3 ms·

Why not encode the starting offset into the type domain? Or at least distinguish between normal and unusual. Then the function signature can restrict to 1-offse

Why not encode the starting offset into the type domain? Or at least distinguish between normal and unusual. Then the function signature can restrict to 1-offset arrays if that is what it assumes internally.

DNF2 4y ago
That means disallowing indexing with integers, I presume? Since an integer can take the values 0 or 1 equally. And what about the other end of the array. Must every index be restricted by type to be located in the acceptable range?
markkitti 4y ago
If the function signature said `Array` rather than `AbstractArray`, then this code would have been fine. `Array` indexing starts at `1`. ``` julia> function f(A::Array) println(A[1:length(A)]) end f (generic function with 1 method) julia> f([1,2,3,4]) [1, 2, 3, 4] julia> f(OffsetArray(1:10, -1)) ERROR: MethodError: no method matching f(::OffsetVector{Int64, UnitRange{Int64}}) ``` You could prevent this problem using Julia's type system. The `AbstractArray` might have been too broad. Based on the chronology of the code that might not have been apparent. See other threads for details. Another way would be to treat `firstindex` as a trait and dispatch on that. ``` julia> f(A::AbstractArray) = f(A, Val(firstindex(A))) f (generic function with 1 method) julia> f(A::AbstractArray, firstindex::Val{1}) = println(A[1:length(A)]) f (generic function with 2 methods) julia> f(A::AbstractArray, firstindex::Val{T}) where T = error("Indexing for array does not start at 1") f (generic function with 3 methods) julia> f(A::AbstractArray, firstindex::Val{0}) = println("So you like 0-based indexing?") f (generic function with 4 methods) julia> f([1,2,3,4]) [1, 2, 3, 4] julia> using OffsetArrays julia> f(OffsetArray(1:10, 1)) ERROR: Indexing for array does not start at 1 Stacktrace: [1] error(s::String) @ Base .\error.jl:33 [2] f(A::OffsetVector{Int64, UnitRange{Int64}}, #unused#::Val{2}) @ Main .\REPL[5]:1 [3] f(A::OffsetVector{Int64, UnitRange{Int64}}) @ Main .\REPL[3]:1 [4] top-level scope @ REPL[9]:1 julia> f(OffsetArray(1:10, -1)) So you like 0-based indexing? ```
DNF2 4y ago
The starting offset is encoded in the type domain, btw, and accessible with the `firstindex` function. But you will still want to calculate indices at runtime, and then out-of-bounds errors will have to be caught at runtime anyway.

DNF2 4y ago

That means disallowing indexing with integers, I presume? Since an integer can take the values 0 or 1 equally. And what about the other end of the array. Must every index be restricted by type to be located in the acceptable range?

markkitti 4y ago

If the function signature said `Array` rather than `AbstractArray`, then this code would have been fine. `Array` indexing starts at `1`. ``` julia> function f(A::Array) println(A[1:length(A)]) end f (generic function with 1 method) julia> f([1,2,3,4]) [1, 2, 3, 4] julia> f(OffsetArray(1:10, -1)) ERROR: MethodError: no method matching f(::OffsetVector{Int64, UnitRange{Int64}}) ``` You could prevent this problem using Julia's type system. The `AbstractArray` might have been too broad. Based on the chronology of the code that might not have been apparent. See other threads for details. Another way would be to treat `firstindex` as a trait and dispatch on that. ``` julia> f(A::AbstractArray) = f(A, Val(firstindex(A))) f (generic function with 1 method) julia> f(A::AbstractArray, firstindex::Val{1}) = println(A[1:length(A)]) f (generic function with 2 methods) julia> f(A::AbstractArray, firstindex::Val{T}) where T = error("Indexing for array does not start at 1") f (generic function with 3 methods) julia> f(A::AbstractArray, firstindex::Val{0}) = println("So you like 0-based indexing?") f (generic function with 4 methods) julia> f([1,2,3,4]) [1, 2, 3, 4] julia> using OffsetArrays julia> f(OffsetArray(1:10, 1)) ERROR: Indexing for array does not start at 1 Stacktrace: [1] error(s::String) @ Base .\error.jl:33 [2] f(A::OffsetVector{Int64, UnitRange{Int64}}, #unused#::Val{2}) @ Main .\REPL[5]:1 [3] f(A::OffsetVector{Int64, UnitRange{Int64}}) @ Main .\REPL[3]:1 [4] top-level scope @ REPL[9]:1 julia> f(OffsetArray(1:10, -1)) So you like 0-based indexing? ```

DNF2 4y ago

The starting offset is encoded in the type domain, btw, and accessible with the `firstindex` function. But you will still want to calculate indices at runtime, and then out-of-bounds errors will have to be caught at runtime anyway.