About RSL

RSL expresses a rating algorithm as typed inputs, constraints, tables, calculations, rounding rules, and source citations. rslc checks that implementation and packages it as a versioned rater bundle. Relativities runs the bundle locally. For comparative rating, the same bundle supports repeated pricing, sensitivity analysis, and inspection of the calculations behind a premium. Start with Compare carriers to use existing programs, or follow this section to build and review a rater.

A small rater

This rater prices a dwelling fire policy from its Coverage A amount and its protection class. It is the running example in this guide:

Records = {
    Policy = {
        inputs {
            dwelling: Dwelling,
            items: List(Item)
                where len(self) in [0, 10] [code: "TOO-MANY-ITEMS", message: "A policy schedules at most ten items.", cite: manual:p6]
        },
        step premium {
            peril_premium[p: Peril]: Premium [cite: manual:p8] =
                round(self.dwelling.coverage_a / 1000 * lookup!(BaseRate, self.dwelling.protection_class).rate[p], 2, half_up),
            item_charge: Premium [cite: manual:p6] = sum(self.items as i, i.charge),
            premium: Premium [cite: manual:p9] =
                round(sum(Peril as p, self.peril_premium[p]) + self.item_charge + expense_fee, 0, half_up)
        }
    },

Each line reads like the manual: a base rate per peril from a table, a charge per scheduled item, a policy fee, and a rounding rule. Each number cites the manual page that prints it.

Why write a rater in RSL

Calculation checks. rslc check verifies accepted-input domains, table coverage, precision, and calculation dependencies before you package the rater.
Explicit refusals. Input constraints carry codes and messages, with citations for filed rules. Refusals report detected violations within documented size limits.
Exact arithmetic. Values stay exact through the declared calculations. Each rounding step states its precision and mode.
Source traceability. Tables, constants, and rounding calculations carry citations or explicit reasons for values outside the filing.

Compiler checks establish properties of the implemented algorithm. Review source interpretations and reconcile independent rating examples to establish fidelity to the filing. Validate a program covers this evidence alongside input mapping and comparison scope. What rslc checks and guarantees lists each promise and the check behind it.

Where to go

You are	Start with
New to RSL and want to see a rater work	Write your first rater
An actuary writing or reviewing a rater	RSL at a glance, then the how-to pages
An engineer running a rater from code	Use the rslc commands
Deciding whether to trust a rater’s results	What rslc checks and guarantees
Looking up a check message or a refusal code	Check messages and Refusals

Get started

Comparative pricing

Prepare and validate

Concepts

Rater development

Python reference

Data reference

A small rater

Why write a rater in RSL

Where to go

​A small rater

​Why write a rater in RSL

​Where to go

A small rater

Why write a rater in RSL

Where to go