CODEWITHSUNDEEP
typesf#match
octopusgrabbus
octopusgrabbus

Reputation: 10695

F# How To Construct match and Satisfy Type

I have a function archive_lb_files, and I want to stub out all but one of the matches using a test function. I am not sure how to correct the stub matches, so I do not get

This expression was expected to have type string -> bool, but here has type unit

errors.

Here is the code snippet, and FileMaint.fm.archive_lb_file lb_dir archive_dir fn returns a bool.

(* Subroutine to process files we fetch from IC, and then need to be uploaded to Munis.  *)
let process_payment_processor_files spool_dir unload_dir paymt_dir report_date =
    use fHLog = FileMaint.fm.open_log_file_out (FileMaint.fm.generate_log_file ())
    // We'll flesh this in when Stratus is installed, and we know the directory names. cmn 5/26/2017
    ignore

let test_print fn =
    printfn "%A" fn

let archive_lb_files lb_dir archive_dir report_date =
    use fHLog = FileMaint.fm.open_log_file_out(FileMaint.fm.generate_log_file () )
    let lb_fnam_template = "*_lockbox.txt"
    let dir_list = Directory.GetFiles(lb_dir, lb_fnam_template)

    for pfn in dir_list do
        let fn = Path.GetFileName(pfn)
        match fn with
        | "cb_lockbox.txt" -> FileMaint.fm.archive_lb_file lb_dir archive_dir fn
        | "ics_lockbox.txt" -> (test_print fn) 
        | "ic_lockbox.txt" -> test_print fn
        | _ -> test_print "does not exist"

In answer to the answers and comments, how does archive_lb_file return a function?

(* Moves a file to a unique file name but not including a central directory in between a source and destination. *)
let archive_lb_file lb_path archive_path lockbox_file_name copy_only =
    use fH = new StreamWriter(base_log_file, true)
    let rc =
        try
            if (0 = String.Compare(copy_only, "copy_only")) then
                File.Move((lb_path + lockbox_file_name), archive_path) |> ignore
                fH.WriteLine(generate_time_stamp + ": " + lb_path + lockbox_file_name + " moved to " + archive_path)
            else
                File.Copy((lb_path + lockbox_file_name), archive_path) |> ignore
                fH.WriteLine(generate_time_stamp + ": " + lb_path + lockbox_file_name + " copied to " + archive_path)
            true 
        with ex -> 
            fH.WriteLine(generate_time_stamp + ": " + "An error occrred trying to move or copy " + 
                         lb_path + lockbox_file_name + " to " + archive_path)
            printfn "%A" ex
            false
    rc

Upvotes: 1

Views: 193

Answers (2)

Tomislav Markovski
Tomislav Markovski

Reputation: 12346

All patterns in your match statement must return the same type. Your test_print returns unit, but your FileMaint.fm.archive_lb_file returns a function string -> bool. If you change your method to

let test_print fn =
    printfn "%A" fn
    true

will fix the error you're getting, even though this looks weird.

Edit: Since you posted the missing function, the error you're seeing is caused by not passing the copy_only argument in your call.

    match fn with
    | "cb_lockbox.txt" -> FileMaint.fm.archive_lb_file lb_dir archive_dir fn true
    | "ics_lockbox.txt" -> (test_print fn) 
    | "ic_lockbox.txt" -> test_print fn
    | _ -> test_print "does not exist"

Adding true will call the function. The reason why you're not getting an error with the call is because of currying functions.

https://fsharpforfunandprofit.com/posts/currying/

Upvotes: 3

Fyodor Soikin
Fyodor Soikin

Reputation: 80765

All branches of the match expression must have the same type. Judging by the error message you posted, the first branch of your match expression has type string -> bool, which means that your call to FileMaint.fm.archive_lb_file is probably missing one argument: it expects another string, and after it gets one, it promises to return a bool.

From this, the solution will have two parts:

  1. Pass the missing argument to FileMaint.fm.archive_lb_file (I can't help you any more with this, because I don't see the definition of that function and therefore don't know what the missing argument is supposed to be).
  2. Make the test_print function return the same type - i.e. bool. To achieve that, just add a return value:

.

 let test_print fn = 
    printfn "%A" fn
    true

That said, I suspect that your program actually has a logical error: FileMaint.fm.archive_lb_file returns a bool, but you're ignoring it. Should you? If the return value doesn't matter to you, should the function return it in the first place?

Upvotes: 3

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