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. -- Conal On Mon, Apr 8, 2013 at 6:11 AM, Jan Stolarek <jan.stola...@p.lodz.pl> wrote: > > You can achieve something similar with the ViewPatterns language > > extension. > > > > member _ [] = False > > member x (((x ==) -> True) : _) = True > > member x (_ : xs) = member x xs > Hi Tillmann, > > there are a couple of ways to achieve this in Haskell, for example using > guards: > > member :: Eq a => a -> [a] -> Bool > member _ [] = False > member y (x:_) | x == y = True > member y (_:xs) = member y xs > > The goal of my proposal is to provide a concise syntax, whereas > ViewPatterns are very verbose and > guards are slightly verbose. I want something simple and something that is > very intuitive if > you've programmed in Prolog :) > > Janek > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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