43. Variable $y not declared. Did you mean $x?

One of the additional user-friendly features of Perl 6 is its great error reporting. It is not a mandatory part of the language grammar or semantics, but it really helps a developer to fix the errors in the code.

Run the following program:

$ perl6 -e'my $var1; say $var2'

It leads to a compile-time error:

===SORRY!=== Error while compiling -e
Variable '$var2' is not declared. Did you mean '$var1'?
at -e:1
------> my $var1; say ⏏$var2

Not only Perl 6 says that there is no such variable but it also suggests a correct candidate.

The smartness does not stop here. Rakudo Perl 6 offers you more variants if it sees other options:

$ perl6 -e'my $var1; my $var2; say $var3'
===SORRY!=== Error while compiling -e
Variable '$var3' is not declared. Did you mean any of these?
    $var2
    $var1

at -e:1
------> my $var1; my $var2; say ⏏$var3

Let us see how Perl 6 gets an idea of how to correct our typo. The exception is easy to spot:

my class X::Undeclared does X::Comp {
    has $.what = 'Variable';
    has $.symbol;
    has @.suggestions;
    method message() {
        my $message := "$.what '$.symbol' is not declared";
        if +@.suggestions == 1 {
            $message := "$message. Did you mean '@.suggestions[0]'?";
        } elsif +@.suggestions > 1 {
            $message := "$message. Did you mean any of these?\n { @.suggestions.join("\n ") }\n";
        }
        $message;
    }
}

First of all, we see both cases there: when there are only one or more suggestions. They come from the @.suggestions attribute. The error $message is formed accordingly. By the way, notice how a code block is interpolated in the double-quoted string:

" . . . { @.suggestions.join("\n ") }\n";

The X::Undeclared class is only a part of the whole family of exceptions:

my class X::Attribute::Undeclared is X::Undeclared
my class X::Attribute::Regex is X::Undeclared 
my class X::Undeclared::Symbols does X::Comp

For the unknown variable, the exception is thrown from the check_variable method in the Grammar. I will not copy it here as the method is quite big and will only show the relevant lines so that you can see the picture:

method check_variable($var) {
    my $varast := $var.ast;
    . . .
    my $name := $varast.name;
    . . .
    my @suggestions := $*W.suggest_lexicals($name);
    . . .
    $*W.throw($var, ['X', 'Undeclared'], 
              symbol => $name,
              suggestions => @suggestions,
              precursor => '1');
    . . .
    self
}

So, an exception is thrown for the missing symbol $name with one ore more @suggestions.

Now move on to the suggest_lexicals method that finds similar names. The $*W variable is Rakudo Perl 6’s World object, so search for it in the src/Perl6/World.nqp file:

method suggest_lexicals($name) {
    my @suggestions;
    my @candidates := [[], [], []];
    my &inner-evaluator := make_levenshtein_evaluator($name, @candidates);
    . . .
    levenshtein_candidate_heuristic(@candidates, @suggestions);
    return @suggestions;
}

Once again, only the most significant code is shown. As you might guess, Rakudo is using the Levenshtein distance to find the closest matches. Roughly speaking, it counts how many letters you need to replace in a word A to get another word B. The bigger the distance, the less similar are the words.

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