CODEWITHSUNDEEP
f#
anon
anon

Reputation:

F# incorrect type and another incorrect type in the function call

Not sure what's off about this code. I want to use this as the main function to print a string or return not enough arguments. What's incorrect about this VS15 isn't very helpful in this instance.

[<EntryPoint>]
let main argv =
    if (List.length argv) >= 1 then
        printfn "Hello %s" argv.[0]; 0
    else
        printfn "Not enough arguments"; 1

main ["Test"]

Upvotes: 0

Views: 113

Answers (1)

Fyodor Soikin
Fyodor Soikin

Reputation: 80915

Even though you didn't specify what error you get and where it appears (please always do that when asking questions), I can see what's wrong with your code: you're treating argv as if it was a List, but a .NET program entry point must accept an argument of type Array - specifically, array of strings - string[].

If you switch out List.length for Array.length, the function will compile.

[<EntryPoint>]
let main argv =
  if (Array.length argv) >= 1 then
      printfn "Hello %s" argv.[0]; 0
  else
      printfn "Not enough arguments"; 1

Now, if you want to call this function, you would want to supply an argument that is an array, not List. In F#, the brackets are used to denote a list. If you want to denote an array, you need to use bracket-pipes instead:

main [| "Test |]

EDIT in response to comment:

Normally you wouldn't need to "call" the entry point function explicitly. Entry point is the "start" of the program, there are no other functions calling it. This is why the entry point function must be the last function in the last file of the program. If you do place any code after the entry point, the compiler will give you an error.
The way you wrote the call main ["Test"], I assumed you just wanted to execute this call in F# interactive, which is a popular way of verifying your code without building and running it. Once you are ready to compile, you should remove this call.

Finally, I'd like to point out that you're actually accessing the array twice: first to check its length, then to fetch its first item. You can do both in one step using pattern matching:

[<EntryPoint>]
let main argv =
  match argv with
  | [|name|] -> printfn "Hello %s" name; 0
  | _ -> printfn "Not enough or too many arguments"; 1

And look: this way, the program actually became slightly more valid. If you look closer, you'll notice that your original program accepts any number of arguments, but only actually uses the first one. This is a bit "unclean", so to say. The above version using pattern matching does one better: it will take exactly as many arguments as is needed for its function, or print an error message otherwise.

Upvotes: 6

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