pattern_match.hs

data Box = Box { box_x1 :: Integer, box_y1 :: Integer, box_x2 :: Integer, box_y2 :: Integer } deriving Show

subdivideNW Box { box_x1 = x1', box_y1 = y1', box_x2 = x2', box_y2 = y2'} = Box {
        box_x1 = x1',
        box_y1 = y1',
        box_x2 = x1' + half_width,
        box_y2 = y1' + half_height
    }
    where
        half_width = (x2' - x1') `div` 2
        half_height = (y2' - y1') `div` 2

-- could I do something like this instead?  where I redundantly assign variables?

half_width Box { box_x1 = x1', box_y1 = _, box_x2 = x2', box_y2 = _ } = (x2' - x1') `div` 2
half_height Box { box_x1 = _, box_y1 = y1', box_x2 = _, box_y2 = y2' } = (y2' - y1') `div` 2

subdivideNW Box box' { box_x1 = x1', box_y1 = y1', box_x2 = x2', box_y2 = y2'} = Box {
        box_x1 = x1',
        box_y1 = y1',
        box_x2 = x1' + (half_width box'),
        box_y2 = y1' + (half_height box')
    }
    
-- hell yea I can!
-- 13:57 < dhrosa> felixn: maybe you want myvar@Box {...}
-- 13:57 < dhrosa> felixn: that will let you bind the inner stuff to variables,
--                 and also the entire  structure
-- 13:58 < dhrosa> felixn: you can also use that syntax on any constructor, like f
--                 x@(y:z)

half_width Box { box_x1 = x1', box_y1 = _, box_x2 = x2', box_y2 = _ } = (x2' - x1') `div` 2
half_height Box { box_x1 = _, box_y1 = y1', box_x2 = _, box_y2 = y2' } = (y2' - y1') `div` 2

subdivideNW box@Box { box_x1 = x1', box_y1 = y1', box_x2 = x2', box_y2 = y2'} = Box {
        box_x1 = x1',
        box_y1 = y1',
        box_x2 = x1' + (half_width box),
        box_y2 = y1' + (half_height box)
    }