JuliaLang/julia

unary `+` acts differently for `Array` and `AbstractArray`

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#58,295 opened on May 1, 2025

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arraysdesigngood first issue

Description

(This issue is tangentially related to #58281.)

The return type of unary + with Bool argument is not the same for arrays and views:

julia> v = [true, false]; +v
2-element Vector{Int64}:
 1
 0

julia> w = view(v, :); +w
2-element view(::Vector{Bool}, :) with eltype Bool:
 1
 0

The reason is that unary + for AbstractArray is simply the identity map:

https://github.com/JuliaLang/julia/blob/748775b5689deaa4acac7929e0a31a80d1c564f4/base/abstractarraymath.jl#L285

Hence it misses the convention that + transforms Bool to Int.

Another consequence of this definition is that one doesn't know whether +v is identical to v for mutable v:

julia> v = [1, 2]; +v === v
false

julia> w = view(v, :); +w === w
true

In my opinion, it would be good to have a consistent answer. However, it may be to late for that.

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backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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1-3 hours
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active
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clear
Prerequisites
Julia programmingAbstractArray concepttype promotion rules
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Examine the definition of unary `+` for AbstractArray in base/abstractarraymath.jl at line 285. The current implementation is `+(x::AbstractArray) = x`, which does not convert Bool to Int. Consider modifying it to `+(x::AbstractArray) = map(+, x)` or similar to ensure consistent behavior, but be cautious about the identity property. Also check related issue #58281 for context. Discuss in the issue before implementing.
unary `+` acts differently for `Array` and `AbstractArray` · JuliaLang/julia#58295 | Good First Issue