Re: [Haskell-cafe] ANN: monad-bool 0.1

23 Jan 2013

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John Wiegley  writes:
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Never mind, when/unless + left work just fine for this.
You know, it's been a humorous day.

First ekmett showed that I can't make a sane Monad instance for AndM or OrM.

Then I discovered I can't make a reasonable Monoid (no mempty, given only
'toBool'), so I dropped down to a Semigroup.  Further, my combinators for
EitherT can be implemented using just 'when' and 'left' to provide the
short-circuiting.

Already I had very little code left, until he showed me the Applicative
instance for Either, plus a little trick:

    >>> Right 1 *> Right 2 *> Left 2 *> Right 5
    Left 2              -- same functionality as my And semigroup

    >>> let Left x |> y = y; x |> _ = x in Left 1 |> Right 2 |> Right 3
    Right 2            -- same functionality as my Or semigroup

And poof, all my code just disappeared...

-- 
John Wiegley
FP Complete                         Haskell tools, training and consulting
http://fpcomplete.com               johnw on #haskell/irc.freenode.net