sorbet-coerce

A type coercion lib works with Sorbet's static type checker and type definitions; raises an error if the coercion fails.

It provides a simple and generic way of coercing types in a sorbet-typed project. It is particularly useful when we're dealing with external API responses and controller parameters.

1. Follow the steps here to set up the latest version of Sorbet and run srb tc.
2. Add sorbet-coerce to your Gemfile and install them with Bundler.

# -- Gemfile --

gem 'sorbet-coerce'

❯ bundle install

TypeCoerce takes a valid sorbet type and coerce the input value into that type. It'll return a statically-typed object or throws errors when the coercion process cannot be handled as expected (more details in the Errors section).

converted = TypeCoerce[<Type>].new.from(<value>)

T.reveal_type(converted) # <Type>

• Simple Types
• Custom Types: If the values can be coerced by .new
• T.untyped (an escape hatch to ignore & return the given value)
• T::Boolean
• T::Enum
• T.nilable(<supported type>)
• T::Array[<supported type>]
• T::Hash[<supported type>, <supported type>]
• T::Set[<supported type>]
• T.any(<supported type>, ...)
• Subclasses of T::Struct

We don't support

• Experimental features (tuples and shapes)
• passing in variables as types (ex TypeCoerce[var]) - please use at your own risk!
  • for more information, see this Slack thread in the Sorbet Slack

• Simple Types

TypeCoerce[T::Boolean].new.from('false')
# => false

TypeCoerce[T::Boolean].new.from('true')
# => true

TypeCoerce[Date].new.from('2019-08-05')
# => #<Date: 2019-08-05 ((2458701j,0s,0n),+0s,2299161j)>

TypeCoerce[DateTime].new.from('2019-08-05')
# => #<DateTime: 2019-08-05T00:00:00+00:00 ((2458701j,0s,0n),+0s,2299161j)>

TypeCoerce[Float].new.from('1')
# => 1.0

TypeCoerce[Integer].new.from('1')
# => 1

TypeCoerce[String].new.from(1)
# => "1"

TypeCoerce[Symbol].new.from('a')
# => :a

TypeCoerce[Time].new.from('2019-08-05')
# => 2019-08-05 00:00:00 -0700

• T.nilable

TypeCoerce[T.nilable(Integer)].new.from('')
# => nil
TypeCoerce[T.nilable(Integer)].new.from(nil)
# => nil
TypeCoerce[T.nilable(Integer)].new.from('')
# => nil

The behaviour for converting '' for the T.nilable(String) type depends on an option flag called coerce_empty_to_nil (new in v0.6.0):

# default behaviour
TypeCoerce[T.nilable(String)].new.from('')
# => ""

# using the coerce_empty_to_nil flag
TypeCoerce[T.nilable(String)].new.from('', coerce_empty_to_nil: true)
# => nil

• T::Array

TypeCoerce[T::Array[Integer]].new.from([1.0, '2.0'])
# => [1, 2]

• T::Struct

class Params < T::Struct
  const :id, Integer
  const :role, String, default: 'wizard'
end

TypeCoerce[Params].new.from({id: '1'})
# => <Params id=1, role="wizard">

More examples: nested params

We will get CoercionError, ShapeError, or TypeError when the coercion doesn't work successfully.

It raises a coercion error when it fails to convert a value into the specified type (i.e. 'bad string args' to Integer). This can be configured globally or at each call-site. When configured to true, it will fill the result with nil instead of raising the errors.

TypeCoerce::Configuration.raise_coercion_error = false # default to true

We can use an inline flag to overwrite the global configuration:

TypeCoerce[T.nilable(Integer)].new.from('abc', raise_coercion_error: false)
# => nil

It raises a shape error when the shape of the input does not match the shape of input type (i.e. '1' to T::Array[Integer] or to T::Struct). This cannot be configured and always raise an error.

It raises a type error when the coerced input does not match the input type. This error is raised by Sorbet and can be configured through T::Configuration.

In an environment where type errors and coercion errors are configured to be silent (referred to as soft errors), when the coercion fails, TypeCoerce will fill the result with nil instead of actually raising the errors (referred to hard errors).

With hard errors,

class Params < T::Struct
  const :a, Integer
end

TypeCoerce[Integer].new.from(nil)
# => TypeError Exception: T.let: Expected type Integer, got type NilClass

TypeCoerce[Integer].new.from('abc')
# => TypeCoerce::CoercionError Exception: Could not coerce value ("abc") of type (String) to desired type (Integer)

TypeCoerce[T.nilable(Integer)].new.from('abc', raise_coercion_error: false)
# => nil

TypeCoerce[Params].new.from({a: 'abc'}, raise_coercion_error: false)
# => TypeError Exception: Parameter 'a': Can't set Params.a to nil (instance of NilClass) - need a Integer

With soft errors,

TypeCoerce[Integer].new.from('abc', raise_coercion_error: false)
# => nil

TypeCoerce[Params].new.from({a: 'abc'}, raise_coercion_error: false) # require sorbet version ~> 0.4.4948
# => <Params a=nil>

TypeCoerce[Params].new.from({a: 'abc'}, raise_coercion_error: true)
# TypeCoerce::CoercionError Exception: Could not coerce value ("abc") of type (String) to desired type (Integer)

Sorbet-coerce is designed in the context of web development. When coercing into a T::Struct, the values that need to be coerced are often JSON-like. Suppose we send a JavaScript object

json_js = {"a": "1", "null_field": null, "blank_field": "", "missing_key": undefined} // javascript

to the server side and get a JSON hash

json_rb = {"a" => "1", "null_field" => nil, "blank_field" => ""} # ruby, note `missing_key` is removed from the hash

We expect the object to have shape

class Params < T::Struct
  const :a, Integer
  const :null_field, T.nilable(Integer)
  const :blank_field, T.nilable(Integer)
  const :missing_key, T::Array[Integer], default: []
end

Then we coerce the object json_rb into an instance of Params.

params = TypeCoerce[Params].new.from(json_rb)
# => <Params a=1, blank_field=nil, missing_key=[], null_field=nil>

• When json_js["null_field"] is null, params.null_field is nil
• When json_js["blank_field"] is "", params.blank_field is nil
• When json_js["missing_key"] is undefined, params.missing_key will use the default value []

Contributions and ideas are welcome! Please see our contributing guide and don't hesitate to open an issue or send a pull request to improve the functionality of this gem.

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to opensource@chanzuckerberg.com.

This project is licensed under MIT.