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.
--
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