Simplify Channel constructors, change kwargs to positional args. (#32818)

"Soft" deprecation of the old `Channel(f::Function; csize=0, ctype=Any)`
style in favor of the new `Channel{T=Any}(f::Function, size=0)`
interface.

Removed the old interface from the docs, so that it is not discoverable,
and included a `!!! compat` note that the new interface is new in Julia
1.3, and the old interface should not be used.

Author: NHDaly

NEWS.md

@@ -20,6 +20,8 @@ Multi-threading changes
   ([#32309], [#32174], [#31981], [#32421]).
 * The global random number generator (`GLOBAL_RNG`) is now thread-safe (and thread-local) ([#32407]).
 * New experimental `Threads.@spawn` macro that runs a task on any available thread ([#32600]).
+* Simplified the `Channel` constructor, which is now easier to read and more idiomatic julia.
+  The old constructor (which used kwargs) is still available, but use is discouraged ([#30855], [#32818]).
 
 Build system changes
 --------------------

base/channels.jl

@@ -8,9 +8,9 @@ Representation of a channel passing objects of type `T`.
 abstract type AbstractChannel{T} end
 
 """
-    Channel{T}(sz::Int=0)
+    Channel{T=Any}(size::Int=0)
 
-Constructs a `Channel` with an internal buffer that can hold a maximum of `sz` objects
+Constructs a `Channel` with an internal buffer that can hold a maximum of `size` objects
 of type `T`.
 [`put!`](@ref) calls on a full channel block until an object is removed with [`take!`](@ref).
 
@@ -24,7 +24,7 @@ Other constructors:
 * `Channel(sz)`: equivalent to `Channel{Any}(sz)`
 
 !!! compat "Julia 1.3"
-  The default constructor `Channel()` and default `sz=0` were added in Julia 1.3.
+  The default constructor `Channel()` and default `size=0` were added in Julia 1.3.
 """
 mutable struct Channel{T} <: AbstractChannel{T}
     cond_take::Threads.Condition                 # waiting for data to become available
@@ -51,26 +51,27 @@ function Channel{T}(sz::Float64) where T
     sz = (sz == Inf ? typemax(Int) : convert(Int, sz))
     return Channel{T}(sz)
 end
-Channel(sz) = Channel{Any}(sz)
-Channel() = Channel{Any}(0)
+Channel(sz=0) = Channel{Any}(sz)
 
 # special constructors
 """
-    Channel(func::Function; ctype=Any, csize=0, taskref=nothing)
+    Channel{T=Any}(func::Function, size=0; taskref=nothing)
 
 Create a new task from `func`, bind it to a new channel of type
-`ctype` and size `csize`, and schedule the task, all in a single call.
+`T` and size `size`, and schedule the task, all in a single call.
 
 `func` must accept the bound channel as its only argument.
 
 If you need a reference to the created task, pass a `Ref{Task}` object via
-keyword argument `taskref`.
+the keyword argument `taskref`.
 
 Return a `Channel`.
 
 # Examples
 ```jldoctest
-julia> chnl = Channel(c->foreach(i->put!(c,i), 1:4));
+julia> chnl = Channel() do ch
+           foreach(i -> put!(ch, i), 1:4)
+       end;
 
 julia> typeof(chnl)
 Channel{Any}
@@ -89,7 +90,9 @@ Referencing the created task:
 ```jldoctest
 julia> taskref = Ref{Task}();
 
-julia> chnl = Channel(c -> println(take!(c)); taskref=taskref);
+julia> chnl = Channel(taskref=taskref) do ch
+           println(take!(ch))
+       end;
 
 julia> istaskdone(taskref[])
 false
@@ -102,11 +105,8 @@ true
 ```
 
 !!! compat "Julia 1.3"
-  The following constructors were added in Julia 1.3.
-
-Other constructors:
-* `Channel{T}(func::Function, sz=0)`
-* `Channel{T}(func::Function; csize=0, taskref=nothing)`
+  This constructor was added in Julia 1.3. Earlier versions of Julia used kwargs
+  to set `size` and `T`, but those constructors are deprecated.
 
 ```jldoctest
 julia> chnl = Channel{Char}(1) do ch
@@ -120,22 +120,21 @@ julia> String(collect(chnl))
 "hello world"
 ```
 """
-function Channel(func::Function; ctype=Any, csize=0, taskref=nothing)
-    chnl = Channel{ctype}(csize)
+function Channel{T}(func::Function, size=0; taskref=nothing) where T
+    chnl = Channel{T}(size)
     task = Task(() -> func(chnl))
     bind(chnl, task)
     yield(task) # immediately start it
 
     isa(taskref, Ref{Task}) && (taskref[] = task)
     return chnl
 end
-function Channel{T}(f::Function, sz=0) where T
-    return Channel(f, csize=sz, ctype=T)
-end
-function Channel{T}(f::Function; csize=0, taskref=nothing) where T
-    return Channel(f, csize=csize, ctype=T, taskref=taskref)
-end
+Channel(func::Function, args...; kwargs...) = Channel{Any}(func, args...; kwargs...)
 
+# This constructor is deprecated as of Julia v1.3, and should not be used.
+function Channel(func::Function; ctype=Any, csize=0, taskref=nothing)
+    return Channel{ctype}(func, csize; taskref=taskref)
+end
 
 closed_exception() = InvalidStateException("Channel is closed.", :closed)
 

test/channels.jl

@@ -44,12 +44,6 @@ end
 end
 
 @testset "Task constructors" begin
-    c = Channel(ctype=Float32, csize=2) do c; map(i->put!(c,i), 1:100); end
-    @test eltype(c) == Float32
-    @test c.sz_max == 2
-    @test isopen(c)
-    @test collect(c) == 1:100
-
     c = Channel{Int}() do c; map(i->put!(c,i), 1:100); end
     @test eltype(c) == Int
     @test c.sz_max == 0
@@ -59,18 +53,28 @@ end
     @test c.sz_max == 0
     @test collect(c) == [1, "hi"]
 
+    c = Channel(Inf) do c; put!(c,1); end
+    @test eltype(c) == Any
+    @test c.sz_max == typemax(Int)
     c = Channel{Int}(Inf) do c; put!(c,1); end
     @test eltype(c) == Int
     @test c.sz_max == typemax(Int)
 
     taskref = Ref{Task}()
-    c = Channel{Int}(csize=0, taskref=taskref) do c; put!(c, 0); end
+    c = Channel{Int}(0, taskref=taskref) do c; put!(c, 0); end
     @test eltype(c) == Int
     @test c.sz_max == 0
     @test istaskstarted(taskref[])
     @test !istaskdone(taskref[])
     take!(c); wait(taskref[])
     @test istaskdone(taskref[])
+
+    # Legacy constructor
+    c = Channel(ctype=Float32, csize=2) do c; map(i->put!(c,i), 1:100); end
+    @test eltype(c) == Float32
+    @test c.sz_max == 2
+    @test isopen(c)
+    @test collect(c) == 1:100
 end
 
 @testset "multiple concurrent put!/take! on a channel for different sizes" begin
@@ -230,7 +234,7 @@ using Distributed
 
     for T in [Any, Int]
         taskref = Ref{Task}()
-        chnl = Channel(tf6, ctype=T, csize=N, taskref=taskref)
+        chnl = Channel{T}(tf6, N, taskref=taskref)
         put!(chnl, 2)
         yield()
         @test_throws ErrorException wait(chnl)