From fb6973def07359d1879117ec26a2577f85cc7273 Mon Sep 17 00:00:00 2001
From: Takafumi Arakaki <aka.tkf@gmail.com>
Date: Mon, 4 Nov 2019 01:12:49 -0800
Subject: [PATCH] Add `copy(xf, [T,] foldable)`

---
 Project.toml       |  2 +-
 docs/src/manual.md |  1 +
 src/Transducers.jl |  2 +-
 src/processes.jl   | 20 ++++++++++++++++++++
 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/Project.toml b/Project.toml
index b7ff1dc7e6..ee166b39cb 100644
--- a/Project.toml
+++ b/Project.toml
@@ -15,7 +15,7 @@ Setfield = "efcf1570-3423-57d1-acb7-fd33fddbac46"
 
 [compat]
 ArgCheck = "1"
-BangBang = "0.2, 0.3"
+BangBang = "0.3.2"
 InitialValues = "0.2"
 Requires = "0.5"
 Setfield = "0.3, 0.4, 0.5"
diff --git a/docs/src/manual.md b/docs/src/manual.md
index 6a992795df..f62ad26531 100644
--- a/docs/src/manual.md
+++ b/docs/src/manual.md
@@ -19,6 +19,7 @@ dtransduce
 eduction
 map!
 copy!
+copy
 append!
 Transducers.append!!
 collect
diff --git a/src/Transducers.jl b/src/Transducers.jl
index f5d24e0b6b..d5e6af172f 100644
--- a/src/Transducers.jl
+++ b/src/Transducers.jl
@@ -14,7 +14,7 @@ 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
diff --git a/src/processes.jl b/src/processes.jl
index 1a112992f6..7f57cb2c46 100644
--- a/src/processes.jl
+++ b/src/processes.jl
@@ -590,6 +590,26 @@ 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 there is no consistent interface to create an empty
+container given its type and not all containers support creating an empty
+container.)
+"""
+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)