Re: [Haskell-cafe] Re: How to make code least strict?

I second Ryan's recommendation of using unamb [1,2,3] to give you unbiased (symmetric) laziness. The zip definition could also be written as zip xs@(x:xs') ys@(y:ys') = assuming (xs == []) [] `unamb` assuming (ys == []) [] `unamb` (x,y) : zip xs' ys' The 'assuming' function yields a value if a condition is true and otherwise is bottom: assuming :: Bool -> a -> a assuming True a = a assuming False _ = undefined This zip definition is a special case of the annihilator pattern, so zip = parAnnihilator (\ (x:xs') (y:ys') -> (x,y) : zip xs' ys') [] where 'parAnnihilator' is defined in Data.Unamb (along with other goodies) as follows: parAnnihilator :: Eq a => (a -> a -> a) -> a -> (a -> a -> a) parAnnihilator op ann x y = assuming (x == ann) ann `unamb` assuming (y == ann) ann `unamb` (x `op` y) [1] http://haskell.org/haskellwiki/Unamb [2] http://hackage.haskell.org/packages/archive/unamb/latest/doc/html/Data-Unamb... [3] http://conal.net/blog/tag/unamb/ - conal On Mon, Jan 19, 2009 at 12:27 PM, Ryan Ingram wrote:

On Mon, Jan 19, 2009 at 9:10 AM, ChrisK wrote:

...

Consider that the order of pattern matching can matter as well, the simplest common case being zip:

zip xs [] = [] zip [] ys = [] zip (x:xs) (y:ys) = (x,y) : zip xs ys

If you are obsessive about least-strictness and performance isn't a giant concern, this seems like a perfect use for Conal's unamb[1] operator.

zipR xs [] = [] zipR [] ys = [] zipR (x:xs) (y:ys) = (x,y) : zip xs ys

zipL [] ys = [] zipL xs [] = [] zipL (x:xs) (y:ys) = (x,y) : zip xs ys

zip xs ys = unamb (zipL xs ys) (zipR xs ys)

This runs both zipL and zipR in parallel until one of them gives a result; if neither of them is _|_ they are guaranteed to be identical, so we can "unambiguously choose" whichever one gives a result first.

-- ryan

[1] http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe