Context

While working on #2398 I noticed that we are not validating if config provided has correct structure at all, in this pr i added basic validation for resolved configuration as doing unsafe type casting is really bad idea

Proposed implementation in not ideal, and requires more refining.

Proposal

Create new package responsible for validation of configuration and rules @commitlint/config-validator and use ajv to validate configurations loaded by @commitlint/load and @commitlint/resolve-extends

Validation should be done in 2 steps: config and rules

Config validation #2412

this should be first step of validation of config structure and type check properties, but it should not validate structure of specific rules as by default we should accept any configuration that matches

'string': [RuleConfigSeverity /* 0, 1, 2 */, RuleConfigCondition /* when */, unknown /* options */]

Rules validation

after resolving and merging all extended configs we should proceed to validate options for rules:
this is going to require change in structure of rule definition, we should add support for object and deprecate raw functions

in case if we don't have schema for rule, we should skip validation of options

Rule as object

schema property is optional, but recommended,

• if provided we should validate options against it
• if not provided we should threat options as any/unknown

{
  schema: { ... },
  handler (parsed, when = 'always', options) { } // I'm not sure about this name, but this can be refined/changed
}

Rule as function - backward compatibility

It should be threated as there is no schema provided and we should accept any options

function rule (parsed, when = 'always', options) {}

Why we should change default structure of rules

with moving away of simple function structure it gives us possibility:

• to extend rules to provide more functionality in feature like requested fixing behavior. ([FeatureReq] ability to fix warnings #584)
• provide meaningful configuration errors (Unable to commit when type-enum is overridden #2384)
• add ability to validate configurations provided by this project: config-angular-type-enum, config-angular, config-conventional, config-lerna-scopes, config-patternplate (Add tests to all packages #889)
• ability to expose json-schema for IDE to provide syntax autocompletion
• This feature was requested before: Validate config better #263, Improve error handling for misconfigured instances #835

this is my general idea for this, its not written in best way, but I'd like to hear your opinion on this