Add specialization of `retract` for `Iter ~ IterT Identity`

[vlopezj]

When using the Iter monad, it is common to, at some point, run the computation until it terminates by using runIdentity . retract.

This could be implemented as a separate function, named, for example unsafeIter :: Iter a -> a, in the same spirit as fromMaybe :: Maybe a -> a or unsafePerformIO :: IO a -> a. The unsafe monicker warns that the safety guarantees from the Iter monad are lost, namely, evaluation might not terminate.

• Implementation
• Usage example

Any input on the name, the implementation or the usefulness of the proposed function is welcome.

[ekmett]

I'm not so keen on the 'unsafe' name, but I have no objection to adding an operation with a name that clearly indicates it runs to completion.

[vlopezj]

How about whileRight or untilLeft? Either standalone, or prefixed with run, runIter or iter.

The probability package uses untilLeft [1]. According to Google, the other alternatives are not used by any other Hackage package.

[1] https://hackage.haskell.org/package/probability-0.2.4.1/docs/Numeric-Probability-Transition.html