tail in the DList instance is not efficient

[andreasabel]

tail :: DList a -> DList a is essentially DList.fromList . Data.List.tail . DList.toList in dlist-1.0, which is losing efficiency over (Data.List.tail .). The latter cannot be implemented outside of dlist though, due to data abstraction.

See 8c20780#r55694269 and spl/dlist#98.

[spl]

A couple of things:

• As I mentioned on Restore tail :: DList a -> DList a? spl/dlist#98, I changed tail to the current one since the previous tail was inefficient, and I think the current one is slightly better than toList . tail. The listlike implementation of tail is the same as the previous dlist implementation.
• You could use the UnsafeDList approach, that @andreasabel mentioned, in listlike. I don't like it myself, but the option is there and perfectly acceptable.

[andreasabel]

@spl wrote:

• You could use the UnsafeDList approach, that @andreasabel mentioned, in listlike. I don't like it myself, but the option is there and perfectly acceptable.

I don't think this is an option here because the constructor for DList is not exported ("data abstraction").

[spl]

I don't think this is an option here because the constructor for DList is not exported ("data abstraction").

It's exported from Data.DList.Internal.

[andreasabel]

@spl: I tried import Data.DList.Internal (DList(..)) but this gives me

src/Data/ListLike/DList.hs:11:1: error:
    Could not load module ‘Data.DList.Internal’
    it is a hidden module in the package ‘dlist-1.0’

That's to be expected when looking at the docs at https://hackage.haskell.org/package/dlist-1.0:

Modules
Data
Data.DList
Data.DList.DNonEmpty
Data.DList.Unsafe

(That's also the convention: Internal modules are not exported.)

[spl]

@andreasabel Sorry, I misremembered. Use Data.DList.Unsafe. It's been over a year since I did it. And I surprised myself to find that I actually have a test for that.

[spl]

@andreasabel Also, there's documentation on this under the heading of Abstraction at the link you provided: https://hackage.haskell.org/package/dlist-1.0.

[andreasabel]

@andreasabel Sorry, I misremembered. Use Data.DList.Unsafe. It's been over a year since I did it. And I surprised myself to find that I actually have a test for that.

Thanks for the guidance. I'll use this backdoor, then.

@andreasabel Also, there's documentation on this under the heading of Abstraction at the link you provided: https://hackage.haskell.org/package/dlist-1.0.

Thanks, I see this. (Actually, I do not want to do anything unsafe, the (Data.List.tail .) function does not break the invariant mentioned there.)

There is still a problem with accessing the documentation of Data.List.Unsafe, see spl/dlist#100.