Add copy(xf, [T,] foldable)

src/Transducers.jl

@@ -6,15 +6,15 @@ export Transducer, Map, Filter, Cat, MapCat, Take, PartitionBy, Scan, Zip,
     TakeLast, FlagFirst, MapSplat, ScanEmit, Enumerate, NotA, OfType,
     transduce, eduction, setinput, Reduced, reduced, unreduced, ifunreduced,
     Completing, OnInit, CopyInit, right, reducingfunction, dreduce, dtransduce,
-    withprogress, AdHocFoldable
+    withprogress, AdHocFoldable, into
 
 # Deprecated:
 export Distinct
 
 using Base.Broadcast: Broadcasted
 
 using ArgCheck
-using BangBang: BangBang, append!!, empty!!, push!!, setindex!!
+using BangBang: BangBang, Empty, append!!, empty!!, push!!, setindex!!
 using Distributed: Distributed, @everywhere
 using Logging: LogLevel, @logmsg
 using Requires

src/processes.jl

@@ -590,6 +590,25 @@ function Base.collect(xf::Transducer, coll)
 end
 # Base.collect(xf, coll) = append!([], xf, coll)
 
+"""
+    copy(xf::Transducer, T, foldable) :: Union{T, Nothing}
+    copy(xf::Transducer, foldable::T) :: Union{T, Nothing}
+
+Process `foldable` with a transducer `xf` and then create a container of type `T`
+filled with the result.  Return `nothing` if the transducer does not produce
+anything.  This is because not all containers support creating an empty
+container in a type-stable manner.
+"""
+function Base.copy(xf::Transducer, ::Type{T}, foldable) where T
+    result = append!!(Empty(T), foldable)
+    if result isa Empty
+        return nothing
+    end
+    return result
+end
+
+Base.copy(xf::Transducer, foldable::T) where T = copy(xf, T, foldable)
+
 """
     map!(xf::Transducer, dest, src; simd)