Using Arrows for Dependency Handling

http://jaspervdj.be/posts/2010-03-26-arrows-dependencies.html

by nudded • 16 years ago

14 3 16 years ago

leif2• 16 years ago

If anyone can explain to me why monads are insufficient, that would be swell. I'm really not going to get it on my own any time soon.

jaspervdj• 16 years ago

Author of the article here. I'll try to give a quick informal explanation. If you look at the type signature of `>>=`:

  m a -> (a -> m b) -> m b

Let's suppose there's a HakyllAction monad. The >>= signature would be:

  HakyllAction a -> (a -> HakyllAction b) -> HakyllAction b

If we implement this operator, we get a value `HakyllAction a` and a function `a -> HakyllAction b`. We can look at the dependencies of the value, but we don't have access to the dependencies of the `a -> HakyllAction b`. If we want that, we need to apply it to the first value.

This goes against the idea of dependency handling -- we want to know the dependencies first, and then only execute the function if needed.

Hope this helps.

leif2• 16 years ago

This helps very much, thanks! :)

Using Arrows for Dependency Handling

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