On Fri, 2012-12-21 at 04:36 -0900, Christopher Howard wrote:
Using a simple type I gave earlier from my monadic type question...
code: -------- data Socket3 a b c = Socket3 a b c deriving (Show) --------
Is it possible somehow to layer on record syntax onto a synonym of the type?
The idea would be something like this...
code: -------- type SpaceShip = Socket3 { engine :: Last Engine , hull :: Last Hull , guns :: [Guns] } --------
...purely for the convenience. But this doesn't seem to work with "type" as it assumes you are referring to already made constructors, and evidently "newtype" only allows use of a single record. I could wrap it in a normal "data" declaration but that would add an extra layer of complexity I think.
Although this 'Socket3' data type which all of a sudden should be aliased as 'SpaceShip' feels/looks really strange (are you sure that's the right way to reach whatever the goal is?), you could use lenses: import Control.Lens data Socket3 a b c = Socket3 a b c deriving (Show) data Last a = Last a deriving Show data Engine = Engine deriving Show data Hull = Hull deriving Show data Gun = Gun deriving Show type SpaceShip = Socket3 (Last Engine) (Last Hull) [Gun] engine :: Simple Lens SpaceShip (Last Engine) engine = lens get lset where get (Socket3 a _ _) = a lset (Socket3 _ b c) a' = Socket3 a' b c hull :: Simple Lens SpaceShip (Last Hull) hull = lens get lset where get (Socket3 _ b _ ) = b lset (Socket3 a _ c) b' = Socket3 a b' c guns :: Simple Lens SpaceShip [Gun] guns = lens get lset where get (Socket3 _ _ c) = c lset (Socket3 a b _) = Socket3 a b main :: IO () main = do print $ s0 ^. engine print $ s0 ^. guns let s1 = guns .~ [Gun, Gun] $ s0 print s1 print $ s1 ^. guns where s0 :: SpaceShip s0 = Socket3 (Last Engine) (Last Hull) [] (I'm no Lens expert so maybe there are better ways than manually creating these Lens instances, or make them shorter/abstract something out) Nicolas