Maybe
The Maybe monad is used when a series of computations could return nil at any point.
bind
Applies a block to a monadic value. If the value is Some then calls the block passing the unwrapped value as an argument. Returns itself if the value is None.
require 'dry-monads'
M = Dry::Monads
maybe_user = M.Maybe(user).bind do |u|
M.Maybe(u.address).bind do |a|
M.Maybe(a.street)
end
end
# If user with address exists
# => Some("Street Address")
# If user or address is nil
# => None()
# You also can pass a proc to #bind
add_two = -> (x) { M.Maybe(x + 2) }
M.Maybe(5).bind(add_two).bind(add_two) # => Some(9)
M.Maybe(nil).bind(add_two).bind(add_two) # => None()
fmap
Similar to bind but works with blocks/methods that returns unwrapped values (i.e. not Maybe instances).
require 'dry-monads'
Dry::Monads::Maybe(user).fmap(&:address).fmap(&:street)
# If user with address exists
# => Some("Street Address")
# If user or address is nil
# => None()
value!
You always can extract the result by calling value!. It will raise an error if you call it on None. You can use value_or for safe unwrapping.
require 'dry-monads'
Dry::Monads::Some(5).fmap(&:succ).value! # => 6
Dry::Monads::None().fmap(&:succ).value!
# => Dry::Monads::UnwrapError: value! was called on None
value_or
Has one argument, unwraps the value in case of Some or returns the argument value back in case of None. It's a safe and recommended way of extracting values.
require 'dry-monads'
M = Dry::Monads
add_two = -> (x) { M.Maybe(x + 2) }
M.Maybe(5).bind(add_two).value_or(0) # => 7
M.Maybe(nil).bind(add_two).value_or(0) # => 0
M.Maybe(nil).bind(add_two).value_or { 0 } # => 0
or
The opposite of bind.
require 'dry-monads'
M = Dry::Monads
add_two = -> (x) { M.Maybe(x + 2) }
M.Maybe(5).bind(add_two).or(M.Some(0)) # => Some(7)
M.Maybe(nil).bind(add_two).or(M.Some(0)) # => Some(0)
M.Maybe(nil).bind(add_two).or { M.Some(0) } # => Some(0)