only (#662)

* JuliaLang/julia#33129

Author: mcabbott

README.md

@@ -114,6 +114,8 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 ## New functions, macros, and methods
 
+* `only(x)` returns the one-and-only element of a collection `x`. ([#33129])
+
 * `mod` now accepts a unit range as the second argument ([#32628]).
 
 * `eachrow`, `eachcol`, and `eachslice` to iterate over first, second, or given dimension
@@ -576,3 +578,5 @@ includes this fix. Find the minimum version from there.
 [#29259]: https://github.com/JuliaLang/julia/issues/29259
 [#29674]: https://github.com/JuliaLang/julia/issues/29674
 [#29749]: https://github.com/JuliaLang/julia/issues/29749
+[#33129]: https://github.com/JuliaLang/julia/issues/33129
+[#32628]: https://github.com/JuliaLang/julia/issues/32628

src/Compat.jl

@@ -1866,6 +1866,37 @@ if v"0.7.0" <= VERSION < v"1.1.0-DEV.594"
     Base.merge(a::NamedTuple) = a
 end
 
+# https://github.com/JuliaLang/julia/pull/33129
+if v"0.7.0" <= VERSION < v"1.4.0-DEV.142"
+    export only
+
+    Base.@propagate_inbounds function only(x)
+        i = iterate(x)
+        @boundscheck if i === nothing
+            throw(ArgumentError("Collection is empty, must contain exactly 1 element"))
+        end
+        (ret, state) = i
+        @boundscheck if iterate(x, state) !== nothing
+            throw(ArgumentError("Collection has multiple elements, must contain exactly 1 element"))
+        end
+        return ret
+    end
+
+    # Collections of known size
+    only(x::Ref) = x[]
+    only(x::Number) = x
+    only(x::Char) = x
+    only(x::Tuple{Any}) = x[1]
+    only(x::Tuple) = throw(
+        ArgumentError("Tuple contains $(length(x)) elements, must contain exactly 1 element")
+    )
+    only(a::AbstractArray{<:Any, 0}) = @inbounds return a[]
+    only(x::NamedTuple{<:Any, <:Tuple{Any}}) = first(x)
+    only(x::NamedTuple) = throw(
+        ArgumentError("NamedTuple contains $(length(x)) elements, must contain exactly 1 element")
+    )
+end
+
 # https://github.com/JuliaLang/julia/pull/32628
 if v"0.7.0" <= VERSION < v"1.3.0-alpha.8"
     Base.mod(i::Integer, r::Base.OneTo) = mod1(i, last(r))

test/runtests.jl

@@ -1499,6 +1499,43 @@ end
     @test merge((a=1,), (b=2,), (c=3,)) == (a=1,b=2,c=3)
 end
 
+@static if VERSION >= v"0.7.0"
+    @testset "only" begin
+        @test only([3]) === 3
+        @test_throws ArgumentError only([])
+        @test_throws ArgumentError only([3, 2])
+
+        @test @inferred(only((3,))) === 3
+        @test_throws ArgumentError only(())
+        @test_throws ArgumentError only((3, 2))
+
+        @test only(Dict(1=>3)) === (1=>3)
+        @test_throws ArgumentError only(Dict{Int,Int}())
+        @test_throws ArgumentError only(Dict(1=>3, 2=>2))
+
+        @test only(Set([3])) === 3
+        @test_throws ArgumentError only(Set(Int[]))
+        @test_throws ArgumentError only(Set([3,2]))
+
+        @test @inferred(only((;a=1))) === 1
+        @test_throws ArgumentError only(NamedTuple())
+        @test_throws ArgumentError only((a=3, b=2.0))
+
+        @test @inferred(only(1)) === 1
+        @test @inferred(only('a')) === 'a'
+        if  VERSION >= v"1.0"
+            @test @inferred(only(Ref([1, 2]))) == [1, 2] # Fails on v0.7, depwarn "`Ref(x::AbstractArray)` is deprecated, use `Ref(x, 1)` instead."
+        end
+        @test_throws ArgumentError only(Pair(10, 20))
+
+        @test only(1 for ii in 1:1) === 1
+        @test only(1 for ii in 1:10 if ii < 2) === 1
+        @test_throws ArgumentError only(1 for ii in 1:10)
+        @test_throws ArgumentError only(1 for ii in 1:10 if ii > 2)
+        @test_throws ArgumentError only(1 for ii in 1:10 if ii > 200)
+    end
+end
+
 # https://github.com/JuliaLang/julia/pull/32628
 if VERSION >= v"0.7"
     @testset "mod with ranges" begin