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add one arg function composition #34251

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Jan 13, 2020
Merged

add one arg function composition #34251

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85a4924
add zero and one arg function composition
jw3126 Jan 3, 2020
326b1fe
add function composition news and compat
jw3126 Jan 4, 2020
5289aab
remove empty function composition
jw3126 Jan 9, 2020
6458eb9
fix
jw3126 Jan 9, 2020
ae03f24
fix
jw3126 Jan 9, 2020
1630d32
fix
jw3126 Jan 10, 2020
fffce8a
fix
jw3126 Jan 12, 2020
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1 change: 1 addition & 0 deletions NEWS.md
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Expand Up @@ -23,6 +23,7 @@ New library functions

New library features
--------------------
* Function composition now works also on one argument `∘(f) = f` (#34251)


Standard library changes
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13 changes: 13 additions & 0 deletions base/operators.jl
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Expand Up @@ -837,6 +837,10 @@ and splatting `∘(fs...)` for composing an iterable collection of functions.
!!! compat "Julia 1.4"
Multiple function composition requires at least Julia 1.4.

!!! compat "Julia 1.5"
Composition of one function ∘(f) requires at least Julia 1.5.


# Examples
```jldoctest
julia> map(uppercase∘first, ["apple", "banana", "carrot"])
Expand All @@ -856,6 +860,15 @@ julia> ∘(fs...)(3)
3.0
```
"""
function ∘()
# Like +() and *() we leave ∘() undefined.
# While `∘() = identity` is a reasonable definition for functions, this
# would cause headaches for composition of user defined morphisms.
# See also #34251
msg = """Empty composition ∘() is undefined."""
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Why define it at all then? Why not just let it be a method error?

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So that we don't get a stream of PRs that want to define it 😄

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I think it's better to leave it undefined.

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I see the benefit of making it very clear that not defining it is intentional with this approach. But I think leaving a comment is enough to prevent future PRs.

throw(ArgumentError(msg))
end
∘(f) = f
∘(f, g) = (x...)->f(g(x...))
∘(f, g, h...) = ∘(f ∘ g, h...)

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18 changes: 18 additions & 0 deletions test/operators.jl
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Expand Up @@ -111,7 +111,25 @@ Base.promote_rule(::Type{T19714}, ::Type{Int}) = T19714
@test ∘(x -> x-2, x -> x-3, x -> x+5)(7) == 7
fs = [x -> x[1:2], uppercase, lowercase]
@test ∘(fs...)("ABC") == "AB"

@test_throws ArgumentError ∘()
@test ∘(x -> (x, 1))(0) === (0, 1)
@test ∘(x -> (x, 2), x -> (x, 1))(0) === ((0, 1), 2)
@test ∘(x -> (x, 3), x -> (x, 2), x->(x,1))(0) === (((0, 1), 2), 3)
@test ∘(x -> (x, 4), x -> (x, 3), x->(x,2), x-> (x, 1))(0) === ((((0, 1), 2), 3), 4)

# test that user defined functors only need to overload the two arg version
struct FreeMagma
word
end
Base.:(∘)(a::FreeMagma, b::FreeMagma) = FreeMagma((a.word, b.word))

@test ∘(FreeMagma(1)) === FreeMagma(1)
@test ∘(FreeMagma(1), FreeMagma(2)) === FreeMagma((1,2))
@test ∘(FreeMagma(1), FreeMagma(2), FreeMagma(3)) === FreeMagma(((1,2), 3))
@test ∘(FreeMagma(1), FreeMagma(2), FreeMagma(3), FreeMagma(4)) === FreeMagma((((1,2), 3), 4))
end

@testset "function negation" begin
str = randstring(20)
@test filter(!isuppercase, str) == replace(str, r"[A-Z]" => "")
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