Add pretty printing for splat(f)

NEWS.md

@@ -44,6 +44,8 @@ New library functions
 ---------------------
 
 * `Iterators.flatmap` was added ([#44792]).
+* New helper `Splat(f)` which acts like `x -> f(x...)`, with pretty printing for
+  inspecting which function `f` was originally wrapped. ([#42717])
 
 Library changes
 ---------------
@@ -120,6 +122,7 @@ Standard library changes
 Deprecated or removed
 ---------------------
 
+* Unexported `splat` is deprecated in favor of exported `Splat`, which has pretty printing of the wrapped function. ([#42717])
 
 External dependencies
 ---------------------

base/deprecated.jl

@@ -294,3 +294,9 @@ const var"@_noinline_meta" = var"@noinline"
 @deprecate getindex(t::Tuple, i::Real) t[convert(Int, i)]
 
 # END 1.8 deprecations
+
+# BEGIN 1.9 deprecations
+
+@deprecate splat(x) Splat(x) false
+
+# END 1.9 deprecations

base/exports.jl

@@ -807,6 +807,7 @@ export
     atreplinit,
     exit,
     ntuple,
+    Splat,
 
 # I/O and events
     close,

base/iterators.jl

@@ -300,7 +300,7 @@ the `zip` iterator is a tuple of values of its subiterators.
     `zip` orders the calls to its subiterators in such a way that stateful iterators will
     not advance when another iterator finishes in the current iteration.
 
-See also: [`enumerate`](@ref), [`splat`](@ref Base.splat).
+See also: [`enumerate`](@ref), [`Splat`](@ref Base.Splat).
 
 # Examples
 ```jldoctest

base/operators.jl

@@ -1185,27 +1185,39 @@ used to implement specialized methods.
 <(x) = Fix2(<, x)
 
 """
-    splat(f)
+    Splat(f)
 
-Defined as
+Equivalent to
 ```julia
-    splat(f) = args->f(args...)
+    my_splat(f) = args->f(args...)
 ```
 i.e. given a function returns a new function that takes one argument and splats
 its argument into the original function. This is useful as an adaptor to pass
 a multi-argument function in a context that expects a single argument, but
-passes a tuple as that single argument.
+passes a tuple as that single argument. Additionally has pretty printing.
 
 # Example usage:
 ```jldoctest
-julia> map(Base.splat(+), zip(1:3,4:6))
+julia> map(Base.Splat(+), zip(1:3,4:6))
 3-element Vector{Int64}:
  5
  7
  9
+
+julia> my_add = Base.Splat(+)
+Splat(+)
+
+julia> my_add((1,2,3))
+6
 ```
 """
-splat(f) = args->f(args...)
+struct Splat{F} <: Function
+    f::F
+    Splat(f) = new{Core.Typeof(f)}(f)
+end
+(s::Splat)(args) = s.f(args...)
+print(io::IO, s::Splat) = print(io, "Splat(", s.f, ')')
+show(io::IO, s::Splat) = print(io, s)
 
 ## in and related operators
 

base/show.jl

@@ -46,6 +46,7 @@ end
 
 show(io::IO, ::MIME"text/plain", c::ComposedFunction) = show(io, c)
 show(io::IO, ::MIME"text/plain", c::Returns) = show(io, c)
+show(io::IO, ::MIME"text/plain", s::Splat) = show(io, s)
 
 function show(io::IO, ::MIME"text/plain", iter::Union{KeySet,ValueIterator})
     isempty(iter) && get(io, :compact, false) && return show(io, iter)

base/strings/search.jl

@@ -179,7 +179,7 @@ function _searchindex(s::Union{AbstractString,ByteArray},
         if i === nothing return 0 end
         ii = nextind(s, i)::Int
         a = Iterators.Stateful(trest)
-        matched = all(splat(==), zip(SubString(s, ii), a))
+        matched = all(Splat(==), zip(SubString(s, ii), a))
         (isempty(a) && matched) && return i
         i = ii
     end
@@ -435,7 +435,7 @@ function _rsearchindex(s::AbstractString,
         a = Iterators.Stateful(trest)
         b = Iterators.Stateful(Iterators.reverse(
             pairs(SubString(s, 1, ii))))
-        matched = all(splat(==), zip(a, (x[2] for x in b)))
+        matched = all(Splat(==), zip(a, (x[2] for x in b)))
         if matched && isempty(a)
             isempty(b) && return firstindex(s)
             return nextind(s, popfirst!(b)[1])::Int

doc/src/base/base.md

@@ -256,7 +256,7 @@ new
 Base.:(|>)
 Base.:(∘)
 Base.ComposedFunction
-Base.splat
+Base.Splat
 Base.Fix1
 Base.Fix2
 ```

doc/src/devdocs/ast.md

@@ -425,7 +425,7 @@ These symbols appear in the `head` field of [`Expr`](@ref)s in lowered form.
   * `splatnew`
 
     Similar to `new`, except field values are passed as a single tuple. Works similarly to
-    `Base.splat(new)` if `new` were a first-class function, hence the name.
+    `Base.Splat(new)` if `new` were a first-class function, hence the name.
 
   * `isdefined`
 

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -18,7 +18,7 @@ import Base: USE_BLAS64, abs, acos, acosh, acot, acoth, acsc, acsch, adjoint, as
     vec, zero
 using Base: IndexLinear, promote_eltype, promote_op, promote_typeof,
     @propagate_inbounds, reduce, typed_hvcat, typed_vcat, require_one_based_indexing,
-    splat
+    Splat
 using Base.Broadcast: Broadcasted, broadcasted
 using OpenBLAS_jll
 using libblastrampoline_jll

stdlib/LinearAlgebra/src/qr.jl

@@ -159,7 +159,7 @@ function Base.hash(F::QRCompactWY, h::UInt)
     return hash(F.factors, foldr(hash, _triuppers_qr(F.T); init=hash(QRCompactWY, h)))
 end
 function Base.:(==)(A::QRCompactWY, B::QRCompactWY)
-    return A.factors == B.factors && all(splat(==), zip(_triuppers_qr.((A.T, B.T))...))
+    return A.factors == B.factors && all(Splat(==), zip(_triuppers_qr.((A.T, B.T))...))
 end
 function Base.isequal(A::QRCompactWY, B::QRCompactWY)
     return isequal(A.factors, B.factors) && all(zip(_triuppers_qr.((A.T, B.T))...)) do (a, b)

test/broadcast.jl

@@ -870,13 +870,13 @@ end
     ys = 1:2:20
     bc = Broadcast.instantiate(Broadcast.broadcasted(*, xs, ys))
     @test IndexStyle(bc) == IndexLinear()
-    @test sum(bc) == mapreduce(Base.splat(*), +, zip(xs, ys))
+    @test sum(bc) == mapreduce(Base.Splat(*), +, zip(xs, ys))
 
     xs2 = reshape(xs, 1, :)
     ys2 = reshape(ys, 1, :)
     bc = Broadcast.instantiate(Broadcast.broadcasted(*, xs2, ys2))
     @test IndexStyle(bc) == IndexCartesian()
-    @test sum(bc) == mapreduce(Base.splat(*), +, zip(xs, ys))
+    @test sum(bc) == mapreduce(Base.Splat(*), +, zip(xs, ys))
 
     xs = 1:5:3*5
     ys = 1:4:3*4

test/compiler/inference.jl

@@ -2842,7 +2842,7 @@ j30385(T, y) = k30385(f30385(T, y))
 @test @inferred(j30385(:dummy, 1)) == "dummy"
 
 @test Base.return_types(Tuple, (NamedTuple{<:Any,Tuple{Any,Int}},)) == Any[Tuple{Any,Int}]
-@test Base.return_types(Base.splat(tuple), (typeof((a=1,)),)) == Any[Tuple{Int}]
+@test Base.return_types(Base.Splat(tuple), (typeof((a=1,)),)) == Any[Tuple{Int}]
 
 # test that return_type_tfunc isn't affected by max_methods differently than return_type
 _rttf_test(::Int8) = 0

test/iterators.jl

@@ -584,7 +584,7 @@ end
             @test length(I) == iterate_length(I) == simd_iterate_length(I) == simd_trip_count(I)
             @test collect(I) == iterate_elements(I) == simd_iterate_elements(I) == index_elements(I)
         end
-        @test all(Base.splat(==), zip(Iterators.flatten(map(collect, P)), iter))
+        @test all(Base.Splat(==), zip(Iterators.flatten(map(collect, P)), iter))
     end
 end
 @testset "empty/invalid partitions" begin