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Comment by smadge | Hacker News Reader

I’m having trouble understanding what you mean by an option type that is not a monad.

I thought the value of an option type was being able to “program the semicolon” to do automatic null checks for you.

Option type is just an algebraic data type.

    datatype 'a option = None | Some of 'a

Which is awfully similar to Haskell's Maybe. Ignoring boilerplate and comments, the entire module is just [0]:

  data Maybe a = Nothing | Just a
    deriving ( Eq, Ord )

And the deriving line isn't really nessasary.

The Monad module is larger and contains code specific to Maybe [1]:

  instance  Monad Maybe  where
    (Just x) >>= k      = k x
    Nothing  >>= _      = Nothing
    (>>) = (*>)
    fail _              = Nothing

(Only the first 2 lines are needed. The 3rd is an optimization, the 4th is a historical wart that will be removed in future versions (moved to MonadFail)

[0] https://hackage.haskell.org/package/base-4.12.0.0/docs/src/G...

[1] https://hackage.haskell.org/package/base-4.12.0.0/docs/src/G...

And this is meaningful only in the context of a lazy language.

In the context of strict, statically typed language an algebraic Option type is good-enough.

Maybe is an algebraic Option type. It just happens to implement an interface that abstracts away the if(None) checks that you would be doing anyway. Lazyness has nothing to do with it.

The option monad is literally isomorphic to maybe. It 100% follows the definition of a monad and is an instance of the monad type class in Cats and Scalaz.

Many algebraic data types are monads. I’m not sure why you think that is a contradiction.

This isomorphism has no meaning. It is also isomorphic to logical OR, if you wish. Shall we wrap OR into a Monad?

I think you're overthinking it. The isomorphism simply exists. And you don't wrap things into a monad, you just write an instance for it -- it has no effect on the data type itself, it simply allows to use the data type in monadic contexts.

I think you mean of Either. Either is isomorphic to logical OR in type theory.

Yes, and that algebraic data type you defined is a monad, whether you acknowledge and use that fact or not.

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