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Performing a "while" on sequence contents
f#web

Hi -

I'm brand new to F# so this is probably a dumb question, but here goes...

In my brief time with F# it seems like there's all sorts of ways to do things with everything in a sequence. In fact the aggregate operators seem to operate entirely on that basis. There's also a way to truncate a sequence so you could force these to act on the first n elements for a predetermined n. What there doesn't seem to be is a way to limit aggregates based on conditions of the elements of the sequence - at least not so far as I can see. Every time I think I've almost got a way to do it, something seems to keep me from what I want.

For instance, my latest guess was something like the following:

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seq {
    for i in my_seq do
        if myBooleanFunction i then yield i
        else break
    };;

hoping that there would be a break statement in F# but of course, when I typed it in, I found that "break" is "reserved for future use" so once again, I come tantalizingly close but am again denied.

I keep looking for a foldWhile or foldUntil or TruncateAtCondition or some such sequence function, but there doesn't seem to be any. The Seq.generated function seems like it may be right but I can't figure out how it works nor find any examples. If I read it right, it takes a parameterless function which returns a boolean and a sequence and produces another sequence. The description reads "Generate a sequence by repeatedly calling the given function and concatenating the results (similar to a while loop)". The only function I see is the paremeterless boolean so I assume that's the one that's "repeatedly called". When they say "concatenate the results" the only "result" that I can see is the result of calling the boolean function which doesn't seem right - concatenating the results of calling the paremeterless boolean function - but how else can it be interpreted? Or is the boolean like the condition of the while loop? if so, how does it know when to quit with no parameters? And are elements just peeled from the sequence argument or what? I have no idea. The fact that they say it's "similar to a while loop" is the only clue I have that maybe this is the right function.

I think I could probably jury rig it using fold since it will end a seq when a "None" is returned but that seems like a really contrived way to do this. For that matter, just what would the implementation of fold look like? It seems to be doing something very similar to what I want to do except it's test is whether the result is "None" rather than a general test. How can I go through the elements of a sequence and produce a new sequence that ends when a while condition is not met?

Oh, I guess I should mention that I want to only go through the necessary elements one time. There are ways I can see of turning it into an array after counting the necessary number of elements but I want something that's going to basically do what I'd do - go through the sequence one element at a time doing some sort of aggregation/iteration until I find an element that doesn't fit my criteria.

I'm sure there's a simple solution to this but I've spent all day looking through web sites and "Expert F#" without recognizing it - undoubtedly a failing on my part, but could somebody steer me in the right direction?

Thanks!

Darrell Plank

.

What there doesn't seem to be is a way to limit aggregates based on conditions of the elements of the sequence - at least not so far as I can see. Every time I think I've almost got a way to do it, something seems to keep me from what I want.

Wow, this took longer to figure out than expected! Apparently, there is no "Seq.tl" function. Hrumpf.

Anyway, here's a simple, non-recursive solution:

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let truncateIf p (s:seq<'a>) =
    seq { let e = s.GetEnumerator()
          while e.MoveNext() && not (p e.Current) do
            yield e.Current
        }
By on 6/5/2008 7:05 PM ()

Well, this is obviously what I wanted but I can't seem to type it into to F# interactive in VS 2008 with the latest version of F#. I can type it into an FS file and compile the file with no errors but if I type in:

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let truncateIf p (s:seq<'a>) =
    seq { let e = s.GetEnumerator()
          while e.MoveNext() && not (p e.Current) do

to the interactive window, then as soon as I hit return it tells me that the "while" is a syntax error. Sorry to pester after the great answer you gave, but this is very confusing and keeps me from trying this out in the interactive window.

Thanks for any ideas.

Darrell

By on 6/5/2008 11:17 PM ()

[...] then as soon as I hit return it tells me that the "while" is a syntax error.

There are two styles of F#: the "light" style, and the OCaml-compatible style. Mostly everyone uses the light style (by typing "#light" at the top of the source, or via command line switches) in which white-space is significant and several language constructs are optional, but a few people prefer the control of OCaml style. Best choice is to be compatible with both, but I still often have trouble with the latter style.

Here it is again without assuming #light:

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> let truncateIf p (s:seq<'a>) =
-     seq { let e = s.GetEnumerator() in
-           while e.MoveNext() && not (p e.Current) do
-               yield e.Current
-           done }
- ;;

val truncateIf : ('a -> bool) -> seq<'a> -> seq<'a>

Now that I think of it (OK, so after seeing the name in someone else's post), "takeWhile" sounds more natural than "truncateIf":

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> let takeWhile p (s:seq<'a>) =
-     seq { let e = s.GetEnumerator() in
-           while e.MoveNext() && p e.Current do
-               yield e.Current
-           done
-         }
- ;;

val takeWhile : ('a -> bool) -> seq<'a> -> seq<'a>
By on 6/6/2008 6:20 PM ()

Hi,

Don't forget to use light mode!
Type #light in the interactive mode, it should work.

By on 6/6/2008 3:40 PM ()

Oh - of course - the seq is really a .NET enumerable so you're just using the IEnumerable member function. I guess that's what comes of me being a greenhorn. I'll get the hang of this eventually. Thanks again!

Darrell

By on 6/5/2008 10:08 PM ()

Thanks so much! That's perfect! In fact, in an earlier draft I had asked about how to get the IEnumerator from the sequence's IEnumerable which is apparently what GetEnumerator does. It's not listed in the online documentation under the Sequence members so it's no wonder I didn't find it. Anyway, I'm much obliged for the help - exactly what I was looking for!

Darrell

By on 6/5/2008 9:56 PM ()

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