"Samuel E. Moelius III" wrote:
What is the reasoning for the requirement that patterns be linear? Am I correct to say that if non-linear patterns were allowed, it would require equality to be defined for all data types? Is this the reason, or are there others?
If that were the reason, then to be consistent, there would be no literals in patterns, as these are tested using equality. Here is a reason. Some people believe that the equations that form a function definition should be disjoint. A good reason for this view is that fold/unfold transformations can be then performed without reference to the other cases in the definition - if it matches, it is correct to use it. Although Haskell does not require disjoint cases, and defines the semantics in terms of a top-to-bottom testing order, the rejection of non-linear patterns was largely because it is not generally possible to specify, as a pattern, the complement of the set of values that match a non-linear pattern, and so not possible to write a set of disjoint case equations if non-linear patterns occur. In any case, the same effect can be obtained using guards, where the extra power of expression syntax solves the problem. Instead of f x x = e1 f (illegal pattern) = e2 ... you can write the single equation f x y | x == y = e1 | x /= y = e2 --brian