On Nov 20, 2008, at 1:27 AM, David Menendez wrote:
There are also possible space issues, since "min a b" will end up re- creating much of "a" or "b" if they share a large common prefix.
Thanks! I had put off writing the version I needed until seeing your code, which inspired me:
Compute the least list in a list of equal length lazy lists
least :: forall a. Ord a => [[a]] -> [a]
least ((x : xs) : xss) = w : least wss where (w, wss) = foldl' f (x, [xs]) xss
f :: (a, [[a]]) -> [a] -> (a, [[a]]) f v@(y, yss) (z : zs) = case compare y z of LT -> v EQ -> (y, zs : yss) GT -> (z, [zs]) f v _ = v
least _ = []
In my applications, there are usually many instances of the minimum, so peeling off a reference copy isn't all that wasteful. I just want to avoid evaluating everything else. So to summarize the responses, there's no GHC language support for introspecting lazy structures, allowing one to write a generic bounded compare that only evaluates lazy structures to a specified depth. One can however write a class, and solve this problem type-by-type with a common interface. I can see how providing language support for this could be controversial. Functions would remain referentially transparent within a given compilation, but their values would depend non-portably on the compiler. (I use a "code is indented, comments are flush" literate preprocessor (http://hackage.haskell.org/trac/ghc/ticket/2776 ) so I don't have to look at punctuation for either code or comments; syntax coloring makes it obvious which is which. The 1980's email quote symbols > in the default literate Haskell reminds me of being introduced to an IBM 360 in college, and being told I was working with virtual card images. I gasped. I remember punched cards, and it isn't an experience I want to relive. I also remember 1980's email, so my first reaction to literate Haskell was "You've got to be kidding!" Ahh, but better to propose a fix than to gripe.)