Re: [Haskell-cafe] Error monads (was: Call for comments: neither package)

2010/6/29 Roman Cheplyaka :

* Stephen Tetley [2010-06-29 12:02:45+0100]

...

The "Applicative Programming with Effects Paper" has the "monodial accumulating" applicative instance on a sum type Conor McBride and Ross Paterson call Except:

data Except err a = OK a | Failed err

The names are nice and to the point, but they would equally describes the other exception model (monadic - exit on first fail) and having both might be particularly confusing to newcomers: two error types - one with an obvious name, one with an unfamiliar one, the unfamiliar one might be the one they need most often...

On a slightly related note, in my projects I tend to define my own Error-like types with descriptive names, often more than one failure mode and apropriate instances. For an example of this approach, see http://github.com/feuerbach/loker/blob/master/testingtool.hs#L33

From a quick glance at it, I believe it would be easier to use

Either MyError MyResult and then have data MyError = All | The | Possible | Descriptive | Errors Cheers, Thu