Raku Books / Creating a Compiler in Raku / A Better Translator

Using variables

In our expressions, only numbers can be used. It would be much handier if we can use variables there too. To achieve that, a small change to the grammar is needed. In terms of the grammar rules, to have a variable in an expression means to allow variable names in it:

rule value {
    | <number>
    | <variable-name>
    | '(' <expression> ')'
}

There is no made attribute associated with the variable-name token, so we have to make some simple work to access the storage:

method value($/) {
    if $<number> {
        $/.make($<number>.made);
    }
    elsif $<variable-name> {
        $/.make(%var{$<variable-name>});
    }
    else {
        $/.make($<expression>.made);
    }
}

The value method consists of three branches, whose task is to find an appropriate data to be passed further. In the case of variables, a hash lookup is done.

Having all that, the translator can now process the following program and compute the length of Earth’s equator (assuming we already assigned the value to pi earlier):

my r;
r = 6371; # km

my d;
d = 2 * pi * r;
say d;

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