Getting Started

You can import the necessary types and functions from the effect/Schema module:

Example (Namespace Import)

import * as Schema from "effect/Schema"

Example (Named Import)

import { Schema } from "effect"

Defining a schema

One common way to define a Schema is by utilizing the Struct constructor. This constructor allows you to create a new schema that outlines an object with specific properties. Each property in the object is defined by its own schema, which specifies the data type and any validation rules.

Example (Defining a Simple Object Schema)

This Person schema describes an object with a name (string) and age (number) property:

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})

Extracting Inferred Types

Type

Once you’ve defined a schema (Schema<Type, Encoded, Context>), you can extract the inferred type Type in two ways:

Using the Schema.Type utility
Accessing the Type field directly on your schema

Example (Extracting Inferred Type)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
// 1. Using the Schema.Type utility
9
type Person = Schema.Schema.Type<typeof Person>
10

11
// 2. Accessing the Type field directly
12
type Person2 = typeof Person.Type

The resulting type will look like this:

type Person = {
  readonly name: string
  readonly age: number
}

Alternatively, you can extract the Person type using the interface keyword, which may improve readability and performance in some cases.

Example (Extracting Type with an Interface)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
interface Person extends Schema.Schema.Type<typeof Person> {}

Both approaches yield the same result, but using an interface provides benefits such as performance advantages and improved readability.

Encoded

In a Schema<Type, Encoded, Context>, the Encoded type can differ from the Type type, representing the format in which data is encoded. You can extract the Encoded type in two ways:

Using the Schema.Encoded utility
Accessing the Encoded field directly on the schema

Example (Extracting the Encoded Type)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  // a schema that decodes a string to a number
6
  age: Schema.NumberFromString,
7
})
8

9
// 1. Using the Schema.Encoded utility
10
type PersonEncoded = Schema.Schema.Encoded<typeof Person>
11

12
// 2. Accessing the Encoded field directly
13
type PersonEncoded2 = typeof Person.Encoded

The resulting type is:

type PersonEncoded = {
  readonly name: string
  readonly age: string
}

Note that age is of type string in the Encoded type of the schema and is of type number in the Type type of the schema.

Alternatively, you can define the PersonEncoded type using the interface keyword, which can enhance readability and performance.

Example (Extracting Encoded Type with an Interface)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  // a schema that decodes a string to a number
6
  age: Schema.NumberFromString,
7
})
8

9
interface PersonEncoded extends Schema.Schema.Encoded<typeof Person> {}

Both approaches yield the same result, but using an interface provides benefits such as performance advantages and improved readability.

Context

In a Schema<Type, Encoded, Context>, the Context type represents any external data or dependencies that the schema requires to perform encoding or decoding. You can extract the inferred Context type in two ways:

Using the Schema.Context utility.
Accessing the Context field on the schema.

Example (Extracting the Context Type)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
// 1. Using the Schema.Context utility
9
type PersonContext = Schema.Schema.Context<typeof Person>
10

11
// 2. Accessing the Context field directly
12
type PersonContext2 = typeof Person.Context

Schemas with Opaque Types

When defining a schema, you may want to create a schema with an opaque type. This is useful when you want to hide the internal structure of the schema and only expose the type of the schema.

Example (Creating an Opaque Schema)

To create a schema with an opaque type, you can use the following technique that re-declares the schema:

1
import { Schema } from "effect"
2

3
// Define the schema structure
4
const _Person = Schema.Struct({
5
  name: Schema.String,
6
  age: Schema.Number,
7
})
8

9
// Declare the type interface to make it opaque
10
interface Person extends Schema.Schema.Type<typeof _Person> {}
11

12
// Re-declare the schema as opaque
13
const Person: Schema.Schema<Person> = _Person

Alternatively, you can use the Class APIs (see the Class APIs section for more details).

Note that the technique shown above becomes more complex when the schema is defined such that Type is different from Encoded.

Example (Opaque Schema with Different Type and Encoded)

1
import { Schema } from "effect"
2

3
// Define the schema structure, with a field that
4
// decodes a string to a number
5
const _Person = Schema.Struct({
6
  name: Schema.String,
7
  age: Schema.NumberFromString,
8
})
9

10
// Create the `Type` interface for an opaque schema
11
interface Person extends Schema.Schema.Type<typeof _Person> {}
12

13
// Create the `Encoded` interface for an opaque schema
14
interface PersonEncoded extends Schema.Schema.Encoded<typeof _Person> {}
15

16
// Re-declare the schema with opaque Type and Encoded
17
const Person: Schema.Schema<Person, PersonEncoded> = _Person

In this case, the field "age" is of type string in the Encoded type of the schema and is of type number in the Type type of the schema. Therefore, we need to define two interfaces (PersonEncoded and Person) and use both to redeclare our final schema Person.

Readonly Types by Default

It’s important to note that by default, most constructors exported by effect/Schema return readonly types.

Example (Readonly Types in a Schema)

For instance, in the Person schema below:

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})

the resulting inferred Type would be:

{
  readonly name: string;
  readonly age: number;
}

Decoding

When working with unknown data types in TypeScript, decoding them into a known structure can be challenging. Luckily, effect/Schema provides several functions to help with this process. Let’s explore how to decode unknown values using these functions.

API	Description
`decodeUnknownSync`	Synchronously decodes a value and throws an error if parsing fails.
`decodeUnknownOption`	Decodes a value and returns an Option type.
`decodeUnknownEither`	Decodes a value and returns an Either type.
`decodeUnknownPromise`	Decodes a value and returns a `Promise`.
`decodeUnknown`	Decodes a value and returns an Effect.

decodeUnknownSync

The Schema.decodeUnknownSync function is useful when you want to parse a value and immediately throw an error if the parsing fails.

Example (Using decodeUnknownSync for Immediate Decoding)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
// Simulate an unknown input
9
const input: unknown = { name: "Alice", age: 30 }
10

11
// Example of valid input matching the schema
12
console.log(Schema.decodeUnknownSync(Person)(input))
13
// Output: { name: 'Alice', age: 30 }
14

15
// Example of invalid input that does not match the schema
16
console.log(Schema.decodeUnknownSync(Person)(null))
17
/*
18
throws:
19
ParseError: Expected { readonly name: string; readonly age: number }, actual null
20
*/

decodeUnknownEither

The Schema.decodeUnknownEither function allows you to parse a value and receive the result as an Either, representing success (Right) or failure (Left). This approach lets you handle parsing errors more gracefully without throwing exceptions.

Example (Using Schema.decodeUnknownEither for Error Handling)

1
import { Schema } from "effect"
2
import { Either } from "effect"
3

4
const Person = Schema.Struct({
5
  name: Schema.String,
6
  age: Schema.Number,
7
})
8

9
const decode = Schema.decodeUnknownEither(Person)
10

11
// Simulate an unknown input
12
const input: unknown = { name: "Alice", age: 30 }
13

14
// Attempt decoding a valid input
15
const result1 = decode(input)
16
if (Either.isRight(result1)) {
17
  console.log(result1.right)
18
  /*
19
  Output:
20
  { name: "Alice", age: 30 }
21
  */
22
}
23

24
// Simulate decoding an invalid input
25
const result2 = decode(null)
26
if (Either.isLeft(result2)) {
27
  console.log(result2.left)
28
  /*
29
  Output:
30
  {
31
    _id: 'ParseError',
32
    message: 'Expected { readonly name: string; readonly age: number }, actual null'
33
  }
34
  */
35
}

decodeUnknown

If your schema involves asynchronous transformations, the Schema.decodeUnknownSync and Schema.decodeUnknownEither functions will not be suitable. In such cases, you should use the Schema.decodeUnknown function, which returns an Effect.

Example (Handling Asynchronous Decoding)

1
import { Schema } from "effect"
2
import { Effect } from "effect"
3

4
const PersonId = Schema.Number
5

6
const Person = Schema.Struct({
7
  id: PersonId,
8
  name: Schema.String,
9
  age: Schema.Number,
10
})
11

12
const asyncSchema = Schema.transformOrFail(PersonId, Person, {
13
  strict: true,
14
  // Decode with simulated async transformation
15
  decode: (id) => Effect.succeed({ id, name: "name", age: 18 }).pipe(Effect.delay("10 millis")),
16
  encode: (person) => Effect.succeed(person.id).pipe(Effect.delay("10 millis")),
17
})
18

19
// Attempting to use a synchronous decoder on an async schema
20
console.log(Schema.decodeUnknownEither(asyncSchema)(1))
21
/*
22
Output:
23
{
24
  _id: 'Either',
25
  _tag: 'Left',
26
  left: {
27
    _id: 'ParseError',
28
    message: '(number <-> { readonly id: number; readonly name: string; readonly age: number })\n' +
29
      '└─ cannot be be resolved synchronously, this is caused by using runSync on an effect that performs async work'
30
  }
31
}
32
*/
33

34
// Decoding asynchronously with `Schema.decodeUnknown`
35
Effect.runPromise(Schema.decodeUnknown(asyncSchema)(1)).then(console.log)
36
/*
37
Output:
38
{ id: 1, name: 'name', age: 18 }
39
*/

In the code above, the first approach using Schema.decodeUnknownEither results in an error indicating that the transformation cannot be resolved synchronously. This occurs because Schema.decodeUnknownEither is not designed for async operations. The second approach, which uses Schema.decodeUnknown, works correctly, allowing you to handle asynchronous transformations and return the expected result.

Encoding

The Schema module provides several encode* functions to encode data according to a schema:

API	Description
`encodeSync`	Synchronously encodes data and throws an error if encoding fails.
`encodeOption`	Encodes data and returns an Option type.
`encodeEither`	Encodes data and returns an Either type representing success or failure.
`encodePromise`	Encodes data and returns a `Promise`.
`encode`	Encodes data and returns an Effect.

Example (Using Schema.encodeSync for Immediate Encoding)

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  // Ensure name is a non-empty string
5
  name: Schema.NonEmptyString,
6
  // Allow age to be decoded from a string and encoded to a string
7
  age: Schema.NumberFromString,
8
})
9

10
// Valid input: encoding succeeds and returns expected types
11
console.log(Schema.encodeSync(Person)({ name: "Alice", age: 30 }))
12
// Output: { name: 'Alice', age: '30' }
13

14
// Invalid input: encoding fails due to empty name string
15
console.log(Schema.encodeSync(Person)({ name: "", age: 30 }))
16
/*
17
throws:
18
ParseError: { readonly name: NonEmptyString; readonly age: NumberFromString }
19
└─ ["name"]
20
   └─ NonEmptyString
21
      └─ Predicate refinement failure
22
         └─ Expected a non empty string, actual ""
23
*/

Note that during encoding, the number value 30 was converted to a string "30".

Handling Unsupported Encoding

In certain cases, it may not be feasible to support encoding for a schema. While it is generally advised to define schemas that allow both decoding and encoding, there are situations where encoding a particular type is either unsupported or unnecessary. In these instances, the Forbidden issue can signal that encoding is not available for certain values.

Example (Using Forbidden to Indicate Unsupported Encoding)

Here is an example of a transformation that never fails during decoding. It returns an Either containing either the decoded value or the original input. For encoding, it is reasonable to not support it and use Forbidden as the result.

1
import { Either, ParseResult, Schema } from "effect"
2

3
// Define a schema that safely decodes to Either type
4
export const SafeDecode = <A, I>(self: Schema.Schema<A, I, never>) => {
5
  const decodeUnknownEither = Schema.decodeUnknownEither(self)
6
  return Schema.transformOrFail(
7
    Schema.Unknown,
8
    Schema.EitherFromSelf({
9
      left: Schema.Unknown,
10
      right: Schema.typeSchema(self),
11
    }),
12
    {
13
      strict: true,
14
      // Decode: map a failed result to the input as Left,
15
      // successful result as Right
16
      decode: (input) =>
17
        ParseResult.succeed(Either.mapLeft(decodeUnknownEither(input), () => input)),
18
      // Encode: only support encoding Right values,
19
      // Left values raise Forbidden error
20
      encode: (actual, _, ast) =>
21
        Either.match(actual, {
22
          onLeft: () =>
23
            ParseResult.fail(new ParseResult.Forbidden(ast, actual, "cannot encode a Left")),
24
          // Successfully encode a Right value
25
          onRight: ParseResult.succeed,
26
        }),
27
    },
28
  )
29
}

Explanation

Decoding: The SafeDecode function ensures that decoding never fails. It wraps the decoded value in an Either, where a successful decoding results in a Right and a failed decoding results in a Left containing the original input.
Encoding: The encoding process uses the Forbidden error to indicate that encoding a Left value is not supported. Only Right values are successfully encoded.

ParseError

The Schema.decodeUnknownEither and Schema.encodeEither functions returns a Either:

Either<Type, ParseError>

where ParseError is defined as follows (simplified):

interface ParseError {
  readonly _tag: "ParseError"
  readonly issue: ParseIssue
}

In this structure, ParseIssue represents an error that might occur during the parsing process. It is wrapped in a tagged error to make it easier to catch errors using Effect.catchTag. The result Either<Type, ParseError> contains the inferred data type described by the schema (Type). A successful parse yields a Right value with the parsed data Type, while a failed parse results in a Left value containing a ParseError.

Parse Options

The options below provide control over both decoding and encoding behaviors.

Managing Excess properties

By default, any properties not defined in the schema are removed from the output when parsing a value. This ensures the parsed data conforms strictly to the expected structure.

If you want to detect and handle unexpected properties, use the onExcessProperty option (default value: "ignore"), which allows you to raise an error for excess properties. This can be helpful when you need to validate and catch unanticipated properties.

Example (Setting onExcessProperty to "error")

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
// Excess properties are ignored by default
9
console.log(
10
  Schema.decodeUnknownSync(Person)({
11
    name: "Bob",
12
    age: 40,
13
    email: "bob@example.com", // Ignored
14
  }),
15
)
16
/*
17
Output:
18
{ name: 'Bob', age: 40 }
19
*/
20

21
// With `onExcessProperty` set to "error",
22
// an error is thrown for excess properties
23
Schema.decodeUnknownSync(Person)(
24
  {
25
    name: "Bob",
26
    age: 40,
27
    email: "bob@example.com", // Will raise an error
28
  },
29
  { onExcessProperty: "error" },
30
)
31
/*
32
throws
33
ParseError: { readonly name: string; readonly age: number }
34
└─ ["email"]
35
   └─ is unexpected, expected: "name" | "age"
36
*/

To retain extra properties, set onExcessProperty to "preserve".

Example (Setting onExcessProperty to "preserve")

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
// Excess properties are preserved in the output
9
console.log(
10
  Schema.decodeUnknownSync(Person)(
11
    {
12
      name: "Bob",
13
      age: 40,
14
      email: "bob@example.com",
15
    },
16
    { onExcessProperty: "preserve" },
17
  ),
18
)
19
/*
20
{ email: 'bob@example.com', name: 'Bob', age: 40 }
21
*/

Receive all errors

The errors option enables you to retrieve all errors encountered during parsing. By default, only the first error is returned. Setting errors to "all" provides comprehensive error feedback, which can be useful for debugging or offering detailed validation feedback.

Example (Setting errors to "all")

1
import { Schema } from "effect"
2

3
const Person = Schema.Struct({
4
  name: Schema.String,
5
  age: Schema.Number,
6
})
7

8
// Attempt to parse with multiple issues in the input data
9
Schema.decodeUnknownSync(Person)(
10
  {
11
    name: "Bob",
12
    age: "abc",
13
    email: "bob@example.com",
14
  },
15
  { errors: "all", onExcessProperty: "error" },
16
)
17
/*
18
throws
19
ParseError: { readonly name: string; readonly age: number }
20
├─ ["email"]
21
│  └─ is unexpected, expected: "name" | "age"
22
└─ ["age"]
23
   └─ Expected number, actual "abc"
24
*/

Managing Property Order

The propertyOrder option provides control over the order of object fields in the output. This feature is particularly useful when the sequence of keys is important for the consuming processes or when maintaining the input order enhances readability and usability.

By default, the propertyOrder option is set to "none". This means that the internal system decides the order of keys to optimize parsing speed. The order of keys in this mode should not be considered stable, and it’s recommended not to rely on key ordering as it may change in future updates.

Setting propertyOrder to "original" ensures that the keys are ordered as they appear in the input during the decoding/encoding process.

Example (Synchronous Decoding)

1
import { Schema } from "effect"
2

3
const schema = Schema.Struct({
4
  a: Schema.Number,
5
  b: Schema.Literal("b"),
6
  c: Schema.Number,
7
})
8

9
// Default decoding, where property order is system-defined
10
console.log(Schema.decodeUnknownSync(schema)({ b: "b", c: 2, a: 1 }))
11
// Output may vary: { a: 1, b: 'b', c: 2 }
12

13
// Decoding while preserving input order
14
console.log(Schema.decodeUnknownSync(schema)({ b: "b", c: 2, a: 1 }, { propertyOrder: "original" }))
15
// Output preserves input order: { b: 'b', c: 2, a: 1 }

Example (Asynchronous Decoding)

1
import type { Duration } from "effect"
2
import { Effect, ParseResult, Schema } from "effect"
3

4
// Helper function to simulate an async operation in schema
5
const effectify = (duration: Duration.DurationInput) =>
6
  Schema.Number.pipe(
7
    Schema.transformOrFail(Schema.Number, {
8
      strict: true,
9
      decode: (x) => Effect.sleep(duration).pipe(Effect.andThen(ParseResult.succeed(x))),
10
      encode: ParseResult.succeed,
11
    }),
12
  )
13

14
// Define a structure with asynchronous behavior in each field
15
const schema = Schema.Struct({
16
  a: effectify("200 millis"),
17
  b: effectify("300 millis"),
18
  c: effectify("100 millis"),
19
}).annotations({ concurrency: 3 })
20

21
// Default decoding, where property order is system-defined
22
Schema.decode(schema)({ a: 1, b: 2, c: 3 }).pipe(Effect.runPromise).then(console.log)
23
// Output decided internally: { c: 3, a: 1, b: 2 }
24

25
// Decoding while preserving input order
26
Schema.decode(schema)({ a: 1, b: 2, c: 3 }, { propertyOrder: "original" })
27
  .pipe(Effect.runPromise)
28
  .then(console.log)
29
// Output preserving input order: { a: 1, b: 2, c: 3 }

Customizing Parsing Behavior at the Schema Level

The parseOptions annotation allows you to customize parsing behavior at different schema levels, enabling you to apply unique parsing settings to nested schemas within a structure. Options defined within a schema override parent-level settings and apply to all nested schemas.

Example (Using parseOptions to Customize Error Handling)

1
import { Schema } from "effect"
2
import { Either } from "effect"
3

4
const schema = Schema.Struct({
5
  a: Schema.Struct({
6
    b: Schema.String,
7
    c: Schema.String,
8
  }).annotations({
9
    title: "first error only",
10
    // Limit errors to the first in this sub-schema
11
    parseOptions: { errors: "first" },
12
  }),
13
  d: Schema.String,
14
}).annotations({
15
  title: "all errors",
16
  // Capture all errors for the main schema
17
  parseOptions: { errors: "all" },
18
})
19

20
// Decode input with custom error-handling behavior
21
const result = Schema.decodeUnknownEither(schema)({ a: {} }, { errors: "first" })
22
if (Either.isLeft(result)) {
23
  console.log(result.left.message)
24
}
25
/*
26
all errors
27
├─ ["a"]
28
│  └─ first error only
29
│     └─ ["b"]
30
│        └─ is missing
31
└─ ["d"]
32
   └─ is missing
33
*/

Detailed Output Explanation:

In this example:

The main schema is configured to display all errors. Hence, you will see errors related to both the d field (since it’s missing) and any errors from the a subschema.
The subschema (a) is set to display only the first error. Although both b and c fields are missing, only the first missing field (b) is reported.

Type Guards

The Schema.is function provides a way to verify if a value conforms to a given schema. It acts as a type guard, taking a value of type unknown and determining if it matches the structure and type constraints defined in the schema.

Here’s how the Schema.is function works:

Schema Definition: Define a schema to describe the structure and constraints of the data type you expect. For instance, Schema<Type, Encoded, Context>, where Type is the target type you want to validate against.
Type Guard Creation: Use the schema to create a user-defined type guard, (u: unknown) => u is Type. This function can be used at runtime to check if a value meets the requirements of the schema.

Example (Creating and Using a Type Guard)

1
import { Schema } from "effect"
2

3
// Define a schema for a Person object
4
const Person = Schema.Struct({
5
  name: Schema.String,
6
  age: Schema.Number,
7
})
8

9
// Generate a type guard from the schema
10
const isPerson = Schema.is(Person)
11

12
// Test the type guard with various inputs
13
console.log(isPerson({ name: "Alice", age: 30 }))
14
// Output: true
15

16
console.log(isPerson(null))
17
// Output: false
18

19
console.log(isPerson({}))
20
// Output: false

The generated isPerson function has the following signature:

const isPerson: (
  u: unknown,
  overrideOptions?: number | ParseOptions,
) => u is {
  readonly name: string
  readonly age: number
}

Assertions

While type guards verify whether a value conforms to a specific type, the Schema.asserts function goes further by asserting that an input matches the schema type Type (from Schema<Type, Encoded, Context>). If the input does not match the schema, it throws a detailed error, making it useful for runtime validation.

Example (Creating and Using an Assertion)

1
import { Schema } from "effect"
2

3
// Define a schema for a Person object
4
const Person = Schema.Struct({
5
  name: Schema.String,
6
  age: Schema.Number,
7
})
8

9
// Generate an assertion function from the schema
10
const assertsPerson: Schema.Schema.ToAsserts<typeof Person> = Schema.asserts(Person)
11

12
try {
13
  // Attempt to assert that the input matches the Person schema
14
  assertsPerson({ name: "Alice", age: "30" })
15
} catch (e) {
16
  console.error("The input does not match the schema:")
17
  console.error(e)
18
}
19
/*
20
throws:
21
The input does not match the schema:
22
{
23
  _id: 'ParseError',
24
  message: '{ readonly name: string; readonly age: number }\n' +
25
    '└─ ["age"]\n' +
26
    '   └─ Expected number, actual "30"'
27
}
28
*/
29

30
// This input matches the schema and will not throw an error
31
assertsPerson({ name: "Alice", age: 30 })

The assertsPerson function generated from the schema has the following signature:

const assertsPerson: (
  input: unknown,
  overrideOptions?: number | ParseOptions,
) => asserts input is {
  readonly name: string
  readonly age: number
}

Managing Missing Properties

When decoding, it’s important to understand how missing properties are processed. By default, if a property is not present in the input, it is treated as if it were present with an undefined value.

Example (Default Behavior of Missing Properties)

1
import { Schema } from "effect"
2

3
const schema = Schema.Struct({ a: Schema.Unknown })
4
const input = {}
5

6
console.log(Schema.decodeUnknownSync(schema)(input))
7
// Output: { a: undefined }

In this example, although the key "a" is not present in the input, it is treated as { a: undefined } by default.

If you need your validation logic to differentiate between genuinely missing properties and those explicitly set to undefined, you can enable the exact option.

Example (Setting exact: true to Distinguish Missing Properties)

1
import { Schema } from "effect"
2

3
const schema = Schema.Struct({ a: Schema.Unknown })
4
const input = {}
5

6
console.log(Schema.decodeUnknownSync(schema)(input, { exact: true }))
7
/*
8
throws
9
ParseError: { readonly a: unknown }
10
└─ ["a"]
11
   └─ is missing
12
*/

For the APIs Schema.is and Schema.asserts, however, the default behavior is to treat missing properties strictly, where the default for exact is true:

Example (Strict Handling of Missing Properties with Schema.is and Schema.asserts)

1
import type { SchemaAST } from "effect"
2
import { Schema } from "effect"
3

4
const schema = Schema.Struct({ a: Schema.Unknown })
5
const input = {}
6

7
console.log(Schema.is(schema)(input))
8
// Output: false
9

10
console.log(Schema.is(schema)(input, { exact: false }))
11
// Output: true
12

13
const asserts: (
14
  u: unknown,
15
  overrideOptions?: SchemaAST.ParseOptions,
16
) => asserts u is {
17
  readonly a: unknown
18
} = Schema.asserts(schema)
19

20
try {
21
  asserts(input)
22
  console.log("asserts passed")
23
} catch (e: any) {
24
  console.error("asserts failed")
25
  console.error(e.message)
26
}
27
/*
28
Output:
29
asserts failed
30
{ readonly a: unknown }
31
└─ ["a"]
32
  └─ is missing
33
*/
34

35
try {
36
  asserts(input, { exact: false })
37
  console.log("asserts passed")
38
} catch (e: any) {
39
  console.error("asserts failed")
40
  console.error(e.message)
41
}
42
// Output: asserts passed

Naming Conventions

The naming conventions in effect/Schema are designed to be straightforward and logical, focusing primarily on compatibility with JSON serialization. This approach simplifies the understanding and use of schemas, especially for developers who are integrating web technologies where JSON is a standard data interchange format.

Overview of Naming Strategies

JSON-Compatible Types

Schemas that naturally serialize to JSON-compatible formats are named directly after their data types.

For instance:

Schema.Date: serializes JavaScript Date objects to ISO-formatted strings, a typical method for representing dates in JSON.
Schema.Number: used directly as it maps precisely to the JSON number type, requiring no special transformation to remain JSON-compatible.

Non-JSON-Compatible Types

When dealing with types that do not have a direct representation in JSON, the naming strategy incorporates additional details to indicate the necessary transformation. This helps in setting clear expectations about the schema’s behavior:

For instance:

Schema.DateFromSelf: indicates that the schema handles Date objects, which are not natively JSON-serializable.
Schema.NumberFromString: this naming suggests that the schema processes numbers that are initially represented as strings, emphasizing the transformation from string to number when decoding.

The primary goal of these schemas is to ensure that domain objects can be easily serialized (“encoded”) and deserialized (“decoded”) for transmission over network connections, thus facilitating their transfer between different parts of an application or across different applications.

Rationale

While JSON’s ubiquity justifies its primary consideration in naming, the conventions also accommodate serialization for other types of transport. For instance, converting a Date to a string is a universally useful method for various communication protocols, not just JSON. Thus, the selected naming conventions serve as sensible defaults that prioritize clarity and ease of use, facilitating the serialization and deserialization processes across diverse technological environments.