I agree that if (n+k) patterns go, then so should k patterns. Both are overloaded, and that's the root of their complexity. Personally I think ~ patterns are great, and we are now talking about ! patterns, a kind of dual to ~ patterns. So at least I think we should un-couple the two discussions. Simon | -----Original Message----- | From: haskell-prime-bounces@haskell.org [mailto:haskell-prime-bounces@haskell.org] On Behalf Of | Olaf Chitil | Sent: 26 January 2006 17:01 | To: haskell-prime@haskell.org | Subject: Removal candidates in patterns | | | I am very please to see on the Wiki also a list of removal candidates | and that these include n+k patterns and ~ patterns. | | I'd like to add one pattern to this list of removal candiates: k | patterns, that is, numeric literals. | | Why do I want to get rid of these three patterns? Because all three | caused me no end of trouble when implementing the program transformation | of the Haskell tracer Hat. Hat actually still doesn't handle nested ~ | patterns. | | Why are these patterns so hard to implement for Hat? Surely the Haskell | report gives a translation into simple core Haskell. Well, Hat does not | use this translation because it does not want to be an inefficient | pattern matcher (leave that job to the compiler) but produce a trace of | the Haskell program as it is written. However, both n+k and k patterns | cause calls of functions ( (-), (==) etc) that Hat has to record in its | trace. Also ~ patterns do not fit the simple rewriting semantics of the | Hat trace and hence have to be recorded specially. While in simple cases | that occur in practice it is pretty straightforward to remove n+k, k and | ~ patterns from a larger pattern while keeping the rest of the larger | pattern intact, in the general case this is incredibly hard. | | Iff n+k patterns are removed, there is little good use for k patterns | either. Since the introduction of monadic IO the ~ pattern is hardly | used in practice either. In all the simple cases that these three are | currently used in practice, it is easy for the programmer to define | their function in an alternative way. | | So get rid of these three and pattern matching becomes so much more simple. | | Ciao, | Olaf | _______________________________________________ | Haskell-prime mailing list | Haskell-prime@haskell.org | http://haskell.org/mailman/listinfo/haskell-prime