dry-types 1.7

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Maybe

The dry-monads gem provides an approach to handling optional values by returning a Maybe object from operations that can return nil.

dry-types has an extension that can return Maybes from calls to types. That is, it wraps the return result in either:

  • a Some object (with the resulting value)
  • a None object (when the value would be nil)

NOTE: You must require 'dry-monads', and include Dry::Monads[:maybe]

  1. Load dry-monads with Maybe and the dry-types :maybe extension in your application.
require 'dry-monads'
include Dry::Monads[:maybe] # This should be inside your class
require 'dry-types'

Dry::Types.load_extensions(:maybe)
Types = Dry.Types()
  1. Append .maybe to a Type to return a Maybe object
Types::Strict::Integer.maybe[nil]     # => None
Types::Strict::Integer.maybe[123]     # => Some(123)

Types::Coercible::String.maybe[nil]   # => None
Types::Coercible::String.maybe[123]   # => Some("123")

Types::Coercible::Float.maybe[nil]    # => None
Types::Coercible::Float.maybe['12.3'] # => Some(12.3)

Types::Strict::String.maybe[123]      # => raises Dry::Types::ConstraintError
Types::Strict::Integer.maybe["foo"]   # => raises Dry::Types::ConstraintError

If you want to capture the errors (e.g. to return messages) instead of raising them, you may want to use the :monads extension instead, which returns a Result.

Or, if you prefer, instead of calling .maybe you can use the Maybe:: namespaced types instead.

The following examples are identical to the ones above:

Types::Maybe::Strict::Integer[nil]     # => None
Types::Maybe::Strict::Integer[123]     # => Some(123)

Types::Maybe::Coercible::String[nil]   # => None
Types::Maybe::Coercible::String[123]   # => Some("123")

Types::Maybe::Coercible::Float[nil]    # => None
Types::Maybe::Coercible::Float['12.3'] # => Some(12.3)

Types::Maybe::Strict::String[123]      # => raises Dry::Types::ConstraintError
Types::Maybe::Strict::Integer["foo"]   # => raises Dry::Types::ConstraintError

Mapping methods on Maybe

Since these are dry-monads Maybe objects, you can #fmap methods to them: applying the method to the value when the value is Some (and keeping the None when the value is None).

You can #fmap these, and then use #value_or to return the underlying value out of a Some or return a default when the value is None.

maybe_string = Types::Strict::String.maybe
maybe_string[nil]                 # => None
maybe_string[nil].fmap(&:upcase)  # => None
maybe_string['something']                                    # => Some('something')
maybe_string['something'].fmap(&:upcase)                     # => Some('SOMETHING')
maybe_string['something'].fmap(&:upcase).value_or('NOTHING') # => "SOMETHING"

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