Hi Jan,

What you're suggesting is called "non-linear patterns", and it's a perfectly sensible, well-defined feature in a language with pattern-matching. As you point out, non-linearity allows for more direct & succinct programming. I've often wished for this feature when writing optimizations on data types, especially for syntactic types (languages).

As Ivan mentioned, there is some danger that people may accidentally a non-linear pattern accidentally, and perhaps the early Haskell designers chose the linearity restriction out of this worry. The importance of such dangers is a subjective call, and certainly not one carried out consistently in Haskell. Consider, for instance, the choice that let & where bindings are recursive by default in Haskell, unlike ML and Lisp. I like this choice, but I can understand objections that it leads to accidental recursions, especially for non-functions.