Add MonadDeclare about mtl HOT 5 CLOSED

It is possible, when working directly in DeclareT, to erase declared values.

ghci> execDeclare [] $ do
  declare [1]
  DeclareT $ const $ return ([], ())
[]

Now, I see that the laws are carefully constructed such that this isn't a violation of any sort. But it does make me question the usefulness of those laws. When given an arbitrary MonadDeclare d m => m a, you have no idea if it behaves as a state monad or a declare monad, as it might wipe any declarations you make before using it. I imagine this should be seen as a problem?

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@ElvishJerricco I suspect you didn't actually run that in GHCi:

• execDeclare receives arguments in different order;
• DeclareT $ const $ return ([], ()) is essentially declare [];
• the output is actually [1], as expected.

Here's what I get:

$ stack ghci
...
>>> flip execDeclare [] $ do { declare [1]; DeclareT $ const $ return ([], ()); }
[1]

I believe the laws are useful and although you made me doubt it, I still think they don't allow any MonadDeclare instance to erase output. However they probably might be better and/or clearer and I am open to suggestions!

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Oh geez you're right. That's what I get for browsing github issues on my phone =P My bad!

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Starting with transformers-0.5.4.0 there's Control.Monad.Trans.Accum which is DeclareT under a different (more general) name — AccumT.

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Since AccumT is now in transformers, and #44 is an open issue, I am going to close this. Anyone feel free to re-open or ping to re-open if I'm missing something.

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