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copied!<p>I'm guessing you're using <code>monads-fd</code>.</p> <pre><code>$ ghci t.hs -hide-package mtl *Main Data.List> testMyDiv1 1 0 "*** Exception: My divison by zero *Main Data.List> :i Either ... instance Monad (Either e) -- Defined in Control.Monad.Trans.Error ... </code></pre> <p>Looking in the <a href="http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/Control-Monad-Trans-Error.html" rel="nofollow">transformers</a> package, which is where <code>monads-fd</code> gets the instance, we see:</p> <pre><code>instance Monad (Either e) where return = Right Left l >>= _ = Left l Right r >>= k = k r </code></pre> <p>So, no definition for Fail what-so-ever. In general, <code>fail</code> is discouraged as it isn't always guaranteed to fail cleanly in a monad (many people would like to see <code>fail</code> removed from the Monad class).</p> <p>EDIT: I should add that it certainly isn't clear <code>fail</code> was intentioned to be left as the default <code>error</code> call. A ping to haskell-cafe or the maintainer might be worth while.</p> <p>EDIT2: The <code>mtl</code> instance has been <a href="http://hackage.haskell.org/trac/ghc/ticket/4159#comment:1" rel="nofollow">moved to base</a>, this move includes removing the definition of <code>fail = Left</code> and discussion as to why that decision was made. Presumably, they want people to use ErrorT more when monads fail, thus reserving <code>fail</code> for something more catastrophic situations like bad pattern matches (ex: <code>Just x <- e</code> where <code>e -->* m Nothing</code>).</p>

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