Composable runtime validation for Dart. Build readable schemas from tiny validator functions & operators and get precise, structured errors (message, code, path, data) for untyped JSON, configs, and user input. Fast sync path, seamless async, no boilerplate.

• Validate untyped JSON & config fast
• Concise composition (&, |, not(), nullable(), optional())
• Sync fast path, auto async when needed
• Rich Expectation objects: message, code, path, data
• Extensively tested

dart pub add eskema

import 'package:eskema/eskema.dart';

void main() {
  final userValidator = eskema({
    'username': isString(),                  // basic type check
    'lastname': $isString,                   // cached zero‑arg variant
    'age': all([isInt(), isGte(0)]),         // multi validator (AND)
    'theme': isString() & (isEq('light') | isEq('dark')), // operators
    'premium': nullable($isBool),            // key must exist; value may be null
    'birthday': optional(isDate()),          // key may be absent
  });

  final ok = userValidator.validate({
    'username': 'bob', 'lastname': 'builder', 'theme': 'light', 'age': 42,
  });
  print(ok); // missing premium (nullable != optional)

  final res = userValidator.validate({'username': 'alice', 'age': -1});
  print(res.isValid);        // false
  print(res.expectations);   // structured reasons (age invalid, missing keys, etc.)
}

1. Build a schema with eskema({...}) (map of field -> validator)
2. Compose validators with functions (isString(), isGte(0), all([...])) or operators (&, |)
3. Add modifiers: nullable() (value may be null) vs optional() (key may be absent)
4. Call validate(value) (sync chain) or validateAsync(value) if any link is async
5. Inspect Result.isValid & Result.expectations or throw via validateOrThrow()

Each failure is an Expectation having:

• message – human text
• code – stable identifier (see expectation codes doc)
• path – location (e.g. .user.address[0].city)
• data – structured metadata

See the example/ directory for:

• transformers (toInt, defaultTo, trim)
• contextual & polymorphic validation (resolve, when, switchBy)
• custom validators & builders
• list & strict schema validation

• Docs & guides
• API reference
• Wiki
• Issues / questions: https://github.com/nombrekeff/eskema/issues

Open an issue for discussion, then PR with tests. Keep additions composable. (Detailed guidelines live in the wiki / future CONTRIBUTING doc.)

MIT – see LICENSE.

Star the repo if it helps you. ⭐