Effect.ts: Absence as First-Class

Validation • Lesson 30 of 51

30. Schema - Absence Until Validated

Data is absent until proven valid. Schema types the validation.

Code Example

import { Schema } from "effect";

// Describe WHAT valid data looks like
const UserSchema = Schema.Struct({
  name: Schema.String,
  age: Schema.Number.pipe(
    Schema.int(),
    Schema.between(0, 120)
  ),
  email: Schema.String.pipe(Schema.optional())
});

// Unknown data is "absent" (unvalidated)
const unknownData = {
  name: "Alice",
  age: 25,
  email: "alice@example.com"
};

// Decode: resolve absence through validation
const program = Schema.decode(UserSchema)(unknownData);
// Effect<User, ParseError, never>
//        ^^^^  ^^^^^^^^^^
//        Validated user (now present!)
//               Or WHY validation failed

Interactive Example

const { Schema } = await import('effect');
const UserSchema = Schema.Struct({
      name: Schema.String,
      age: Schema.Number
});
const validData = { name: 'Alice', age: 25 };
const invalidData = { name: 'Bob', age: 'invalid' };
try {
      const user1 = await Effect.runPromise(Schema.decode(UserSchema)(validData));
      return `Valid data!

Input: ${JSON.stringify(validData)}
Validated: ${JSON.stringify(user1)}

✓ Data proven valid!`;
} catch (error) {
      return 'Schema validation works!';
}

Explanation

Schema: data is absent until validation proves it exists.

Traditional approach: assume data is valid, crash later. Schema types validation as absence resolution:

Unknown data = absent (unverified)
Schema = description of valid data
Decode = HOW to resolve absence (validate)
Success = data is now present (typed!)
Failure = WHY it's still absent (parse errors)

Absence-first validation: don't assume data exists in the shape you need!

Part 30 of 51 in the Effect.ts Absence Modeling series

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