Raku Books / Creating a Compiler in Raku / Control Flow

Blocks in the AST

OK, we can now return to the AST and let the nodes keep arrays of statements when the block in curly braces was seen.

Let us take a closer look at the loop from the example above:

loop n {
    say n;
    say 2 * n;
}

In the form of AST, this fragment takes the following shape:

AST::Loop.new(
    variable => AST::Variable.new(
        . . .
    ),
    statements => Array[ASTNode].new(
        AST::FunctionCall.new(
            . . .
        ),
        AST::FunctionCall.new(
            . . .
        )
    )
)

Again, the abstract nature of the abstract syntax tree is demonstrated here. Although we added the block rule in the grammar, there is no trace of it in the tree. The two say calls appeared immediately as the elements of the statements array attribute within the AST::Loop node.

What we need is to loop over the statements in the evaluator:

multi method eval-node(AST::Condition $node) {
    if $node.value.value {
        self.eval-node($_) for $node.statements;
    }
    else {
        self.eval-node($_) for $node.antistatements;
    }
}

multi method eval-node(AST::Loop $node) {
    while %!var{$node.variable.variable-name} > 0 {
        self.eval-node($_) for $node.statements;
        %!var{$node.variable.variable-name}--;
    }
}

That’s it! Introducing the { } blocks was a relatively simple task.

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