuple(Returns(1), Val(N)))
 
     # arithmetic, min/max
     @inline (-)(index::CartesianIndex{N}) where {N} =
@@ -457,7 +457,7 @@ module IteratorsMD
 
     # Split out the first N elements of a tuple
     @inline function split(t, V::Val)
-        ref = ntuple(d->true, V)  # create a reference tuple of length N
+        ref = ntuple(Returns(true), V)  # create a reference tuple of length N
         _split1(t, ref), _splitrest(t, ref)
     end
     @inline _split1(t, ref) = (t[1], _split1(tail(t), tail(ref))...)
@@ -684,10 +684,10 @@ checkindex(::Type{Bool}, inds::Tuple, I::CartesianIndices) = all(checkindex.(Boo
 # rather than returning N, it returns an NTuple{N,Bool} so the result is inferrable
 @inline index_ndims(i1, I...) = (true, index_ndims(I...)...)
 @inline function index_ndims(i1::CartesianIndex, I...)
-    (map(x->true, i1.I)..., index_ndims(I...)...)
+    (map(Returns(true), i1.I)..., index_ndims(I...)...)
 end
 @inline function index_ndims(i1::AbstractArray{CartesianIndex{N}}, I...) where N
-    (ntuple(x->true, Val(N))..., index_ndims(I...)...)
+    (ntuple(Returns(true), Val(N))..., index_ndims(I...)...)
 end
 index_ndims() = ()
 
@@ -697,7 +697,7 @@ index_ndims() = ()
 @inline index_dimsum(::Colon, I...) = (true, index_dimsum(I...)...)
 @inline index_dimsum(::AbstractArray{Bool}, I...) = (true, index_dimsum(I...)...)
 @inline function index_dimsum(::AbstractArray{<:Any,N}, I...) where N
-    (ntuple(x->true, Val(N))..., index_dimsum(I...)...)
+    (ntuple(Returns(true), Val(N))..., index_dimsum(I...)...)
 end
 index_dimsum() = ()
 

base/operators.jl

@@ -915,6 +915,42 @@ julia> [1:5;] |> x->x.^2 |> sum |> inv
 """
 |>(x, f) = f(x)
 
+"""
+    f = Returns(value)
+
+Create a callable `f` such that `f(args...; kw...) === value` holds.
+
+# Examples
+
+```jldoctest
+julia> f = Returns(42);
+
+julia> f(1)
+42
+
+julia> f("hello", x=32)
+42
+
+julia> f.value
+42
+```
+
+!!! compat "Julia 1.7"
+    Returns requires at least Julia 1.7.
+"""
+struct Returns{V} <: Function
+    value::V
+    Returns{V}(value) where {V} = new{V}(value)
+    Returns(value) = new{Core.Typeof(value)}(value)
+end
+
+(obj::Returns)(args...; kw...) = obj.value
+function show(io::IO, obj::Returns)
+    show(io, typeof(obj))
+    print(io, "(")
+    show(io, obj.value)
+    print(io, ")")
+end
 # function composition
 
 """

base/reshapedarray.jl

@@ -146,7 +146,7 @@ end
 # product of trailing dims into the last element
 rdims_trailing(l, inds...) = length(l) * rdims_trailing(inds...)
 rdims_trailing(l) = length(l)
-rdims(out::Val{N}, inds::Tuple) where {N} = rdims(ntuple(i -> OneTo(1), Val(N)), inds)
+rdims(out::Val{N}, inds::Tuple) where {N} = rdims(ntuple(Returns(OneTo(1)), Val(N)), inds)
 rdims(out::Tuple{}, inds::Tuple{}) = () # N == 0, M == 0
 rdims(out::Tuple{}, inds::Tuple{Any}) = ()
 rdims(out::Tuple{}, inds::NTuple{M,Any}) where {M} = ()

base/show.jl

@@ -45,6 +45,7 @@ function show(io::IO, ::MIME"text/plain", f::Function)
 end
 
 show(io::IO, ::MIME"text/plain", c::ComposedFunction) = show(io, c)
+show(io::IO, ::MIME"text/plain", c::Returns) = show(io, c)
 
 function show(io::IO, ::MIME"text/plain", iter::Union{KeySet,ValueIterator})
     isempty(iter) && get(io, :compact, false) && return show(io, iter)

test/abstractarray.jl

@@ -299,10 +299,10 @@ function test_scalar_indexing(::Type{T}, shape, ::Type{TestAbstractArray}) where
     B = T(A)
     @test A == B
     # Test indexing up to 5 dimensions
-    trailing5 = CartesianIndex(ntuple(x->1, max(ndims(B)-5, 0)))
-    trailing4 = CartesianIndex(ntuple(x->1, max(ndims(B)-4, 0)))
-    trailing3 = CartesianIndex(ntuple(x->1, max(ndims(B)-3, 0)))
-    trailing2 = CartesianIndex(ntuple(x->1, max(ndims(B)-2, 0)))
+    trailing5 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-5, 0)))
+    trailing4 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-4, 0)))
+    trailing3 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-3, 0)))
+    trailing2 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-2, 0)))
     i=0
     for i5 = 1:size(B, 5)
         for i4 = 1:size(B, 4)
@@ -419,10 +419,10 @@ function test_vector_indexing(::Type{T}, shape, ::Type{TestAbstractArray}) where
         N = prod(shape)
         A = reshape(Vector(1:N), shape)
         B = T(A)
-        trailing5 = CartesianIndex(ntuple(x->1, max(ndims(B)-5, 0)))
-        trailing4 = CartesianIndex(ntuple(x->1, max(ndims(B)-4, 0)))
-        trailing3 = CartesianIndex(ntuple(x->1, max(ndims(B)-3, 0)))
-        trailing2 = CartesianIndex(ntuple(x->1, max(ndims(B)-2, 0)))
+        trailing5 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-5, 0)))
+        trailing4 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-4, 0)))
+        trailing3 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-3, 0)))
+        trailing2 = CartesianIndex(ntuple(Returns(1), max(ndims(B)-2, 0)))
         idxs = rand(1:N, 3, 3, 3)
         @test B[idxs] == A[idxs] == idxs
         @test B[vec(idxs)] == A[vec(idxs)] == vec(idxs)

test/arrayops.jl

@@ -1153,17 +1153,17 @@ end
     # issue #5177
 
     c = fill(1,2,3,4)
-    m1 = mapslices(x-> fill(1,2,3), c, dims=[1,2])
-    m2 = mapslices(x-> fill(1,2,4), c, dims=[1,3])
-    m3 = mapslices(x-> fill(1,3,4), c, dims=[2,3])
+    m1 = mapslices(Returns(fill(1,2,3)), c, dims=[1,2])
+    m2 = mapslices(Returns(fill(1,2,4)), c, dims=[1,3])
+    m3 = mapslices(Returns(fill(1,3,4)), c, dims=[2,3])
     @test size(m1) == size(m2) == size(m3) == size(c)
 
-    n1 = mapslices(x-> fill(1,6), c, dims=[1,2])
-    n2 = mapslices(x-> fill(1,6), c, dims=[1,3])
-    n3 = mapslices(x-> fill(1,6), c, dims=[2,3])
-    n1a = mapslices(x-> fill(1,1,6), c, dims=[1,2])
-    n2a = mapslices(x-> fill(1,1,6), c, dims=[1,3])
-    n3a = mapslices(x-> fill(1,1,6), c, dims=[2,3])
+    n1 =  mapslices(Returns(fill(1,6)  ), c, dims=[1,2])
+    n2 =  mapslices(Returns(fill(1,6)  ), c, dims=[1,3])
+    n3 =  mapslices(Returns(fill(1,6)  ), c, dims=[2,3])
+    n1a = mapslices(Returns(fill(1,1,6)), c, dims=[1,2])
+    n2a = mapslices(Returns(fill(1,1,6)), c, dims=[1,3])
+    n3a = mapslices(Returns(fill(1,1,6)), c, dims=[2,3])
     @test size(n1a) == (1,6,4) && size(n2a) == (1,3,6)  && size(n3a) == (2,1,6)
     @test size(n1) == (6,1,4) && size(n2) == (6,3,1)  && size(n3) == (2,6,1)
 
@@ -1659,7 +1659,7 @@ end
 Nmax = 3 # TODO: go up to CARTESIAN_DIMS+2 (currently this exposes problems)
 for N = 1:Nmax
     #indexing with (UnitRange, UnitRange, UnitRange)
-    args = ntuple(d->UnitRange{Int}, N)
+    args = ntuple(Returns(UnitRange{Int}), N)
     @test Base.return_types(getindex, Tuple{Array{Float32, N}, args...}) == [Array{Float32, N}]
     @test Base.return_types(getindex, Tuple{BitArray{N}, args...}) == Any[BitArray{N}]
     @test Base.return_types(setindex!, Tuple{Array{Float32, N}, Array{Int, 1}, args...}) == [Array{Float32, N}]
@@ -1786,7 +1786,7 @@ end
         @test mdsum(A) == 15
         @test mdsum2(A) == 15
         AA = reshape(aa, tuple(2, shp...))
-        B = view(AA, 1:1, ntuple(i->Colon(), i)...)
+        B = view(AA, 1:1, ntuple(Returns(:), i)...)
         @test isa(Base.IndexStyle(B), Base.IteratorsMD.IndexCartesian)
         @test mdsum(B) == 15
         @test mdsum2(B) == 15
@@ -1799,7 +1799,7 @@ end
         A = reshape(a, tuple(shp...))
         @test mdsum(A) == 55
         @test mdsum2(A) == 55
-        B = view(A, ntuple(i->Colon(), i)...)
+        B = view(A, ntuple(Returns(:), i)...)
         @test mdsum(B) == 55
         @test mdsum2(B) == 55
         insert!(shp, 2, 1)

test/bitarray.jl

@@ -1180,8 +1180,8 @@ timesofar("datamove")
 
         @check_bit_operation findfirst(x->x, b1)     Union{Int,Nothing}
         @check_bit_operation findfirst(x->!x, b1)    Union{Int,Nothing}
-        @check_bit_operation findfirst(x->true, b1)  Union{Int,Nothing}
-        @check_bit_operation findfirst(x->false, b1) Union{Int,Nothing}
+        @check_bit_operation findfirst(Returns(true ), b1)  Union{Int,Nothing}
+        @check_bit_operation findfirst(Returns(false), b1) Union{Int,Nothing}
 
         @check_bit_operation findall(b1) Vector{Int}
     end
@@ -1275,49 +1275,51 @@ timesofar("find")
     @test_throws BoundsError findprevnot(b2, 1001)
     @test_throws BoundsError findprev(!, b2, 1001)
     @test_throws BoundsError findprev(identity, b1, 1001)
-    @test_throws BoundsError findprev(x->false, b1, 1001)
-    @test_throws BoundsError findprev(x->true, b1, 1001)
+    @test_throws BoundsError findprev(Returns(false), b1, 1001)
+    @test_throws BoundsError findprev(Returns(true ), b1, 1001)
     @test findprev(b1, 1000) == findprevnot(b2, 1000) == findprev(!, b2, 1000) == 777
     @test findprev(b1, 777)  == findprevnot(b2, 777)  == findprev(!, b2, 777)  == 777
     @test findprev(b1, 776)  == findprevnot(b2, 776)  == findprev(!, b2, 776)  == 77
     @test findprev(b1, 77)   == findprevnot(b2, 77)   == findprev(!, b2, 77)   == 77
     @test findprev(b1, 76)   == findprevnot(b2, 76)   == findprev(!, b2, 76)   == nothing
     @test findprev(b1, -1)   == findprevnot(b2, -1)   == findprev(!, b2, -1)   == nothing
-    @test findprev(identity, b1, -1) == findprev(x->false, b1, -1) == findprev(x->true, b1, -1) == nothing
+    @test findprev(identity, b1, -1) == nothing
+    @test findprev(Returns(false), b1, -1) == nothing
+    @test findprev(Returns(true), b1, -1) == nothing
     @test_throws BoundsError findnext(b1, -1)
     @test_throws BoundsError findnextnot(b2, -1)
     @test_throws BoundsError findnext(!, b2, -1)
     @test_throws BoundsError findnext(identity, b1, -1)
-    @test_throws BoundsError findnext(x->false, b1, -1)
-    @test_throws BoundsError findnext(x->true, b1, -1)
+    @test_throws BoundsError findnext(Returns(false), b1, -1)
+    @test_throws BoundsError findnext(Returns(true), b1, -1)
     @test findnext(b1, 1)    == findnextnot(b2, 1)    == findnext(!, b2, 1)    == 77
     @test findnext(b1, 77)   == findnextnot(b2, 77)   == findnext(!, b2, 77)   == 77
     @test findnext(b1, 78)   == findnextnot(b2, 78)   == findnext(!, b2, 78)   == 777
     @test findnext(b1, 777)  == findnextnot(b2, 777)  == findnext(!, b2, 777)  == 777
     @test findnext(b1, 778)  == findnextnot(b2, 778)  == findnext(!, b2, 778)  == nothing
     @test findnext(b1, 1001) == findnextnot(b2, 1001) == findnext(!, b2, 1001) == nothing
-    @test findnext(identity, b1, 1001) == findnext(x->false, b1, 1001) == findnext(x->true, b1, 1001) == nothing
+    @test findnext(identity, b1, 1001) == findnext(Returns(false), b1, 1001) == findnext(Returns(true), b1, 1001) == nothing
 
     @test findlast(b1) == Base.findlastnot(b2) == 777
     @test findfirst(b1) == Base.findfirstnot(b2) == 77
 
     b0 = BitVector()
-    @test findprev(x->true, b0, -1) == nothing
-    @test_throws BoundsError findprev(x->true, b0, 1)
-    @test_throws BoundsError findnext(x->true, b0, -1)
-    @test findnext(x->true, b0, 1) == nothing
+    @test findprev(Returns(true), b0, -1) == nothing
+    @test_throws BoundsError findprev(Returns(true), b0, 1)
+    @test_throws BoundsError findnext(Returns(true), b0, -1)
+    @test findnext(Returns(true), b0, 1) == nothing
 
     b1 = falses(10)
-    @test findprev(x->true, b1, 5) == 5
-    @test findnext(x->true, b1, 5) == 5
-    @test findprev(x->true, b1, -1) == nothing
-    @test findnext(x->true, b1, 11) == nothing
-    @test findprev(x->false, b1, 5) == nothing
-    @test findnext(x->false, b1, 5) == nothing
-    @test findprev(x->false, b1, -1) == nothing
-    @test findnext(x->false, b1, 11) == nothing
-    @test_throws BoundsError findprev(x->true, b1, 11)
-    @test_throws BoundsError findnext(x->true, b1, -1)
+    @test findprev(Returns(true), b1, 5) == 5
+    @test findnext(Returns(true), b1, 5) == 5
+    @test findprev(Returns(true), b1, -1) == nothing
+    @test findnext(Returns(true), b1, 11) == nothing
+    @test findprev(Returns(false), b1, 5) == nothing
+    @test findnext(Returns(false), b1, 5) == nothing
+    @test findprev(Returns(false), b1, -1) == nothing
+    @test findnext(Returns(false), b1, 11) == nothing
+    @test_throws BoundsError findprev(Returns(true), b1, 11)
+    @test_throws BoundsError findnext(Returns(true), b1, -1)
 
     @testset "issue 32568" for T = (UInt, BigInt)
         for x = (1, 2)
@@ -1382,8 +1384,8 @@ timesofar("reductions")
         b2 = bitrand(l)
         @test map(~, b1) == map(x->~x, b1) == broadcast(~, b1)
         @test map(identity, b1) == map(x->x, b1) == b1
-        @test map(zero, b1) == map(x->false, b1) == falses(l)
-        @test map(one, b1) == map(x->true, b1) == trues(l)
+        @test map(zero, b1) == map(Returns(false), b1) == falses(l)
+        @test map(one, b1) == map(Returns(true), b1) == trues(l)
 
         @test map(&, b1, b2) == map((x,y)->x&y, b1, b2) == broadcast(&, b1, b2)
         @test map(|, b1, b2) == map((x,y)->x|y, b1, b2) == broadcast(|, b1, b2)
@@ -1407,8 +1409,8 @@ timesofar("reductions")
             @test map!(~, b, b1) == map!(x->~x, b, b1) == broadcast(~, b1) == b
             @test map!(!, b, b1) == map!(x->!x, b, b1) == broadcast(~, b1) == b
             @test map!(identity, b, b1) == map!(x->x, b, b1) == b1 == b
-            @test map!(zero, b, b1) == map!(x->false, b, b1) == falses(l) == b
-            @test map!(one, b, b1) == map!(x->true, b, b1) == trues(l) == b
+            @test map!(zero, b, b1) == map!(Returns(false), b, b1) == falses(l) == b
+            @test map!(one, b, b1) == map!(Returns(true), b, b1) == trues(l) == b
 
             @test map!(&, b, b1, b2) == map!((x,y)->x&y, b, b1, b2) == broadcast(&, b1, b2) == b
             @test map!(|, b, b1, b2) == map!((x,y)->x|y, b, b1, b2) == broadcast(|, b1, b2) == b