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2011-06-08T14:02:46.690
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I think this question is about performance. While semantically pattern matching is tested top-to-bottom, most Haskell compilers will optimize matching on an ADT's constructors to the equivalent of a C <code>switch</code> statement. You can think of the data representation for an ADT has a "tag" which says which constructor it was made with, together with a pointer for each argument. So for example, <code>Nothing</code> might be represented as <code>0 (null)</code> and <code>Just 42</code> represented as <code>1 (pointer to 42)</code>. Then in a function like this: <pre><code>squash :: Maybe (Maybe Int) -> Int squash (Just Nothing) = 0 squash Nothing = 0 squash (Just (Just x)) = x </code></pre> The compiler would set up a decision tree: <pre><code>squash x = check tag of x: 0 -> 0 1 y -> check tag of y: 0 -> 0 1 z -> z </code></pre> Where each tag is computed by a jump table or something, so it is no more expensive to check against <code>0</code> as <code>1</code>. Note that this same decision tree would be made no matter what the original order of the patterns in our definition. However, when using guards instead of matching on a constructor, the patterns are most likely checked from top to bottom (the compiler would have to be very smart to optimize that). So if we wrote <code>fromJust</code> in this arcane way: <pre><code>fromJust x | isNothing x = error "fromJust: Nothing" | isJust x = case x of { Just y -> y } </code></pre> Then this would probably check for each constructor in turn, and we could optimize by switching the order of the cases. Fortunately, writing in a way that this matters is cumbersome :-).
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