CODEWITHSUNDEEP
haskell
user2975699
user2975699

Reputation: 595

How should a error monad be defined

data Error a = OK a | Error String
instance Monad Error where
  return = OK

How should i define bind in this case? >>= : Error a -> (a->error b) -> error b this is the type of the function bind for this case i hope it's correct.

 OK a >>= f       =               (don't know where to begin need some help) 
 Error err >>=f   =               (in this case can i just return error like = Error err)

I also want to know if someone can explain why these usage of seq are problematic. You can find them on slide(page) 10 Usage of seq http://fileadmin.cs.lth.se/cs/Education/EDAN40/lectures/Parallelism.pdf hittendInside xy=someFunc(x 'seq' y) why is this problematic forexamble is it because seq won't return x it will only evaluate it and will only return y. Is this the case in the other two exambles also?

Upvotes: 1

Views: 158

Answers (2)

jamshidh
jamshidh

Reputation: 12060

Think of how you would want a pipeline of functions to work

---->f---->g---->h---->
     |     |     |
     V     V     V
    Err   Err   Err

If no error occurs, the data should just be a straight shot through

OK x---->remove OK---->f---->remove OK---->g---->remove OK---->h---->

This can be realized by

OK x >>= f = f x

If at any point you hit an error, the calculation should stop

OK x---->remove OK---->f---->Error (Stop!)

This can be realized by

Error message >>= f = Error message

Incidentally, you don't have to write this, it already exists under the name "Either"

data Either a b = Left a | Right b

The monad was set up so that "Right" passes data through (get it- "Right" means "correct"), and Left stops the data (ie- it is "Error"). If you don't want a message, use "Maybe".

Note that this works a lot like exceptions in Java.

Upvotes: 9

Lee
Lee

Reputation: 144136

For your OK case, you have an a and a function (a -> Error b) and need to return a value of type Error b. You can simply apply your function f to the value a of your OK:

OK a >>= f = f a
Error err >>= f = Error err

Upvotes: 1

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