I've got these data types:

data PointPlus = PointPlus
    { coords :: Point
    , velocity :: Vector
    } deriving (Eq)

data BodyGeo = BodyGeo
    { pointPlus :: PointPlus
    , size :: Point
    } deriving (Eq)

data Body = Body
    { geo :: BodyGeo
    , pict :: Color
    } deriving (Eq)

It's the base datatype for characters, enemies, objects, etc. in my game (well, I just have two rectangles as the player and the ground right now :p).

When a key, the characters moves right, left or jumps by changing its velocity. Moving is done by adding the velocity to the coords. Currently, it's written as follows:

move (PointPlus (x, y) (xi, yi)) = PointPlus (x + xi, y + yi) (xi, yi)

I'm just taking the PointPlus part of my Body and not the entire Body, otherwise it would be:

move (Body (BodyGeo (PointPlus (x, y) (xi, yi)) wh) col) = (Body (BodyGeo (PointPlus (x + xi, y + yi) (xi, yi)) wh) col)

Is the first version of move better? Anyway, if move only changes PointPlus, there must be another function that calls it inside a new Body. I explain: there's a function update which is called to update the game state; it is passed the current game state, a single Body for now, and returns the updated Body.

update (Body (BodyGeo (PointPlus xy (xi, yi)) wh) pict) = (Body (BodyGeo (move (PointPlus xy (xi, yi))) wh) pict)

That tickles me. Everything is kept the same within Body except the PointPlus. Is there a way to avoid this complete "reconstruction" by hand? Like in:

update body = backInBody $ move $ pointPlus body

Without having to define backInBody, of course.