New trait for collections that change sizes (issue #22)

README.md

@@ -11,6 +11,15 @@ serve as a staging ground for ideas before they merged into Base Julia. For this
 reason, no functionality is exported so that way if such functions are added
 and exported in a future Base Julia there will be no issues with the upgrade.
 
+## parent_type(x)
+
+Returns the parent array that `x` wraps.
+
+## can_change_size(x)
+
+Returns `true` if the size of `T` can change, in which case operations
+such as `pop!` and `popfirst!` are available for collections of type `T`.
+
 ## ismutable(x)
 
 A trait function for whether `x` is a mutable or immutable array. Used for

src/ArrayInterface.jl

@@ -22,6 +22,18 @@ parent_type(::Type{Symmetric{T,S}}) where {T,S} = S
 parent_type(::Type{<:LinearAlgebra.AbstractTriangular{T,S}}) where {T,S} = S
 parent_type(::Type{T}) where {T} = T
 
+"""
+    can_change_size(::Type{T}) -> Bool
+
+Returns `true` if the size of `T` can change, in which case operations
+such as `pop!` and `popfirst!` are available for collections of type `T`.
+"""
+can_change_size(x) = can_change_size(typeof(x))
+can_change_size(::Type{T}) where {T} = false
+can_change_size(::Type{<:Vector}) = true
+can_change_size(::Type{<:AbstractDict}) = true
+can_change_size(::Type{<:Base.ImmutableDict}) = false
+
 function ismutable end
 
 """

test/runtests.jl

@@ -182,9 +182,17 @@ end
 
     @test isnothing(ArrayInterface.known_last(1:4))
     @test isnothing(ArrayInterface.known_last(typeof(1:4)))
-    
+
     @test isnothing(ArrayInterface.known_step(typeof(1:0.2:4)))
     @test isone(ArrayInterface.known_step(1:4))
     @test isone(ArrayInterface.known_step(typeof(1:4)))
 end
 
+@testset "can_change_size" begin
+    @test ArrayInterface.can_change_size([1])
+    @test ArrayInterface.can_change_size(Vector{Int})
+    @test ArrayInterface.can_change_size(Dict{Symbol,Any})
+    @test !ArrayInterface.can_change_size(Base.ImmutableDict{Symbol,Int64})
+    @test !ArrayInterface.can_change_size(Tuple{})
+end
+