Try

The Try monad is a layer between object-oriented exceptions and functional-programming monads. You give it a proc that might raise; Try runs it and captures either the result or the exception. You then convert the outcome into a Maybe or an Either and chain from there.

Creating a Try

Pass a proc to new. It is executed immediately:

Monads::Try(Float64).new(->{ 10.0 / 2 })

Converting to Either

to_either produces a Right(Exception, T) on success or a LeftException(T) if the proc raised:

Monads::Try(Float64).new(->{ 10.0 / 2 }).to_either
# => Right(Exception, Float64) with value 5.0

Monads::Try(Float64).new(->{ raise DivisionByZeroError.new }).to_either
# => LeftException(Float64) carrying the exception

Converting to Maybe

to_maybe produces a Just on success or Nothing if the proc raised:

Monads::Try(Int32).new(->{ 42 }).to_maybe       # => Just(42)
Monads::Try(Int32).new(->{ raise "boom" }).to_maybe # => Nothing(Int32)

Why use it?

Try lets you bring exception-throwing code (file I/O, parsing, arithmetic) into a monadic pipeline without scattering begin/rescue. Convert once, then use the shared chaining interface:

Monads::Try(Int32).new(->{ Int32.new("123") })
  .to_maybe
  .fmap(->(x : Int32) { x * 2 })
  .value_or(0) # => 246

For concurrent execution of the same idea, see Task. For the full method list, see the Try API reference.