Try
Rescues a block from an exception. The Try
monad is useful when you want to wrap some code that can raise exceptions of certain types. A common example is making an HTTP request or querying a database.
require 'dry/monads'
class ExceptionalLand
include Dry::Monads[:try]
def call
res = Try { 10 / 2 }
res.value! if res.value?
# => 5
res = Try { 10 / 0 }
res.exception if res.error?
# => #<ZeroDivisionError: divided by 0>
# By default Try catches all exceptions inherited from StandardError.
# However you can catch only certain exceptions like this
Try[NoMethodError, NotImplementedError] { 10 / 0 }
# => raised ZeroDivisionError: divided by 0 exception
end
end
It is better if you pass a list of expected exceptions which you are sure you can process. Catching exceptions of all types is considered bad practice.
The Try
monad consists of two types: Value
and Error
. The first is returned when code did not raise an error and the second is returned when the error was captured.
bind
Allows you to chain blocks that can raise exceptions.
Try[NetworkError, DBError] { grap_user_by_making_request }.bind { |user| user_repo.save(user) }
# Possible outcomes:
# => Value(persisted_user)
# => Error(NetworkError: request timeout)
# => Error(DBError: unique constraint violated)
fmap
Works exactly the same way as Result#fmap
does.
require 'dry/monads'
class ExceptionalLand
include Dry::Monads[:try]
def call
Try { 10 / 2 }.fmap { |x| x * 3 }
# => 15
Try[ZeroDivisionError] { 10 / 0 }.fmap { |x| x * 3 }
# => Failure(ZeroDivisionError: divided by 0)
end
end
value!
and exception
Use value!
for unwrapping a Success
and exception
for getting error object from a Failure
.
to_result
and to_maybe
Try
's Value
and Error
can be transformed to Success
and Failure
correspondingly by calling to_result
and to Some
and None
by calling to_maybe
. Keep in mind that by transforming Try
to Maybe
you lose the information about an exception so be sure that you've processed the error before doing so.