Enable multiple function composition in prefix form (#33568)

Document the prefix form and extend the prefix form to support composition of 3 or more functions.

Co-Authored-By: Stefan Karpinski <stefan@karpinski.org>

NEWS.md

@@ -8,6 +8,9 @@ New language features
 
 * `import` now allows quoted symbols, e.g. `import Base.:+` ([#33158]).
 
+* Function composition now supports multiple functions: `∘(f, g, h) = f ∘ g ∘ h`
+and splatting `∘(fs...)` for composing an iterable collection of functions ([#33568]). 
+
 Language changes
 ----------------
 

base/operators.jl

@@ -827,17 +827,34 @@ julia> [1:5;] |> x->x.^2 |> sum |> inv
 Compose functions: i.e. `(f ∘ g)(args...)` means `f(g(args...))`. The `∘` symbol can be
 entered in the Julia REPL (and most editors, appropriately configured) by typing `\\circ<tab>`.
 
+Function composition also works in prefix form: `∘(f, g)` is the same as `f ∘ g`.
+The prefix form supports composition of multiple functions: `∘(f, g, h) = f ∘ g ∘ h`
+and splatting `∘(fs...)` for composing an iterable collection of functions.
+
+!!!compat "Julia 1.4"
+    Multiple function composition requires at least Julia 1.4.    
+
 # Examples
 ```jldoctest
 julia> map(uppercase∘first, ["apple", "banana", "carrot"])
 3-element Array{Char,1}:
  'A'
  'B'
  'C'
+
+julia> fs = [
+           x -> 2x
+           x -> x/2
+           x -> x-1
+           x -> x+1
+       ];
+
+julia> ∘(fs...)(3)
+3.0
 ```
 """
 ∘(f, g) = (x...)->f(g(x...))
-
+∘(f, g, h...) = ∘(f ∘ g, h...)
 
 """
     !f::Function

test/operators.jl

@@ -105,9 +105,12 @@ Base.convert(::Type{T19714}, ::Int) = T19714()
 Base.promote_rule(::Type{T19714}, ::Type{Int}) = T19714
 @test T19714()/1 === 1/T19714() === T19714()
 
-# pr #17155
+# pr #17155 and #33568
 @testset "function composition" begin
     @test (uppercase∘(x->string(x,base=16)))(239487) == "3A77F"
+    @test ∘(x -> x-2, x -> x-3, x -> x+5)(7) == 7
+    fs = [x -> x[1:2], uppercase, lowercase]
+    @test ∘(fs...)("ABC") == "AB"
 end
 @testset "function negation" begin
     str = randstring(20)