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

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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.

Contributor guide

Research direction
Investigate the unary + definition for AbstractArray and propose a fix to ensure consistent behavior between Array and AbstractArray, possibly by transforming Bool to Int.
Tech stack
julia
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Julia
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50