Reading command-line arguments

The Hello World program always prints the same message. We can make it a little more useful by letting the person running it choose a name.

Replace the contents of hello.pop with Listing 1-2:

namespace Hello

function main(arguments: Array<String>)
    if Array.length(arguments) > 0 then
        local name = Array.get(arguments, 1)
        print("Hello")
        print(name)
    else
        print("Hello, stranger")
    end
end

Listing 1-2: Greeting the name supplied on the command line

Run the program and place an argument after --:

pop run hello.pop -- Ada

The program prints:

Hello
Ada

The -- separates options meant for the pop command from arguments meant for your program. Try running the program without an argument:

pop run hello.pop

This time it prints Hello, stranger.

Receiving arguments

Our first main used empty parentheses. The new declaration asks Pop to give the function an array of strings:

function main(arguments: Array<String>)

arguments is the name we chose for the value. Array<String> means an ordered collection whose elements are strings. Pop does not include the program’s own filename in this array, so the first item is the first value written after --.

Before reading an item, the program checks whether the array contains anything:

if Array.length(arguments) > 0 then

Array.length(arguments) gives the number of items. The if expression runs the following block only when that number is greater than zero. If it is not, the else block runs instead.

Inside the first branch, this line obtains the first item:

local name = Array.get(arguments, 1)

local creates a name that can be used within the current block. Pop arrays are numbered from 1, so index 1 is the first item. Array.get expects a valid position; our length check makes that true.

Finally, the program prints a greeting and the supplied name:

print("Hello")
print(name)

The Standard foundation in this release prints each call on its own line and does not yet provide general string formatting or concatenation. Command-line arguments are always strings, even when their text looks like a number. We will learn how arrays, conditions, and types work in detail in later chapters.