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Make Iterators.peel return nothing when itr empty #39607

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Jun 5, 2021
Merged

Make Iterators.peel return nothing when itr empty #39607

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Make Iterators.peel return nothing when itr empty
cmcaine Feb 11, 2021
a2d2d5c
Merge branch 'master' into peelian-principles
vtjnash Jun 3, 2021
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1 change: 1 addition & 0 deletions NEWS.md
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Expand Up @@ -10,6 +10,7 @@ New language features
Language changes
----------------

* `Iterators.peel(itr)` now returns `nothing` when `itr` is empty rather than throwing a `BoundsError`. ([#39569])

Compiler/Runtime improvements
-----------------------------
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4 changes: 3 additions & 1 deletion base/iterators.jl
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Expand Up @@ -556,6 +556,8 @@ rest(itr) = itr

Returns the first element and an iterator over the remaining elements.

If the iterator is empty, returns `nothing` (like `iterate`).

# Examples
```jldoctest
julia> (a, rest) = Iterators.peel("abc");
Expand All @@ -571,7 +573,7 @@ julia> collect(rest)
"""
function peel(itr)
y = iterate(itr)
y === nothing && throw(BoundsError())
y === nothing && return y
val, s = y
val, rest(itr, s)
end
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8 changes: 8 additions & 0 deletions test/iterators.jl
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Expand Up @@ -848,3 +848,11 @@ end
@test cumprod(x + 1 for x in 1:3) == [2, 6, 24]
@test accumulate(+, (x^2 for x in 1:3); init=100) == [101, 105, 114]
end

@testset "Iterators.peel" begin
@test Iterators.peel([]) == nothing
@test Iterators.peel(1:10)[1] == 1
@test Iterators.peel(1:10)[2] |> collect == 2:10
@test Iterators.peel(x^2 for x in 2:4)[1] == 4
@test Iterators.peel(x^2 for x in 2:4)[2] |> collect == [9, 16]
end
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Grep didn't find any existing tests of Iterators.peel, so I added a few simple ones. I'm using the arrow syntax so the characters line up better, making it clearer when I'm calling with the same arguments.