Change semantics for IterT's Alternative and MonadPlus instances about free HOT 9 CLOSED

We can do that with a major version bump and a clearly worded CHANGELOG entry. I don't think anybody but me has adopted IterT yet.

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On the other hand, this new instances are not fully satisfactory. Ideally, if the underlying monad implements MonadPlus, the first computation which completes and does so without errors should be returned.

The instance for plain IterT Identity (which is not MonadPlus) is quite straightforward, and causes no conflicts. I'll add that one first, and then think whether it's possible to make IterT into a true MonadPlus transformer.

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The problem with making instances for IterT Identity is that it precludes any general solution by overlap and requires a FlexibleInstance, which in my experience makes its selection brittle.

Of course we can make a separate Control.Monad.Iter that has a concrete Iter with that instance, which would let us have a nice, explainable

data Iter a = Stop a | Step (Iter a)

with obvious racing.

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The behaviour described above is now in commit 7b3cbcb , on the iter-monadplus branch.

The approach is similar to the one for the ErrorT monad transformer, where the behaviour of the underlying monad is disregarded when implementing MonadPlus.

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I'm cautiously optimistic about the new change. The main question I'd have is how to get the word out about it.

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How about some explanation at the top of the Haddock page, and an usage example? Together with the major version bump, it could be enough for interested users to notice.

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That sounds like a plan to me. Feel free to merge into master.

On Wed, Jul 9, 2014 at 11:21 AM, Víctor López Juan <[email protected]

wrote:

How about some explanation at the top of the Haddock page, and an usage
example? Together with the major version bump, it could be enough for
interested users to notice.

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#52 (comment).

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• Documentation
• Change log and version bump
• New instance

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Have I mentioned that you, sir, are awesome?

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