@gucciogucci/contented

A library to coerce values at run-time.

Table of Contents

• Introduction
• Reference
  • Coercing
    • coerceTo(T, input)
  • Primitive types
    • string
    • number
    • boolean
  • Object types
    • at(path, T)
    • fallback(T, substitute)
  • Array types
    • arrayOf(T)
    • permissiveArrayOf(T)
  • Narrowing
    • match(value)
    • always(value)
  • Combinations & Alternatives
    • combine(fn, ...Ts)
    • combineIntoObject({ [...Keys]: [...Ts] })
    • T1.or(T2)
    • optional(T)
  • Errors
    • InvalidCoercion
    • AtKey<InvalidCoercion>
    • MissingKey
    • Joint<[...Errs]>
  • Utility types
    • Infer<typeof T>
• License

Introduction

Contented is a TypeScript library for performing type coercion at run-time. To this end, Contented introduces run-time representations of primitive types, such as string, which can be then mixed and matched to describe compound types.

import { string, number, at, combine, coerceTo } from '@gucciogucci/contented';

const Image = combine(
  (url, size) => ({ url, size }),
  at('url', string),
  at(['metadata', 'size'], number)
);

const image = coerceTo(Image, data /* arbitrary data */);

Contented may be useful every time there are expectations — but no real guarantees, on the shape of data acquired at run-time. Common use cases include processing data coming over the wire, from files, or any other external source.

Reference

Coercing

coerceTo(T, input)

Attempts to coerce the input data to the type represented by T. Note that the specific return value, whether successful or not, depends on the particular T.

import { string, coerceTo } from '@gucciogucci/contented';

coerceTo(string, 'hello');
// 'hello'

coerceTo(string, 42);
// InvalidCoercion { expected: 'string', got: 42 }

Primitive types

string

A run-time representation of the string type. An attempt to coerce to string may result in either the string itself (if the input data is indeed a string) or an InvalidCoercion error.

import { string, coerceTo } from '@gucciogucci/contented';

coerceTo(string, 'hello');
// 'hello'

coerceTo(string, 42);
// InvalidCoercion { expected: 'string', got: 42 }

number

A run-time representation of the number type. An attempt to coerce to number may result in either the number itself (if the input data is indeed a number) or an InvalidCoercion error.

import { number, coerceTo } from '@gucciogucci/contented';

coerceTo(number, 42);
// 42

coerceTo(number, 'hello');
// InvalidCoercion { expected: 'number', got: 'hello' }

boolean

A run-time representation of the boolean type. An attempt to coerce to boolean may result in either the boolean itself (if the input data is indeed a boolean) or an InvalidCoercion error.

import { boolean, coerceTo } from '@gucciogucci/contented';

coerceTo(boolean, true);
// true

coerceTo(boolean, 'hello');
// InvalidCoercion { expected: 'boolean', got: 'hello' }

Object types

at(path, T)

Constructs a run-time type that expects the input data to be an object such that there exists a value of type T under the keys specified in path.

import { string, at, coerceTo } from '@gucciogucci/contented';

const stringAtAB = at(['a', 'b'], string);

coerceTo(stringAtAB, { a: { b: 'hello' } });
// 'hello'

coerceTo(stringAtAB, { a: { c: 'hello' } });
// MissingKey { missingKey: [ 'a', 'b' ] }

coerceTo(stringAtAB, 'hello');
// InvalidCoercion { expected: 'object', got: 'hello' }

When the path consists of a single key, such a key can be supplied without the need of enclosing it in an array.

coerceTo(at('a', string), { a: 'hello' });
// 'hello'

fallback(T, substitute)

fallback works in tandem with at to provide a fallback value in case the input data does not contain the specified keys. Apart from removing the possibility of a MissingKey error, fallback retains the same behavior as the at it wraps.

import { number, at, fallback, coerceTo } from '@gucciogucci/contented';

const numberAtAB = fallback(at(['a', 'b'], number), 42);

coerceTo(numberAtAB, { a: { c: 3 } });
// 42

coerceTo(numberAtAB, { a: { b: 3 } });
// 3

Array types

arrayOf(T)

A run-time representation of an array of Ts, where T denotes the run-time representation of its element type.

import { number, arrayOf, coerceTo } from '@gucciogucci/contented';

coerceTo(arrayOf(number), [3, 4, 5]);
// [ 3, 4, 5 ]

coerceTo(arrayOf(number), 'hello');
// InvalidCoercion { expected: 'array', got: 'hello' }

coerceTo(arrayOf(number), [3, 'a', 5]);
// AtKey { atKey: [ 1 ], error: InvalidCoercion { expected: 'number', got: 'a' } }

permissiveArrayOf(T)

A run-time representation of an array of Ts, where T denotes the run-time representation of its element type.

The distinctive feature of a permissiveArrayOf(T) is that it skips elements that are not recognized as T. This is different from arrayOf(T), which instead stops as soon as one element is not recognized.

import { number, permissiveArrayOf, coerceTo } from '@gucciogucci/contented';

coerceTo(permissiveArrayOf(number), [3, 4, 5]);
// [ 3, 4, 5 ]

coerceTo(permissiveArrayOf(number), [3, 'a', 5]);
/* [
     [ 3, 5 ],
     [ AtKey { atKey: [ 1 ], error: InvalidCoercion { expected: 'number', got: 'a' } } ]
   ]
*/

Narrowing

match(value)

A run-time representation of the narrowest type that can be constructed from value. Hence, coercions to match(value) succeed only when value is provided as an input.

import { match, coerceTo } from '@gucciogucci/contented';

coerceTo(match('hello'), 'hello');
// 'hello'

coerceTo(match('hello'), 'foo');
// InvalidCoercion { expected: 'hello', got: 'foo' }

always(value)

A run-time type that always succeeds with value regardless of the input data.

import { always, coerceTo } from '@gucciogucci/contented';

coerceTo(always(20), 'hello');
// 20

coerceTo(always(20), false);
// 20

Combinations & Alternatives

combine(fn, ...Ts)

combine constructs a run-time type from some known run-time types Ts and a function fn. Coercing to combine(fn, ...Ts) results in an attempt to coerce the input data to each type specified in Ts; if every coercion ends up successful, the resulting values are passed to the function fn.

import { combine, string, coerceTo } from '@gucciogucci/contented';

const User = combine(
  (name, surname, phone) => ({ fullname: `${name} ${surname}`, phone }),
  at('name', string),
  at('surname', string),
  at(['contacts', 'phone'], string)
);

coerceTo(User, {
  name: 'John',
  surname: 'Smith',
  contacts: {
    phone: '055-123404',
    email: '[e-mail]',
  }
});
// { fullname: 'John Smith', phone: '055-123404' }

coerceTo(User, { name: 42 });
// AtKey { atKey: [ 'name' ], error: InvalidCoercion { expected: 'string', got: 42 } }

combineIntoObject({ [...Keys]: [...Ts] })

combineIntObject is a convenience function that may be used every time there is the need of combining some known run-time types Ts into an object of known keys. In other words, instead of writing the following:

import { string, number, at, combine, coerceTo } from '@gucciogucci/contented';

const Image = combine(
  (url, size) => ({ url, size }),
  at('url', string),
  at(['metadata', 'size'], number)
);

const image = coerceTo(Image, data);

One may simply write:

import { string, number, at, combineIntoObject, coerceTo } from '@gucciogucci/contented';

const Image = combineIntoObject({
  url: at('url', string),
  size: at(['metadata', 'size'], number)
});

const image = coerceTo(Image, data);

T1.or(T2)

A run-time representation of the union type T1 | T2. In case of a failed coercion, the result encloses the errors coming from both T1 and T2.

import { string, number, at, coerceTo } from '@gucciogucci/contented';

coerceTo(string.or(number), 'hello');
// 'hello'

coerceTo(string.or(number), true);
/* Joint {
    errors: [
      InvalidCoercion { expected: 'string', got: true },
      InvalidCoercion { expected: 'number', got: true }
    ]
   }
*/

coerceTo(string.or(at('a', number)), { a: true });
/* Joint {
     errors: [
       InvalidCoercion { expected: 'string', got: { a: true } },
       AtKey { atKey: [ 'a' ], InvalidCoercion { expected: 'number', got: true } }
     ]
   }
*/

optional(T)

Marks the type T as optional. That is, not only optional(T) succeeds for every valid value of type T, but also when undefined is passed as an input. It is equivalent to T.or(always(undefined)).

import { optional, string, coerceTo } from '@gucciogucci/contented';

coerceTo(optional(string), undefined);
// undefined

Errors

InvalidCoercion

When the input data does not conform to the expected primitive type, coerceTo returns a InvalidCoercion, which contains both the expectation and the actual value.

import { string, coerceTo } from '@gucciogucci/contented';

coerceTo(string, 42);
// InvalidCoercion { expected: 'string', got: 42 }

AtKey<InvalidCoercion>

An InvalidCoercion error, together with the path at which to find the non-conforming data.

import { number, arrayOf, at, coerceTo } from '@gucciogucci/contented';

coerceTo(at('x', number), { x: 'hello' });
// AtKey { atKey: [ 'x' ], error: InvalidCoercion { expected: 'number', got: 'hello' } }


coerceTo(arrayOf(number), [3, 'a', 5]);
// AtKey { atKey: [ 1 ], error: InvalidCoercion { expected: 'number', got: 'a' } }

MissingKey

The path at which a non-existing key in the input data was instead expected.

import { number, at, coerceTo } from '@gucciogucci/contented';

coerceTo(at('x', number), { y: 12 });
// MissingKey { missingKey: [ 'x' ] }

Joint<[...Errs]>

When multiple alternatives are provided but none of them is applicable to the input data, coerceTo returns a Joint error, reporting the errors resulting from the different failed attempts.

import { string, number, coerceTo } from '@gucciogucci/contented';

coerceTo(string.or(number), true);
/* Joint {
    errors: [
      InvalidCoercion { expected: 'string', got: true },
      InvalidCoercion { expected: 'number', got: true }
    ]
   }
*/

Utility types

Infer<typeof T>

Infer comes in handy every time it is necessary to infer the compile-time type corresponding to some run-time representation T.

import { Infer, coerceTo } from '@gucciogucci/contented';

const User = combine(
  (name, surname, phone) => ({ fullname: `${name} ${surname}`, phone }),
  at('name', string),
  at('surname', string),
  at(['contacts', 'phone'], string)
);

function fn(user: Infer<typeof User>) {
  // here, user : { fullname: string, phone: string }
}

License

Copyright 2022 Gucci.

Licensed under the GNU Lesser General Public License, Version 3.0