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WIP: Add only function #3

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WIP: Add only function #3

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82bdb7a
Add `only` function
Dec 14, 2017
3218d9d
Update to new iteration interface
nickrobinson251 Aug 31, 2019
14ed6dd
Throw ArgumentError not plain ErrorException
nickrobinson251 Aug 31, 2019
694604d
Add more tests for `only`
nickrobinson251 Aug 31, 2019
e2d543a
Make `only` constant fold more inputs
nickrobinson251 Aug 31, 2019
9235f44
Import `@boundscheck` and `@inbounds` from Base
nickrobinson251 Aug 31, 2019
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3 changes: 2 additions & 1 deletion NEWS.md
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Expand Up @@ -32,8 +32,9 @@ Standard library changes

* The methods of `mktemp` and `mktempdir` which take a function body to pass temporary paths to no longer throw errors if the path is already deleted when the function body returns ([#33091]).

#### Libdl
* New function `only(x)` returns the one-and-only element of a collection `x`, and throws an error if `x` contains zero or multiple elements.

#### Libdl

#### LinearAlgebra

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2 changes: 1 addition & 1 deletion base/Base.jl
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Expand Up @@ -138,7 +138,7 @@ include("ntuple.jl")
include("abstractdict.jl")

include("iterators.jl")
using .Iterators: zip, enumerate
using .Iterators: zip, enumerate, only
using .Iterators: Flatten, Filter, product # for generators

include("namedtuple.jl")
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1 change: 1 addition & 0 deletions base/exports.jl
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Expand Up @@ -628,6 +628,7 @@ export

enumerate, # re-exported from Iterators
zip,
only,

# object identity and equality
copy,
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36 changes: 34 additions & 2 deletions base/iterators.jl
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Expand Up @@ -12,7 +12,7 @@ using .Base:
@inline, Pair, AbstractDict, IndexLinear, IndexCartesian, IndexStyle, AbstractVector, Vector,
tail, tuple_type_head, tuple_type_tail, tuple_type_cons, SizeUnknown, HasLength, HasShape,
IsInfinite, EltypeUnknown, HasEltype, OneTo, @propagate_inbounds, Generator, AbstractRange,
LinearIndices, (:), |, +, -, !==, !, <=, <, missing, map, any
LinearIndices, (:), |, +, -, !==, !, <=, <, missing, map, any, @boundscheck, @inbounds

import .Base:
first, last,
Expand Down Expand Up @@ -929,7 +929,6 @@ julia> collect(Iterators.partition([1,2,3,4,5], 2))
"""
partition(c::T, n::Integer) where {T} = PartitionIterator{T}(c, Int(n))


struct PartitionIterator{T}
c::T
n::Int
Expand Down Expand Up @@ -1095,4 +1094,37 @@ eltype(::Type{Stateful{T, VS}} where VS) where {T} = eltype(T)
IteratorEltype(::Type{Stateful{T,VS}}) where {T,VS} = IteratorEltype(T)
length(s::Stateful) = length(s.itr) - s.taken

"""
only(x)

Returns the one and only element of collection `x`, and throws an `ArgumentError` if the
collection has zero or multiple elements.
"""
@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
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only(x::Ref) = x[]
only(x::Number) = x
only(x::Char) = x
only(x::Tuple{}) = throw(ArgumentError("Tuple is empty, must contain exactly 1 element"))
only(x::Tuple{Any}) = x[1]
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only(x::Tuple) = throw(
ArgumentError("Tuple contains $(length(x)) elements, must contain exactly 1 element")
)
only(a::AbstractArray{<:Any, 0}) = @inbounds return a[]
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Is it not possible to construct a zero length AbstractArray{<:Any, 0} ?

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How do I test that? i.e. How does one create a zero-dim array of any length?

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julia> collect(1)
0-dimensional Array{Int64,0}:
1

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Idk how to create an empty one

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I do not know the answer to this (and Andy Ferris is the wise fellow who actually wrote this)

only(x::NamedTuple{<:Any, <:Tuple{Any}}) = first(x)
only(x::NamedTuple) = throw(
ArgumentError("NamedTuple contains $(length(x)) elements, must contain exactly 1 element")
)
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Not sure of style in Base, but this fits with other files as far as I can tell...

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I am tempted to say this should be a BoundsError or a DimensionMismatch ?

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@nickrobinson251 nickrobinson251 Aug 31, 2019 •
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  • yeah, I can see the case for BoundsError as the error type... but on the other hand the message doesn't really feel right
julia> x = [1, 2];

julia> throw(BoundsError(x, 2))
ERROR: BoundsError: attempt to access 2-element Array{Int64,1} at index [2]

  • Similarly, with DimensionMismatch, what is the "mismatch"?

  • ArgumentError feels better to me... but i'd certainly like to hear other opinions!

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fair


end
1 change: 1 addition & 0 deletions doc/src/base/iterators.md
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Expand Up @@ -15,4 +15,5 @@ Base.Iterators.flatten
Base.Iterators.partition
Base.Iterators.filter
Base.Iterators.reverse
Base.Iterators.only
```
34 changes: 32 additions & 2 deletions test/iterators.jl
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Expand Up @@ -183,7 +183,6 @@ end
@test Base.IteratorEltype(repeated(0, 5)) == Base.HasEltype()
@test Base.IteratorSize(zip(repeated(0), repeated(0))) == Base.IsInfinite()


# product
# -------

Expand Down Expand Up @@ -411,7 +410,6 @@ for n in [5,6]
[(1,1),(2,2),(3,3),(4,4),(5,5)]
end


@test join(map(x->string(x...), partition("Hello World!", 5)), "|") ==
"Hello| Worl|d!"

Expand Down Expand Up @@ -647,3 +645,35 @@ end
@test length(collect(d)) == 2
@test length(collect(d)) == 0
end

@testset "only" begin
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@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'
@test @inferred(only(Ref([1, 2]))) == [1, 2]

@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