structural comparison with List and Array as Sequences

[Robert Kuzelj]

Hi,

can anybody explain what the differences are between an array based sequence and a list based sequence and why the structural comparison works in one but not in the other?

let listCompared = [[1.0; 0.0; 0.0]; [0.0; 1.0; 0.0]] = [[1.0; 0.0; 0.0]; [0.0; 1.0; 0.0]]

let arrayCompared = [|[|1.0; 0.0; 0.0|]; [|0.0; 1.0; 0.0|]|] = [|[|1.0; 0.0; 0.0|]; [|0.0; 1.0; 0.0|]|]

let fromList xs = xs |> List.map List.toSeq |> List.toSeq

let fromArray xs = xs |> Array.map Array.toSeq |> Array.toSeq

let listSeqCompared = (fromList [[1.0; 0.0; 0.0]; [0.0; 1.0; 0.0]]) = (fromList [[1.0; 0.0; 0.0]; [0.0; 1.0; 0.0]])

let arraySeqCompared = (fromArray [|[|1.0; 0.0; 0.0|]; [|0.0; 1.0; 0.0|]|]) = (fromArray [|[|1.0; 0.0; 0.0|]; [|0.0; 1.0; 0.0|]|])

let lazyCompared = seq{yield seq{yield 1.0; yield 0.0; yield 0.0}; yield seq{yield 0.0; yield 1.0; yield 0.0}} = seq{yield seq{yield 1.0; yield 0.0; yield 0.0}; yield seq{yield 0.0; yield 1.0; yield 0.0}}

>>>

val listCompared : bool = true

val arrayCompared : bool = true

val fromList : xs:'a list list -> seq<seq<'a>>

val fromArray : xs:'a [] [] -> seq<seq<'a>>

val listSeqCompared : bool = true

val arraySeqCompared : bool = false

val lazyCompared : bool = false

<<

thx

Robert

[Novel Spin]

I did some testing, and it looks like the list gets casted to a seq, while the array is used to create a seq that can't be casted back to an array.  So I'm guessing when you compare two seqs converted from lists, they use the list comparison, but seqs created on their own or converted from arrays use reference comparison.  This is an inconsistency, and I think it should be made consistent.  If you want fromArray to behave like fromList, you can define it like this:

let fromArray (xs : 'a [] []) = xs |> Array.map seq<'a> |> seq<seq<'a>>