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

Andy Elvey wrote: 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] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH