Re: [Ocaml-lib-devel] Proposal for new list functions

[skaller <sk...@us...>]

On Wed, 2005-08-10 at 00:28 +0900, Janne Hellsten wrote:

> First one is called fold_left_while.  It works the same way as
> fold_left except that it breaks the loop as soon as a "while
> predicate" returns false.=20

Sounds good, but must not be implemented as you=20
have written it. The real implementation must use an
inner exception to stop recursing, we do not want
to uselessly scan the rest of the list after the
termination condition is found.

> The second function is called split. =20

List.split already exists, it splits a list of pairs
into two lists, perhaps call it "part"? (short for
partition .. but that isn't quite accurate).

******************

Consider generalise both functions so that the predicate=20
takes the whole of the remaining list as an argument, not just the next
element. It is easy to write a predicate to only consider
the first element, for example

fun acc (h::_) -> ...

isn't much harder to write than

fun acc h ->

but now we can pattern match to an arbitrary depth in the tail.

Trivial example of tail match in fold_left_while:
a list of characters finding the real content of the C++
code

here / some // C++ code

requires matching on //, which is two characters

The problem is with "part" (your "split") it no longer makes
sense to drop "the separator".



--=20
John Skaller <skaller at users dot sourceforge dot net>