[Haskell-cafe] View patterns

[Ozgur Akgun]

A humble suggestion: Have a *lazy* "to list" method for your *lists, arrays,
sets, etc.* and use the nice list-only version.

On 27 February 2010 18:11, Andrew Coppin <andrewcoppin at btinternet.com>wrote:

> One somewhat neat thing about Haskell is that you can say
>
>  case list of
>   [[x], [y,_], [z,_,_]] -> x + y + z
>   _ -> 0
>
> In Java, you'd have to write something like
>
>  if (list.length() == 3)
>  {
>   List t1 = list.at(0);
>   if (t1.length() == 1)
>   {
>     int x = t1.at(0);
>     List t2 = list.at(1);
>     if (t2.length() == 2)
>     ...
>
> I can't even be bothered to finish typing all that lot!
>
> However, as somebody pointed out, the Java version is polymorphic. Assuming
> that length() is defined for multiple types of container, the Java version
> works with lists, arrays, sets, etc. If you try to do this in Haskell, you
> end up with
>
>  case size c of
>   3 ->
>     case (c ! 0, c ! 1, c ! 2) of
>       (xs, ys, zs) | size x == 1 && size y == 2 & size z == 3 -> (xs ! 0) +
> (ys ! 0) + (zs ! 0)
>       _ -> 0
>   _ -> 0
>
> or similar. Which is shorter than Java, but nowhere near as nice as the
> original list-only version.
>
> Now I was under the impression that "view patterns" fix this problem, but
> it seems they don't:
>
>  case c of
>   (size -> 3) ->
>     case (c ! 0, c ! 1, c ! 2) of
>       (size -> 1, size -> 2, size -> 3) -> (c ! 0 ! 0) + (c ! 1 ! 0) + (c !
> 2 ! 0)
>
> Any suggestions?
>
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>



-- 
Ozgur Akgun
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