Andy Elvey wrote:
> So, I may look at doing what I would call "lpred" and lsucc" - the
> predecessor and successor of a list element. I'm somewhat surprised
> that (from what I can tell) Haskell doesn't seem to have those two
> functions for a list. I may be wrong....
Again, lists don't work that way; a list in Haskell is a single
immutable object, you can pull items from it using (!!), head, etc.,
but you can't have a pointer into the "middle" of a list. You can
have a sublist (for example, `tail ["foo", "bar", "baz"]' = `["bar",
"baz"]') --- but you can't get from there to the "foo", as it isn't
part of that new list. (This despite the fact that what `tail' gives
you is going to be shared in actual storage with the original list.
You don't have a backpointer into that original list to follow.) This
is actually a feature of the Haskell model: it's usually easier to
reason about these kinds of structures, where you can treat any
operation as producing a new object and the Haskell compiler takes
care of optimizations for you (shared representations, possibly
recognizing that it can quietly do an update-in-place because you
can't possibly reach the old value, etc.).

You may want to take a look at the Seq type defined in the Data.Seq
module, though; you have a `view' (what you can think of as a cursor)
on a given Seq which can be moved back and forth.

--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] system administrator [openafs,heimdal,too many hats] electrical and computer engineering, carnegie mellon university KF8NH



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On Sat, Feb 06, 2010 at 01:45:58AM -0500, Brandon S. Allbery KF8NH wrote:
> Andy Elvey wrote:
> >So, I may look at doing what I would call "lpred" and lsucc" - the
> >predecessor and successor of a list element. I'm somewhat
> >surprised that (from what I can tell) Haskell doesn't seem to have
> >those two functions for a list. I may be wrong....
>
> Again, lists don't work that way; a list in Haskell is a single
> immutable object, you can pull items from it using (!!), head, etc.,
> but you can't have a pointer into the "middle" of a list. You can
> have a sublist (for example, `tail ["foo", "bar", "baz"]' = `["bar",
> "baz"]') --- but you can't get from there to the "foo", as it isn't
> part of that new list. (This despite the fact that what `tail'
> gives you is going to be shared in actual storage with the original
> list. You don't have a backpointer into that original list to
> follow.)

Although we can't have a pointer to the middle of a list [a], we
can have another data struture that behaves sort of as if we had
that pointer: a zipper. For example,

To understand a list zipper you could think of it as something like

struct zipper {
list left, right;
}

So we track our "position" by tracking what's on the left and
what's on the right. By using the API of the link above you
could have something similar to what you wanted:

lpred :: Zipper a -> a
lpred = focus . left

lsucc :: Zipper a -> a
lsucc = focus . right

--
Felipe.
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Felipe Lessa wrote:

(snip )
>
> Although we can't have a pointer to the middle of a list [a], we
> can have another data struture that behaves sort of as if we had
> that pointer: a zipper. For example,
>
>

Code:

Content visible to registered users only.

>
> To understand a list zipper you could think of it as something like
>
> struct zipper {
> list left, right;
> }
>
> So we track our "position" by tracking what's on the left and
> what's on the right. By using the API of the link above you
> could have something similar to what you wanted:
>
> lpred :: Zipper a -> a
> lpred = focus . left
>
> lsucc :: Zipper a -> a
> lsucc = focus . right
>
> --
> Felipe.
>
Hi Felipe -
Thanks very much for that! That's very handy to know. I think that
I'll base my lpred and lsucc on this approach.
Bye for now -
- Andy

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