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copied!<p>For lists (such as <code>tossDie</code>), the <code>do</code> notation acts like a list comprehension -- that is to say, as if each variable binding were a nested <code>foreach</code> loop.</p> <p>The do-block expression:</p> <pre><code>toss2Dice = do { n <- tossDie; m <- tossDie; return (n+m) } </code></pre> <p>does the same thing as this list comprehension:</p> <pre><code>toss2Dice = [ n+m | n <- tossDie, m <- tossDie ] </code></pre> <p>The result is comparable to the following imperative pseudocode:</p> <pre><code>toss2Dice = [] foreach n in tossDie: foreach m in tossDie: toss2Dice.push_back(n+m) </code></pre> <p>except that the Haskell examples generate their results lazily, on demand, instead of eagerly and all at once.</p> <hr> <p>If you look at the monad instance for lists, you can see how this works:</p> <pre><code>instance Monad [] where xs >>= f = concat (map f xs) return x = [x] </code></pre> <p>Starting at the beginning of the <code>do</code> block, each variable binding creates a loop over the remainder of the block:</p> <pre><code>do { n <- tossDie; m <- tossDie; return (n+m) } ===> tossDie >>= \n -> do { m <- tossDie; return (n+m) } ===> concat ( map (\n -> do { m <- tossDie; return (n+m) }) tossDie ) </code></pre> <p>Note that <code>map</code> function iterates over the items in the list <code>tossDie</code>, and the results are <code>concat</code>enated. The mapping function is the remainder of the <code>do</code> block, so the first binding effectively creates an outer loop around it.</p> <p>Additional bindings create successively nested loops; and finally, the <code>return</code> function creates a singleton list out of each computed value <code>(n+m)</code> so that the "bind" function <code>>>=</code> (which expects lists) can concatenate them properly.</p>

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