[Haskell-cafe] Data.Tree.Zipper in the standard libraries

[Bayley, Alistair]

> From: haskell-cafe-bounces at haskell.org 
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Jules Bean
> 
> Krasimir Angelov wrote:
> > The monads design is used in Data.Map i.e.
> > 
> > lookup :: (Monad m, Ord k) => k -> Map k a -> m a
> 
> which is widely considered a poor design decision and a wart 
> on Data.Map.
> 
> Seriously, if you don't return a useful error message, then 
> Maybe is as good as it gets, why not use it?
> 
> (And there really is only one kind of error possible here, in 
> each case)
> 
> 'fail' only adds information in the case it has a useful 
> error message (in which case I'd encourage MonadError m =>)

Is there a reason not to recommend Either, if you want to return error
information? It is an instance of MonadError.

Alistair
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