"Ross Paterson"
The proposal is to move the Monad and MonadFix instances for Either (currently in the mtl package) to Control.Monad.Instances and Control.Monad.Fix respectively (both in the base package). The Monad instance is still an orphan, to retain Haskell 98 compatibility, but the MonadFix instance is together with its class. The Error constraint is removed from both instances, and the default definition of fail is used.
Discussion deadline: 20th July 2010
-1, because the default definition of fail is error, which would render the Monad instance useless (unless I'm missing something?). This has never been the Monad instance I wanted for Either (I only use Either if I care about both alternatives, and fail=error would spoil that), nor do I think that there is a single generally useful instance that would improve the existing one in mtl: 0 defining 'instance Fail a => Monad (Either a)' would prevent any instances; but sometimes, Monad (Either String) is useful, so this is no good 1 leaving out the instance would let me define my own; but so could everyone else, so conflicts loom - again, no good 2a I don't like the Error constraint, but I can live with it (almost, because even two libs defining the same instance result in conflict); doesn't work in the long term 2b just moving the existing instance unchanged from Control.Monad.Error into Control.Monad.Instances might be an acceptable improvement 3a limiting the Monad instance to (Either String) with fail as Left would cover the only variant of Monad (Either a) that I've actually used myself; this is no good if there are actually any other Monad (Either a) instances in use 3b even 'fail s = Left (error s)' would be preferable to 'fail = error', if you insist on dropping Error; but no good, that would take away programmer choice (and prevent my only use case!) 4 using 'fail = error' is the worst of all choices, and certainly shouldn't be moved in a position where I cannot avoid it (the other instances in Control.Monad.Instances are occasionally useful, this one wouldn't be); this is also inconsistent with the existing Monad Maybe instance, and doesn't tie in with MonadPlus (Either e): whenever possible, fail s should be mzero; this is no good at all. Of these, only 2b seems acceptable, helping to prevent conflicting equivalent instances for the only use case I've encountered, Monad (Either String). 3b can be recovered from 2b (noMsg defaults to strMsg "", strMsg defaults to const noMsg; one could change the default of strMsg to error, to preserve the error message). In other words, the current design is annoying, but not nearly as bad as the alternatives - the only improvements would be to move the instances out of mtl, to avoid conflicts, and perhaps to change the default of strMsg. Claus