Back to Questions
1.0k
views2
comments
Odd behavior of Seq.length
f#

Hello.

I am working on a F#-program, the relevant parts (for this question) is shown:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37

type NodeDatum ( node_id : Guid, v: float, t : int64) = 
    member r.node_id = node_id
    member r.v = v
    member r.t = t

// a few lines skipped ......

// Create a command to perform the SQL query
    use aims_db = new System.Data.SqlClient.SqlConnection(Config.aims_connection_string)
    use node_query = aims_db.CreateCommand();
    node_query.CommandTimeout <- 300

    let commStr = "SELECT [node_guid]," + quantity_retr + "[resolution_ticks] from [aims].[dbo].[NodeStatistics]"
                 + " WHERE resolution = " + period.to_int16().ToString() +  " ORDER BY [resolution_ticks]" 
    node_query.CommandText <- commStr


    // Run the command and read results
    aims_db.Open()
    use reader = node_query.ExecuteReader()

    let nodestats = seq<NodeDatum>  {
        while reader.Read() do
            yield  NodeDatum( (reader.GetGuid 0), (reader.GetDouble 1), (reader.GetInt64 2) )
    } 

    // start datetime in ticks

    let nd_dat_0 = Seq.nth 0 nodestats
    let t0       = nd_dat_0.t


    let antdat = Seq.length nodestats
    printfn "# of data elements andat = %d" antdat

    let ndat = Seq.length nodestats
    printfn "# of data elements ndat = %d" ndat

which produce this output:

1
2

# of data elements andat = 923
# of data elements ndat = 0

i.e. after the first Seq.length call, the seq nodestat is emptied.

This seems odd, especially for a language that is supposed to be devoid of side effects.

Is there anyone out there who can tell me what happens ?

;.) Cy

.

Because Seq is doing a lazy read, the second time you call .length, there are no more DB records to process.

It's probably not a good idea to use seq/yield like this anyway (at least w/a larger program in an shared environment) , as your DB connection will remain open for the duration if you don't consume the whole db and the seq doesn't go out of scope. (Of course, if you really wan't a new Seq in this case, replace nodestats with notestats() so you get a fresh seq when the fn is eval'd.)

By on 5/26/2011 11:36 AM ()

Language itself does not guarantee an absence of side effects, especially here when you use imperative library. You should cache data somehow. Either via Seq.cache, or Seq.toList, or something similiar.

By on 5/26/2011 6:08 AM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper