You could look into the "Generic Monoid" solution proposed in your
other thread, then you wouldn't need your "Socket" types - you would
use the "Generic Monoid" machinery to make a Monoid instance for
whatever type needed it.
This approach loses some type-safety, as you might pass on version of
a Scoket3 to a function that was meant to take a different type of
Socket3.
On Fri, Dec 21, 2012 at 4:50 PM, Christopher Howard
On 12/21/2012 04:52 AM, Daniel Trstenjak wrote:
Why having a Socket3 in the first place, what's the point of it?
The idea was to have some generic structures (Sockets) which were already instanced into the Monoids-within-Monoids abstraction, yet could still be made concrete into anything more specific.
So, I have...
code: -------- data Socket3 a b c = Socket3 a b c deriving (Show)
instance (Monoid a, Monoid b, Monoid c) => Monoid (Socket3 a b c) where mempty = Socket3 mempty mempty mempty Socket3 a b c `mappend` Socket3 w x y = Socket3 (a <> w) (b <> x) (c <> y)
nullSocket3 :: (Monoid a, Monoid b, Monoid c) => Socket3 a b c nullSocket3 = Socket3 mempty mempty mempty --------
...which allows me to have...
code: -------- type ShipSys = Socket3 (Last Engine) (Last RotThruster) [LinThruster]
nullShipSys :: ShipSys nullShipSys = nullSocket3
setEngineSocket (Socket3 a b c) x = Socket3 x b c
engineSys :: Engine -> ShipSys engineSys a = setEngineSocket nullShipSys (Last (Just a))
mk1Engine = engineSys (Engine 100 1 "Mark I")
-- etc. --------
And so, with each individual component being wrapped as a generic ShipSys (ship system), I can make a complete system simply by composition:
code: -------- h> :t mk1Engine mk1Engine :: ShipSys h> :t stdRearThruster stdRearThruster :: ShipSys h> :t stdFrontThruster stdFrontThruster :: ShipSys h> :t stdRotThruster stdRotThruster :: Power -> ShipSys h> mk1Engine <> stdRearThruster <> stdFrontThruster <> stdRotThruster 10 Socket3 (Last {getLast = Just (Engine 100.0 1.0 "Mark I")}) (Last {getLast = Just (RotThruster 10.0)}) [LinThruster 3.1415927 1.0,LinThruster 0.0 0.5] --------
This seems to work well enough so far. But the issue I was concerned about is: if I can't layer record syntax onto the type synonym, then I have to rewrite a whole bunch of getters / setters each time I want to add an attribute (e.g., requiring a switch from a Socket3 to a Socket4.) If this is the case, then perhaps it would be better just to define the ShipSys type directly, and directly instance it into the monoid abstraction.
-- frigidcode.com
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