Re: [Haskell-cafe] Monadic function fails as Nothing in Maybe context but as Exception in Either context.

30 Nov 2015

      On Mon, 30 Nov 2015 14:25:49 -0800
Jeffrey Brown  wrote:
...
I've written a monadic function which in a Maybe context produces a
Nothing when it fails (as intended), but in an Either context
produces an Exception rather than a Left.
The Monad instance for (Either e) uses the default definition of 'fail',
which is to throw an exception:

instance Monad (Either e) where
    return = Right
    Left  l >>= _ = Left l
    Right r >>= k = k r

In general, 'Left' can't be used because that would only type check
for (Either String), but the Monad instance is more general than that.

-- 
Michael Walker (http://www.barrucadu.co.uk)