Address #22

Author: Tokazama

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
 
+"""
+    is_dynamic(::Type{T}) -> Bool
+
+Returns `true` if the size of `T` is dynamic. If `T` is dynamic then operations
+such as `pop!` and `popfirst!` are available for collections of type `T`.
+"""
+is_dynamic(x) = is_dynamic(typeof(x))
+is_dynamic(::Type{T}) where {T} = false
+is_dynamic(::Type{<:Vector}) = true
+is_dynamic(::Type{<:AbstractDict}) = true
+is_dynamic(::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 "is_dynamic" begin
+    @test ArrayInterface.is_dynamic([1])
+    @test ArrayInterface.is_dynamic(Vector{Int})
+    @test ArrayInterface.is_dynamic(Dict{Symbol,Any})
+    @test !ArrayInterface.is_dynamic(Base.ImmutableDict{Symbol,Int64})
+    @test !ArrayInterface.is_dynamic(Tuple{})
+end
+