Re: #4159: move Monad and MonadFix instances for Either from mtl to base

On 07/02/10 08:13, John Lato wrote:

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From: Ross Paterson 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.

It's not quite as simple as that -- as soon as we start making Either an instance of kind (* -> *) classes, such as Functor and Monad, there inevitably becomes some kind of semantic difference between Left and Right. Still, I support Ross's proposal of fail=error . Either's 'fail' could just as type-correctly be defined 'fail s = Right (error s)', even though no one would be happy about that... which makes me suspicious of defining it as 'fail s = Left (error s)' too. -Isaac