[Haskell-cafe] Fail-back monad

Ross Paterson ross at soi.city.ac.uk
Sat Mar 31 14:00:02 CEST 2012

On Wed, Mar 28, 2012 at 01:34:29AM +0100, Alberto G. Corona  wrote:
> In my package MFlow [1] I program an entire web  navigation in a
> single procedure. That happened  in the good-old WASH web application
> framework.
> The problem is the back button in the Browser.
> To go back in the code to the previous interactions when the data
> input does not match the expected because the user pressed the back
> button one or more times, i came across this Monad specimen,: that
> solves the problem.

This definition does not satisfy the right identity law (m >>= return = m)
included in the monad definition:

*FailBack> BackT [BackPoint 1] >>= return
BackT [NoBack 1]

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