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The Schema module provides powerful runtime validation and type-safe schema definitions for TypeScript applications. It enables you to define data structures once and use them for validation, parsing, serialization, and type inference.

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Overview

A Schema defines both:
  • The encoded representation (e.g., what comes from an API or user input)
  • The type representation (e.g., your application’s domain model)
Schemas automatically provide:
  • Runtime validation and parsing
  • Type inference for TypeScript
  • JSON Schema generation
  • Serialization and deserialization
  • Custom error messages

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Creating Schemas

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Primitive Types

import { Schema } from "effect"

// Basic primitives
const StringSchema = Schema.String
const NumberSchema = Schema.Number
const BooleanSchema = Schema.Boolean
const IntSchema = Schema.Int

// Date and Duration
const DateSchema = Schema.Date
const DurationSchema = Schema.Duration

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Structs and Objects

import { Schema } from "effect"

const PersonSchema = Schema.Struct({
  name: Schema.String,
  age: Schema.Int,
  email: Schema.String
})

// Infer the TypeScript type
type Person = Schema.Schema.Type<typeof PersonSchema>
// { name: string; age: number; email: string }

​
Optional and Nullable Fields

import { Schema } from "effect"

const UserSchema = Schema.Struct({
  id: Schema.Number,
  name: Schema.String,
  // Optional field (may be undefined)
  nickname: Schema.optional(Schema.String),
  // Nullable field (may be null)
  bio: Schema.NullOr(Schema.String),
  // Optional with default value
  role: Schema.optional(Schema.String).pipe(
    Schema.withDefault(() => "user")
  )
})

​
Arrays and Collections

import { Schema } from "effect"

const NumberArraySchema = Schema.Array(Schema.Number)

const TagsSchema = Schema.Array(Schema.String).pipe(
  Schema.minItems(1),
  Schema.maxItems(10)
)

// Non-empty arrays
const NonEmptyArraySchema = Schema.NonEmptyArray(Schema.String)

​
Parsing and Validation

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Using makeUnsafe

import { Schema } from "effect"

const PersonSchema = Schema.Struct({
  name: Schema.String,
  age: Schema.Int
})

// Parse and validate (throws on error)
try {
  const person = PersonSchema.makeUnsafe({
    name: "Alice",
    age: 30
  })
  console.log(person)
} catch (error) {
  console.error("Validation failed:", error)
}

​
Using makeOption

import { Schema } from "effect"
import { Option } from "effect"

const result = PersonSchema.makeOption({ name: "Bob", age: 25 })

if (Option.isSome(result)) {
  console.log("Valid:", result.value)
} else {
  console.log("Invalid input")
}

​
With Effect Integration

import { Effect, Schema } from "effect"

const parseUser = (input: unknown) =>
  Effect.gen(function*() {
    const UserSchema = Schema.Struct({
      name: Schema.String,
      age: Schema.Int
    })
    
    // Decode returns an Effect
    const user = yield* Schema.decode(UserSchema)(input)
    return user
  })

​
Custom Validations

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Refinements

import { Schema } from "effect"

const PositiveInt = Schema.Int.pipe(
  Schema.filter((n) => n > 0, {
    message: () => "Must be positive"
  })
)

const EmailSchema = Schema.String.pipe(
  Schema.filter(
    (s) => s.includes("@"),
    { message: () => "Invalid email format" }
  )
)

​
Transformations

import { Schema } from "effect"

// Transform from string to Date
const DateFromString = Schema.transform(
  Schema.String,
  Schema.Date,
  {
    decode: (s) => new Date(s),
    encode: (d) => d.toISOString()
  }
)

// Transform with validation
const TrimmedString = Schema.transform(
  Schema.String,
  Schema.String,
  {
    decode: (s) => s.trim(),
    encode: (s) => s
  }
)

​
Tagged Errors

import { Schema } from "effect"

// Define custom errors with Schema
export class ValidationError extends Schema.TaggedErrorClass<ValidationError>()("ValidationError", {
  message: Schema.String,
  field: Schema.optional(Schema.String)
}) {}

// Use in Effects
const validateAge = (age: number) =>
  age >= 18
    ? Effect.succeed(age)
    : Effect.fail(new ValidationError({
        message: "Must be 18 or older",
        field: "age"
      }))

​
Union Types

import { Schema } from "effect"

// Simple union
const StringOrNumber = Schema.Union(
  Schema.String,
  Schema.Number
)

// Discriminated union
type Result = Schema.Schema.Type<typeof ResultSchema>

const ResultSchema = Schema.Union(
  Schema.Struct({
    _tag: Schema.Literal("success"),
    value: Schema.String
  }),
  Schema.Struct({
    _tag: Schema.Literal("error"),
    error: Schema.String
  })
)

​
Records and Dictionaries

import { Schema } from "effect"

// Record with string keys and number values
const ScoresSchema = Schema.Record({
  key: Schema.String,
  value: Schema.Number
})

type Scores = Schema.Schema.Type<typeof ScoresSchema>
// Record<string, number>

​
Annotations and Documentation

import { Schema } from "effect"

const PersonSchema = Schema.Struct({
  name: Schema.String.pipe(
    Schema.annotate({
      title: "Full Name",
      description: "The person's full legal name"
    })
  ),
  age: Schema.Int.pipe(
    Schema.annotate({
      title: "Age",
      description: "Age in years",
      minimum: 0,
      maximum: 150
    })
  )
})

​
Type Inference

import { Schema } from "effect"

const UserSchema = Schema.Struct({
  id: Schema.Number,
  name: Schema.String,
  role: Schema.Literal("admin", "user")
})

// Extract the Type (decoded form)
type User = Schema.Schema.Type<typeof UserSchema>
// { id: number; name: string; role: "admin" | "user" }

// Extract the Encoded form
type UserEncoded = Schema.Schema.Encoded<typeof UserSchema>
// { id: number; name: string; role: "admin" | "user" }

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Best Practices

  1. Define schemas at the boundaries: Validate external data at API boundaries
  2. Use Schema.TaggedErrorClass: Define custom errors with schemas for better type safety
  3. Compose schemas: Build complex schemas from simpler ones
  4. Add annotations: Document your schemas for better JSON Schema generation and error messages
  5. Prefer makeOption over makeUnsafe: Use makeOption when validation failure is expected

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Next Steps

  • Learn about Effect for error handling
  • Explore Config for configuration management
  • See how Stream uses schemas for validation

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