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    <p>Here's the source code for <code>now</code>:</p> <pre><code>now arr = do arr `deepSeqArray` return () return arr </code></pre> <p>So it's really just a monadic version of <code>deepSeqArray</code>. You can use either of these to force evaluation, rather than hanging on to a thunk. This "evalulation" is different than the "computation" forced when <code>computeP</code> is called. </p> <p>In your code, <code>now</code> doesn't apply, since you're not in a monad. But in this context <code>deepSeqArray</code> wouldn't help either. Consider this situation:</p> <pre><code>x :: Array U Int Double x = ... y :: Array U Int Double y = computeUnboxedP $ map f x </code></pre> <p>Since <code>y</code> refers to <code>x</code>, we'd like to be sure <code>x</code> is computed before starting to compute <code>y</code>. If not, the available work won't be distributed correctly among the gang of threads. To get this to work out, it's better to write <code>y</code> as</p> <pre><code>y = deepSeqArray x . computeUnboxedP $ map f x </code></pre> <p>Now, for a delayed array, we have</p> <pre><code>deepSeqArray (ADelayed sh f) y = sh `deepSeq` f `seq` y </code></pre> <p>Rather than computing all the elements, this just makes sure the shape is computed, and reduces <code>f</code> to weak-head normal form.</p> <p>As for manifest vs delayed arrays, there are certainly time delayed arrays are preferable. </p> <pre><code>multiplyMM arr brr = [arr, brr] `deepSeqArrays` A.sumP (A.zipWith (*) arrRepl brrRepl) where trr = computeUnboxedP $ transpose2D brr arrRepl = trr `deepSeqArray` A.extend (Z :. All :. colsB :. All) arr brrRepl = trr `deepSeqArray` A.extend (Z :. rowsA :. All :. All) trr (Z :. _ :. rowsA) = extent arr (Z :. colsB :. _ ) = extent brr </code></pre> <p>Here "extend" generates a new array by copying the values across some set of new dimensions. In particular, this means that</p> <pre><code>arrRepl ! (Z :. i :. j :. k) == arrRepl ! (Z :. i :. j' :. k) </code></pre> <p>Thankfully, <code>extend</code> produces a delayed array, since it would be a waste to go through the trouble of all this copying.</p> <p>Delayed arrays also allow the possiblity of fusion, which is impossible if the array is manifest.</p> <p>Finally, <code>computeUnboxedP</code> is just <code>computeP</code> with a specialized type. Giving <code>computeUnboxedP</code> explicitly might allow GHC to optimize better, and makes the code a little clearer.</p>
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