Re: [Haskell-beginners] pattern matching to data inside a list

Is there a way to pattern match to a list argument and get a list? For example, I have this:

-- A LayoutItem has a center position (point), and the four ints are -- "left width", "rigth width", "top height", and "bottom height" data LayoutItem = LayoutItem Point Int Int Int Int deriving Show

-- This computes the left width of a group of composited LayoutItem. compositeLeftWidth :: [LayoutItem] -> Int compositeLeftWidth items = let itemsPosX = [ x | LayoutItem (Point (x,_)) _ _ _ _ <- items ] itemsLW = [ lw | LayoutItem _ lw _ _ _ <- items ] z = zipWith (\x y -> x-y+1) itemsPosX itemsLW in (minimum z) - 1

What I'm wondering is if I could somehow do something like

compositeLeftWidth [ LayoutItem Point( x, _ ) lw _ _ _ ] = ...

and that x and lw would each be the lists I'm calling itemsPosX and itemsLW above.

Thanks, Mike

That would be neat :-). But IIRC there is no syntax for that. However, if you are interested in other suggestions (by a very newbie, so

2009/3/27 Michael Mossey they could be off-mark): 1. if you use a definition like this data LayoutItem = LayoutItem { x :: Point; leftw, rightw, toph, bottomh :: Int } deriving Show then you could do, using the data accessor functions implicitely defined by the above code: xlist = map x items lwlist = map leftw items which is basically the same of your list comprehensions, but takes less characters :-) 2. If I read correctly your code, you are processing a single element of 'items' per time. If so, instead of building lists of attributes and the zipWith them to get your result, you could use this approach: a. define a function tha process a single item of type Layout, like : compLW LayoutItem x lw rw th bh = x - lw + 1 -- not sure it has the same result, but you get the idea b. use map (or list comprehension) to build z from items: compositeLeftWidth items = ( minimum z) -1 where z = map compLW items This way you avoid to scan multiple times the same list. Ciao ------- FB