Re: [Haskell-cafe] Monad transformers

Roman Cheplyaka wrote:

* Andrew Coppin [2010-07-03 15:07:17+0100]

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In my experience, defining a type representing several stacked monad transformers is the easy part.

Of course it is. It wasn't my intention just to show you how easy it is to define a newtype in Haskell :)

I just showed you a monad stack which is successfully used in xmonad -- and you really need to read the code a bit to get the taste of it.

OK, fair enough then.

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The hard part is figuring out how in the name of God to run the resulting computation

It is run just in the one place, so you don't need to think about it each time you do some changes.

As I say, every time I've tried to do this, I end up writing a function to "run this stuff", and it typically takes a few hours to reach the point where it type-checks.

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or how to access functions burried at various levels of the stack.

See above: -- Dynamic components may be retrieved with 'get', static components -- with 'ask'.

So you use ask to get some configuration variable (reader monad is used for configuration in xmonad) and get/put/modify to deal with dynamic state of application. You use liftIO (abbreviated to 'io') to run IO computations.

In other words, somebody has written a combinatorial explosion of class instances to automate some of the lifting.

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From what I've seen, it usually ends up being faster and easier to just define a custom monad that does exactly what you want, and then use that.

In which way is it faster and easier?

It's faster and easier to write the code because I don't have to spend multiple hours trying to work out how to make it type-check. Whether it's any faster at run-time, I have no idea...