Re: [Haskell-cafe] Prolog-style patterns

Hi, I believe one problem with non-linear patterns would be that the compiler has to figure out what notion of equality you want here. An obvious choice is (==), but the Eq instances might not do what you want. Using pattern guards or view patterns explicates this choice. Also, without an explicit call to == the cost model of such a function definition might be harder to see; an innocent looking change to the patterns of a function could cause a considerable amount of extra work if == is expensive. Greetings, Joachim Am Montag, den 08.04.2013, 07:06 -0700 schrieb Conal Elliott:

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 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

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