`EntryPoint`

An F# program must start somewhere—we can use the EntryPoint type annotation for this purpose. EntryPoint designates the initial method of a program.

With this attribute, a method is executed at program initiation. And any arguments to the command-line are passed as a string array to the program.

This program specifies a main function, and uses the EntryPoint type annotation upon it. When we run the program with "dotnet run" main is executed.

Step 1 With a for-do loop, we iterate over the arguments passed after the "dotnet run" command.

Step 2 Inside a string interpolation expression, we invoke the formatArgument function on each string argument.

Step 3 In formatArgument, we call 2 separate functions in a pipeline on the argument.

Step 4 This function (spaceOut) spaces out the argument string with some string logic involving a char array.

Step 5 We write the length of the arguments array (argv) which is 2 in this case (the program's file name is not included in the array).

Step 6 For the main method, we are supposed to return an integer value to indicate success—0 is acceptable.

let uppercase (argument : string) = argument.ToUpper()
let spaceOut (argument : string) =
    // Step 4: place a space in between each character.
    let mutable chars = Array.create (argument.Length * 2) ' '
    for i = 0 to argument.Length - 1 do
        chars[i * 2] <- argument[i]
    new string(chars)

let formatArgument (argument : string) =
    // Step 3: call pipelined functions to format an argument.
    argument
    |> uppercase
    |> spaceOut

[<EntryPoint>]
let main argv = 
    // Step 1: loop over arguments to program.
    for arg in argv do
        // Step 2: format each argument with a function.
        printfn $"ARG IS {formatArgument(arg)}"

    // Step 5: write the Length.
    printfn $"LENGTH: {argv.Length}"

    // Step 6: return 0.
    0dotnet run argument1 argument2
ARG IS A R G U M E N T 1
ARG IS A R G U M E N T 2
LENGTH: 2

Indeterminate type error

Consider this F# compile error—it occurs when a program has no EntryPoint. We can correct the problem with an EntryPoint type annotation.

.../Program.fs(23,24): error FS0072:
Lookup on object of indeterminate type based on information prior to this program point.
A type annotation may be needed prior to this program point to constrain the type of the object.
This may allow the lookup to be resolved.

Many F# programs that are used in the real world will be command-line applications. They receive arguments, and print out values (or write files).