function composition (f∘g) and negation (!f)

base/exports.jl

@@ -233,6 +233,7 @@ export
     ~,
     :,
     =>,
+    ∘,
     A_ldiv_B!,
     A_ldiv_Bc,
     A_ldiv_Bt,

base/operators.jl

@@ -665,6 +665,48 @@ julia> [1:5;] |> x->x.^2 |> sum |> inv
 """
 |>(x, f) = f(x)
 
+# function composition
+
+"""
+    f ∘ g
+
+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>`.
+Example:
+
+```jldoctest
+julia> map(uppercase∘hex, 250:255)
+6-element Array{String,1}:
+ "FA"
+ "FB"
+ "FC"
+ "FD"
+ "FE"
+ "FF"
+```
+"""
+∘(f, g) = (x...)->f(g(x...))
+
+
+"""
+    !f::Function
+
+Predicate function negation: when the argument of `!` is a function, it returns a
+function which computes the boolean negation of `f`. Example:
+
+```jldoctest
+julia> str = "∀ ε > 0, ∃ δ > 0: |x-y| < δ ⇒ |f(x)-f(y)| < ε"
+"∀ ε > 0, ∃ δ > 0: |x-y| < δ ⇒ |f(x)-f(y)| < ε"
+
+julia> filter(isalpha, str)
+"εδxyδfxfyε"
+
+julia> filter(!isalpha, str)
+"∀  > 0, ∃  > 0: |-| <  ⇒ |()-()| < "
+```
+"""
+!(f::Function) = (x...)->!f(x...)
+
 # array shape rules
 
 promote_shape(::Tuple{}, ::Tuple{}) = ()
@@ -989,6 +1031,7 @@ export
     √,
     ∛,
     ⊻,
+    ∘,
     colon,
     hcat,
     vcat,
@@ -1002,6 +1045,6 @@ import ..this_module: !, !=, xor, %, ÷, &, *, +, -,
     /, //, <, <:, <<, <=, ==, >, >=, >>, >>>,
     <|, |>, \, ^, |, ~, !==, ===, >:, colon, hcat, vcat, hvcat, getindex, setindex!,
     transpose, ctranspose,
-    ≥, ≤, ≠, ⋅, ×, ∈, ∉, ∋, ∌, ⊆, ⊈, ⊊, ∩, ∪, √, ∛, ⊻
+    ≥, ≤, ≠, ⋅, ×, ∈, ∉, ∋, ∌, ⊆, ⊈, ⊊, ∩, ∪, √, ∛, ⊻, ∘
 
 end

doc/src/stdlib/base.md

@@ -122,6 +122,7 @@ Base.method_exists
 Core.applicable
 Core.invoke
 Base.:(|>)
+Base.:(∘)
 ```
 
 ## Syntax

test/operators.jl

@@ -79,3 +79,13 @@ Base.:/(::T19714, ::T19714) = T19714()
 Base.convert(::Type{T19714}, ::Int) = T19714()
 Base.promote_rule(::Type{T19714}, ::Type{Int}) = T19714
 @test T19714()/1 === 1/T19714() === T19714()
+
+# pr #17155
+@testset "function composition" begin
+    @test (uppercase∘hex)(239487) == "3A77F"
+end
+@testset "function negation" begin
+    str = randstring(20)
+    @test filter(!isupper, str) == replace(str, r"[A-Z]", "")
+    @test filter(!islower, str) == replace(str, r"[a-z]", "")
+end