Course of Raku / Essentials / Typed variables

Allomorphs

Consider the following program. Before running it, can you tell which input values would break it and at which line?

my $input = prompt 'Enter something: ';
my Int $i = $input;
my Str $s = $input;

say $i;
say $s;

There are three scalar variables created here. Two of them, $i and $s are typed variables. That means that $i can only store integers and $s can only store strings.

The return type of prompt depends on the characters we typed. If the input string can represent an integer number, the result will be of the IntStr type, which is both Int and Str, and thus can be assigned to either an Int or a Str variable. So, if you enter, say, 1234, the program does not break.

$ raku allomorphs.raku
Enter something: 1234
1234
1234

The IntStr type is an example of the so-called allomorph — a data type that combines two other types. Here are a few more examples.

If you enter a string that cannot be an integer number, the program breaks at the moment we assign $input to $i:

$ raku allomorphs.raku
Enter something: Hello, World!
Type check failed in assignment to $i; expected Int but got Str ("Hello, World!")
  in block <unit> at allomorphs.raku line 2

Notice that you get an error even if the input string can be cast to a number, but not an integer. As it is not possible to store a floating-point or a rational number in an integer container, Raku will emit an exception:

$ raku allomorphs.raku
Enter something: 3.14
Type check failed in assignment to $i; expected Int but got RatStr (RatStr.new(3.14, "3....)
  in block <unit> at allomorphs.raku line 2

The second assignment, $s = $input, will never break as a string can take any input.

🦋 You can find the source code of this program in the file allomorphs.raku.