Raku Books / Creating a Compiler in Raku / Control Flow

A real-life test

If you run the loop, you’ll see an error very soon:

5
10
4
Cannot shift from an empty Array[ASTNode]
  in method value at /Users/ash/lingua/LinguaAST.pm    
  (LinguaAST) line 112

The first three lines display the expected beginning of the result, but then everything suddenly stopped. The error message helps to understand that that problem is that we were using shift to compute the expressions. Within a loop, expressions can be reused, so we should not change them. Let us change all shifts to loops. There are two such places in our code. The first one in the AST::MathOperations class:

method value() {
    my $result = @.operands[0].value;

    for 1 .. @.operands.elems - 1 -> $i {
        $result = operation(@.operators[$i - 1], 
                  $result, @.operands[$i].value);
    }

    return $result;
}

The second in one of the multi-methods of the evaluator:

multi method eval-node(AST::HashAssignment $node) {
    %!var{$node.variable-name} = Hash.new;
    for 0 .. $node.keys.elems - 1 -> $i {
        %!var{$node.variable-name}.{$node.keys[$i]} =
            $node.values[$i].value;
    }
}

The test code is working now. Let us fix the behaviour in the test with similar instructions:

my n = 5;
loop n {
    my n2 = n * n;
    say "n = $n, n2 = $n2";
}
## n = 5, n2 = 25
## n = 4, n2 = 16
## n = 3, n2 = 9
## n = 2, n2 = 4
## n = 1, n2 = 1

Don’t forget to test the if and else blocks, too. The program should be able to execute all the statements from each.

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