From: Ross Paterson
On Thu, Jul 01, 2010 at 11:35:24AM +0100, Simon Marlow wrote:
I don't know all the ins and outs of the discussion here, but before proposing to reverse this decision someone should do a careful assessment of the prior rationale and results of subsequent experience.
The general question of fail is a side issue -- the question here is just whether the Monad instance for Either should define fail, and if so to what.
I think it should not define fail (or define it to error). I've never been happy with the semantics of this instance. I thought the point of Either was that neither value had any special meaning attached to it, and in particular that "Left" and "Right" were chosen as being free of any connotations of correct, incorrect, or other. Defining fail as "Left x" seems wrong because it codifies the left value as an error. In my opinion those semantics belong to another type. John