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    <p>Regarding monads:</p> <p>Yes, <code>Either a</code> is a monad. So simplifying the problem, you are basically asking for this:</p> <pre><code>main = print $ magicMonadUnwrap v v :: Either String Int v = Right 3 magicMonadUnwrap :: (Monad m) =&gt; m a -&gt; a magicMonadUnwrap = undefined </code></pre> <p>How do you define <code>magicMonadUnwrap</code>? Well, you see, it's different for each monad. Each one needs its own unwrapper. Many of these have the word "run" in them, for example, <code>runST</code>, <code>runCont</code>, or <code>runEval</code>. However, for some monads, it might not be safe to unwrap them (hence the need for differing unwrappers).</p> <p>One implementation for lists would be <code>head</code>. But what if the list is empty? An unwrapper for <code>Maybe</code> is <code>fromJust</code>, but what if it's <code>Nothing</code>?</p> <p>Similarly, the unwrapper for the <code>Either</code> monad would be something like:</p> <pre><code>fromRight :: Either a b -&gt; b fromRight (Right x) = x </code></pre> <p>But this unwrapper isn't safe: what if you had a <code>Left</code> value instead? (Left usually represents an error state, in your case, a parse error). So the best way to act upon an <code>Either</code> value it is to use the <code>either</code> function, or else use a case statement matching <code>Right</code> and <code>Left</code>, as Daniel Wagner illustrated.</p> <p><em><strong>tl;dr</em></strong>: there is no <code>magicMonadUnwrap</code>. If you're inside that same monad, you can use <code>&lt;-</code>, but to truly extract the value from a monad...well...how you do it depends on which monad you're dealing with.</p>
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    1. POUnwrapping a monad
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    1. USDan Burton
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    1. COThere is with TH, someone posted an implementation of a generic `Monad m => m a -> a`. Link: http://blog.sigfpe.com/2009/01/rewriting-monadic-expressions-with.html
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    2. CO@monadic: that technique is clever, but it's got a problem. It only allows you to use `extract` within a Template Haskell splice which is then interpreted as some monad. It basically just gives you a fancy `(>>=)`, which could be expected since `extract` is simply replaced by `join`.
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      1. USJohn L
    3. CO@Dan: Interesting. I see your point, and it coheres better in my mind when I see what's going on.
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      1. USPaul Nathan
 

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