Backport ComposedFunction (#720)

Author: tkf

Project.toml

@@ -1,6 +1,6 @@
 name = "Compat"
 uuid = "34da2185-b29b-5c13-b0c7-acf172513d20"
-version = "3.16.0"
+version = "3.17.0"
 
 [deps]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"

README.md

@@ -55,6 +55,8 @@ changes in `julia`.
 
 ## Supported features
 
+* The composition operator `∘` now returns a `Compat.ComposedFunction` instead of an anonymous function ([#37517]). (since Compat 3.17)
+
 * New function `addenv` for adding environment mappings into a `Cmd` object, returning the new `Cmd` object ([#37244]). (since Compat 3.16)
 
 * `contains(haystack, needle)` and its one argument partially applied form `contains(haystack)` have been added, it acts like `occursin(needle, haystack)` ([#35132]). (since Compat 3.15)
@@ -200,3 +202,4 @@ Note that you should specify the correct minimum version for `Compat` in the
 [#35132]: https://github.com/JuliaLang/julia/pull/35132
 [#35052]: https://github.com/JuliaLang/julia/pull/35052
 [#37244]: https://github.com/JuliaLang/julia/pull/37244
+[#37517]: https://github.com/JuliaLang/julia/pull/37517

src/Compat.jl

@@ -607,6 +607,37 @@ if VERSION < v"1.5.0-DEV.438" # 0a43c0f1d21ce9c647c49111d93927369cd20f85
     Base.startswith(s) = Base.Fix2(startswith, s)
 end
 
+# https://github.com/JuliaLang/julia/pull/37517
+if VERSION < v"1.6.0-DEV.1037"
+    export ComposedFunction
+    # https://github.com/JuliaLang/julia/pull/35980
+    if VERSION < v"1.6.0-DEV.85"
+        const ComposedFunction = let h = identity ∘ convert
+            Base.typename(typeof(h)).wrapper
+        end
+        @eval ComposedFunction{F,G}(f, g) where {F,G} =
+            $(Expr(:new, :(ComposedFunction{F,G}), :f, :g))
+        ComposedFunction(f, g) = ComposedFunction{Core.Typeof(f),Core.Typeof(g)}(f, g)
+    else
+        using Base: ComposedFunction
+    end
+    function Base.getproperty(c::ComposedFunction, p::Symbol)
+        if p === :f
+            return getfield(c, :f)
+        elseif p === :g
+            return getfield(c, :g)
+        elseif p === :outer
+            return getfield(c, :f)
+        elseif p === :inner
+            return getfield(c, :g)
+        end
+        error("type ComposedFunction has no property ", p)
+    end
+    Base.propertynames(c::ComposedFunction) = (:f, :g, :outer, :inner)
+else
+    using Base: ComposedFunction
+end
+
 # https://github.com/JuliaLang/julia/pull/37244
 if VERSION < v"1.6.0-DEV.873" # 18198b1bf85125de6cec266eac404d31ccc2e65c
     export addenv

test/runtests.jl

@@ -556,6 +556,22 @@ end
     @test endswith("d")("abcd")
 end
 
+# https://github.com/JuliaLang/julia/pull/37517
+@testset "ComposedFunction" begin
+    @test sin ∘ cos isa Compat.ComposedFunction
+    @test sin ∘ cos === Compat.ComposedFunction(sin, cos)
+    c = sin ∘ cos
+    @test c.outer === sin
+    @test c.inner === cos
+    if VERSION < v"1.6.0-DEV.1037"
+        @test c.f === sin
+        @test c.g === cos
+        @test propertynames(c) == (:f, :g, :outer, :inner)
+    else
+        @test propertynames(c) == (:outer, :inner)
+    end
+end
+
 # From spawn.jl
 shcmd = `sh`
 havebb = false