[Haskell-cafe] Tutorial: Haskell for the Evil Genius

[Joel Burget]

I find the Monad instance for Maybe and Either very useful. You can do things like the following (which technically only uses the Applicative instance):

Prelude Control.Applicative> (*3) <$> (+2) <$> Just 1
Just 9
Prelude Control.Applicative> (*3) <$> (+2) <$> Nothing
Nothing
Prelude Control.Applicative> (*3) <$> (+2) <$> Left "error" :: Either String Int
Left "error"
Prelude Control.Applicative> (*3) <$> (+2) <$> Right 1 :: Either String Int
Right 9



Also, Maybe and Either are not "implemented as monads". They are defined using `data` like you suggest:

data Maybe a = Nothing | Just a
data Either a b = Left a | Right b

You can even look up their definitions using ghci using `:i Either` or `:i Maybe`. Monad comes in because they're both instances of the Monad type class.

Take a look at the list monad for another one that doesn't modify state or communicate with an outside interface.

- Joel 

On Friday, September 14, 2012 at 6:08 PM, Andrew Pennebaker wrote:

> > Everyone in the Haskell cafe probably has a secret dream to give the
> > best "five minute monad talk."  Challenge: get someone to have a
> > competition at one of the conferences where students all give their
> > best "five minute monad talk" and try to find the most comprehensible
> > one!
> 
> Haha, maybe that's why I'm writing.
> 
> Agree on all points, not just this quotation.
> 
> Yeah, IO and Maybe are the first monads most new Haskell programmers encounter.  Perhaps a tour of RVars or the accelerate library would give a better impression. I bet a lot of students get the concept of pure functional programming, and if you shock them with: "So how would you implement a PRNG?", they would understand the role monads play. 
> 
> Given that Maybe and Either don't modify state, nor do they communicate with outside interfaces, nor do they specify computation ordering, I don't understand why they're implemented as monads. Why not a primitive typeclass or even datatype declaration? 
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