Default Constructors

When working with data structures, it’s advantageous to be able to construct values that align with a specific schema effortlessly. For this purpose, we offer default constructors for several types of schemas, including Structs, Records, filters, and brands.

Note

Remember, default constructors linked to a schema Schema<A, I, R> specifically relate to the A type rather than the I type.

Structs

Example

import { Schema } from "effect"

const Struct = Schema.Struct({
  name: Schema.NonEmptyString
})

// Successful creation
Struct.make({ name: "a" })

// This will cause an error because the name is empty
Struct.make({ name: "" })
/*
throws
ParseError: { readonly name: NonEmptyString }
└─ ["name"]
   └─ NonEmptyString
      └─ Predicate refinement failure
         └─ Expected NonEmptyString, actual ""
*/

There are scenarios where you might want to bypass validation during instantiation. Although not typically recommended, effect/Schema allows for this flexibility:

import { Schema } from "effect"

const Struct = Schema.Struct({
  name: Schema.NonEmptyString
})

// Bypasses validation, thus avoiding errors
Struct.make({ name: "" }, true)

// or more explicitly
Struct.make({ name: "" }, { disableValidation: true })

Records

Example

import { Schema } from "effect"

const Record = Schema.Record({
  key: Schema.String,
  value: Schema.NonEmptyString
})

// Successful creation
Record.make({ a: "a", b: "b" })

// This will cause an error because 'b' is empty
Record.make({ a: "a", b: "" })
/*
throws
ParseError: { readonly [x: string]: NonEmptyString }
└─ ["b"]
   └─ NonEmptyString
      └─ Predicate refinement failure
         └─ Expected NonEmptyString, actual ""
*/

// Bypasses validation
Record.make({ a: "a", b: "" }, { disableValidation: true })

Filters

Example

import { Schema } from "effect"

const MyNumber = Schema.Number.pipe(Schema.between(1, 10))

// Successful creation
const n = MyNumber.make(5)

// This will cause an error because the number is outside the valid range
MyNumber.make(20)
/*
throws
ParseError: a number between 1 and 10
└─ Predicate refinement failure
   └─ Expected a number between 1 and 10, actual 20
*/

// Bypasses validation
MyNumber.make(20, { disableValidation: true })

Branded Types

Example

import { Schema } from "effect"

const BrandedNumberSchema = Schema.Number.pipe(
  Schema.between(1, 10),
  Schema.brand("MyNumber")
)

// Successful creation
const n = BrandedNumberSchema.make(5)

// This will cause an error because the number is outside the valid range
BrandedNumberSchema.make(20)
/*
throws
ParseError: a number between 1 and 10 & Brand<"MyNumber">
└─ Predicate refinement failure
   └─ Expected a number between 1 and 10 & Brand<"MyNumber">, actual 20
*/

// Bypasses validation
BrandedNumberSchema.make(20, { disableValidation: true })

When utilizing our default constructors, it’s important to grasp the type of value they generate. In the BrandedNumberSchema example, the return type of the constructor is number & Brand<"MyNumber">, indicating that the resulting value is a number with the added branding “MyNumber”.

This differs from the filter example where the return type is simply number. The branding offers additional insights about the type, facilitating the identification and manipulation of your data.

Note that default constructors are “unsafe” in the sense that if the input does not conform to the schema, the constructor throws an error containing a description of what is wrong. This is because the goal of default constructors is to provide a quick way to create compliant values (for example, for writing tests or configurations, or in any situation where it is assumed that the input passed to the constructors is valid and the opposite situation is exceptional). To have a “safe” constructor, you can use Schema.validateEither:

import { Schema } from "effect"

const MyNumber = Schema.Number.pipe(Schema.between(1, 10))

const ctor = Schema.validateEither(MyNumber)

console.log(ctor(5))
/*
{ _id: 'Either', _tag: 'Right', right: 5 }
*/

console.log(ctor(20))
/*
{
  _id: 'Either',
  _tag: 'Left',
  left: {
    _id: 'ParseError',
    message: 'a number between 1 and 10\n' +
      '└─ Predicate refinement failure\n' +
      '   └─ Expected a number between 1 and 10, actual 20'
  }
}
*/

Setting Default Values

When constructing objects, it’s common to want to assign default values to certain fields to simplify the creation of new instances. The Schema.withConstructorDefault function allows you to manage the optionality of a field in your default constructor.

Example (Without Default)

import { Schema } from "effect"

const Person = Schema.Struct({
  name: Schema.NonEmptyString,
  age: Schema.Number
})

// Both name and age are required
Person.make({ name: "John", age: 30 })

Example (With Default)

import { Schema } from "effect"

const Person = Schema.Struct({
  name: Schema.NonEmptyString,
  age: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => 0)
  )
})

// The age field is optional and defaults to 0
console.log(Person.make({ name: "John" }))
/*
Output:
{ age: 0, name: 'John' }
*/

In the second example, notice how the age field is now optional and defaults to 0 when not provided.

Defaults are lazily evaluated, meaning that a new instance of the default is generated every time the constructor is called:

import { Schema } from "effect"

const Person = Schema.Struct({
  name: Schema.NonEmptyString,
  age: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => 0)
  ),
  timestamp: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => new Date().getTime())
  )
})

console.log(Person.make({ name: "name1" }))
// { age: 0, timestamp: 1714232909221, name: 'name1' }

console.log(Person.make({ name: "name2" }))
// { age: 0, timestamp: 1714232909227, name: 'name2' }

Note how the timestamp field varies.

Default values are also “portable”, meaning that if you reuse the same property signature in another schema, the default is carried over:

import { Schema } from "effect"

const Person = Schema.Struct({
  name: Schema.NonEmptyString,
  age: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => 0)
  ),
  timestamp: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => new Date().getTime())
  )
})

const AnotherSchema = Schema.Struct({
  foo: Schema.String,
  age: Person.fields.age
})

console.log(AnotherSchema.make({ foo: "bar" })) // => { foo: 'bar', age: 0 }

Defaults can also be applied using the Class API:

import { Schema } from "effect"

class Person extends Schema.Class<Person>("Person")({
  name: Schema.NonEmptyString,
  age: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => 0)
  ),
  timestamp: Schema.Number.pipe(
    Schema.propertySignature,
    Schema.withConstructorDefault(() => new Date().getTime())
  )
}) {}

console.log(new Person({ name: "name1" }))
// Person { age: 0, timestamp: 1714400867208, name: 'name1' }

console.log(new Person({ name: "name2" }))
// Person { age: 0, timestamp: 1714400867215, name: 'name2' }