Why are bot functions called when the condition is not met?

user_1645737 0
Viewed 42

I have written this F# console application (that should call the run method only if the condition is met - if the directory exists):

open System

let dump (x: string) =
    Console.WriteLine(x)

let validateDirectory directory funToCallWhenOk =
    match (System.IO.Directory.Exists directory) with 
    | true -> funToCallWhenOk
    | false -> dump (sprintf "Problem with '%s'" directory)
    //if System.IO.Directory.Exists directory then
    //    funToCallWhenOk
    //else
    //    dump (sprintf "Problem with '%s'" directory)

let Watch directory =
    let run = 
        dump (sprintf "Now watching '%s'" directory)
        dump "Press 'q' to quit"

    validateDirectory directory run


[<EntryPoint>]
let main argv =

    Watch @"c:\temp\resize1"

    0

The currently specified directory does not exist, but the run function is called anyway, regardles if using match .. when or if then else. The console output:

enter image description here

Why is that?

f#
1 Answers

You haven't defined run as a function but as a unit value. You must define it as a function and call the function in validateDirectory:

let validateDirectory directory funToCallWhenOk =
    match (System.IO.Directory.Exists directory) with 
    | true -> funToCallWhenOk()
    | false -> dump (sprintf "Problem with '%s'" directory)
    //if System.IO.Directory.Exists directory then
    //    funToCallWhenOk
    //else
    //    dump (sprintf "Problem with '%s'" directory)

let Watch directory =
    let run() = 
        dump (sprintf "Now watching '%s'" directory)
        dump "Press 'q' to quit"

    validateDirectory directory run
user_6899230
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