06. Validation and Constraints

This is where the Slangify approach becomes more valuable than plain prompting. Constraints live in the grammar and the Booking class — not in the prompt.

Tightening grammar tokens

Restrict party to one or two digits (no absurdly large parties):

token party { <[1..9]> \d? }   # 1–99

Restrict time to valid 24-hour ranges:

token time { <[0..2]>\d ':' <[0..5]>\d }

Any LLM output that violates these rules causes the grammar to fail — the error surfaces before the result reaches your application.

Constraints in the Booking class

Use Raku's type system and TWEAK for business-rule validation:

class Booking does Actionable {
    has Str $.name;
    has Int $.party where 1 <= * <= 20;   # range constraint
    has Str $.time;
    has Str $.restaurant;
    has Str $.date;
}

If the LLM produces party: 0 or party: 99, object construction fails with a clear type error rather than silently accepting bad data.

Seeing failure in action

# LLM normalized "a crowd" to: party many
my $m = Slangify::Tutorial::Grammar.parse(
    'name "X" party many time 19:00 restaurant "Y" date today'
);
say $m.so;   # False — grammar rejects non-numeric party

Readers love seeing failure and correction. The grammar gives you both: a clear rejection plus a single place to tighten the rule.

Preferred types

integers

Use \d+ in the token and Int in the class attribute.

enums

Use alternation in the token: low | medium | high.

dates

Use a specific pattern token rather than bare \S+ if you need validation.

free strings

Keep \S+ or < <-["]>+ > but add Str typing so at least the attribute is defined.