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copied!<p>It would be very difficult to use do notation without <code>return</code>. Consider the common pattern of:</p> <pre><code>foo = do x <- bar y <- quux return $ x + y </code></pre> <p>Without return it is impossible to define this. Recall that <code>>>=</code> has type <code>(>>=) :: Monad m => m a -> (a -> m b)</code> - it must return a value inside the monad. So even if we write this out without do notation, we have the same problem:</p> <pre><code>foo = bar >>= \x -> quux >>= \y -> (???????) (x + y) </code></pre> <p>So it's not a good idea to use do-notation for this. Particularly, there's nothing to stop the compiler implementation from introducing <code>return</code>s with transformations conforming to the monad laws (which assume the presence of <code>return</code>).</p> <p>Moreover, even defining >>= will be difficult for you. How will you translate it to your imperative language? Remember that >>= takes an arbitrary function on its right argument; you are unable to inspect this function to tell what it might do, and thus cannot translate the body of this function into an imperative language. I suppose you could constrain the type of the argument to some kind of traced datatype and try to figure out the <em>effects</em> of the function, but now this means the function can't inspect its argument. In short, it's just not going to work very well.</p> <p>One alternate approach you might consider is to create a Monad that represents the process of building an imperative function. It might, for example, look like this:</p> <pre><code>emitAddAndPrint varA varB = do varTmp <- allocateTempVariable emitOp (Add varTmp varA varB) -- if you want to be fancy, emitOp (varTmp :=: varA :+: varB) or something emitCall "print" [varTmp] </code></pre>

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