Schema to Pretty Printer

The Pretty.make function is used to create pretty printers that generate a formatted string representation of values based on a schema.

Example (Pretty Printer for a Struct Schema)

1
import { import Pretty
Pretty, import Schema
Schema } from "effect"
2

3
const const Person: Schema.Struct<{
    name: typeof Schema.String;
    age: typeof Schema.Number;
}>
Person = import Schema
Schema.function Struct<{
    name: typeof Schema.String;
    age: typeof Schema.Number;
}>(fields: {
    name: typeof Schema.String;
    age: typeof Schema.Number;
}): Schema.Struct<{
    name: typeof Schema.String;
    age: typeof Schema.Number;
}> (+1 overload)
namespace Struct
Struct({
4
  (property) name: typeof Schema.String
name: import Schema
Schema.(alias) class String
export String
String,
5
  (property) age: typeof Schema.Number
age: import Schema
Schema.(alias) class Number
export Number
Number
6
})
7

8
// Create a pretty printer for the schema
9
const const PersonPretty: (a: {
    readonly name: string;
    readonly age: number;
}) => string
PersonPretty = import Pretty
Pretty.const make: <{
    readonly name: string;
    readonly age: number;
}, {
    readonly name: string;
    readonly age: number;
}, never>(schema: Schema.Schema<{
    readonly name: string;
    readonly age: number;
}, {
    readonly name: string;
    readonly age: number;
}, never>) => (a: {
    readonly name: string;
    readonly age: number;
}) => string
make(const Person: Schema.Struct<{
    name: typeof Schema.String;
    age: typeof Schema.Number;
}>
Person)
10

11
// Format and print a Person object
12
namespace console
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.

The module exports two specific components:

* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.

_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.

Example using the global `console`:

```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```

Example using the `Console` class:

```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```
console.(method) Console.log(message?: any, ...optionalParams: any[]): voidPrints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).

```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```

See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.
log(const PersonPretty: (a: {
    readonly name: string;
    readonly age: number;
}) => string
PersonPretty({ (property) name: string
name: "Alice", (property) age: number
age: 30 }))
13
/*
14
Output:
15
'{ "name": "Alice", "age": 30 }'
16
*/

Customizing Pretty Printer Generation

You can customize how the pretty printer formats output by using the pretty annotation within your schema definition.

The pretty annotation takes any type parameters provided (typeParameters) and formats the value into a string.

Example (Custom Pretty Printer for Numbers)

1
import { import Pretty
Pretty, import Schema
Schema } from "effect"
2

3
// Define a schema with a custom pretty annotation
4
const const schema: Schema.SchemaClass<number, number, never>
schema = import Schema
Schema.(alias) class Number
export Number
Number.(method) Annotable<SchemaClass<number, number, never>, number, number, never>.annotations(annotations: Schema.Annotations.Schema<number, readonly []>): Schema.SchemaClass<number, number, never>Merges a set of new annotations with existing ones, potentially overwriting
any duplicates.
annotations({
5
  (property) Annotations.Schema<number, readonly []>.pretty?: Pretty.PrettyAnnotation<number, readonly []>
pretty: (/**typeParameters**/) => ((parameter) value: number
value) => `my format: ${(parameter) value: number
value}`
6
})
7

8
// Create the pretty printer
9
const const customPrettyPrinter: (a: number) => string
customPrettyPrinter = import Pretty
Pretty.const make: <number, number, never>(schema: Schema.Schema<number, number, never>) => (a: number) => string
make(const schema: Schema.SchemaClass<number, number, never>
schema)
10

11
// Format and print a value
12
namespace console
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.

The module exports two specific components:

* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.

_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.

Example using the global `console`:

```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```

Example using the `Console` class:

```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```
console.(method) Console.log(message?: any, ...optionalParams: any[]): voidPrints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).

```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```

See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.
log(const customPrettyPrinter: (a: number) => string
customPrettyPrinter(1))
13
// Output: "my format: 1"