Re: [Haskell-cafe] Data types and field labels and "Show"

27 Sep 2007


      On Thu, 27 Sep 2007 11:41:00 -0700, Don Stewart wrote
...
bbrown:
...
I am trying to print the data from a data type and also get the field 
values.  How would I reference those values if I am declaring a Show 
function.
I should probably use a class for this, but so far it is working.
I have something along the lines of this.
data SimplePlayer = SimplePlayer { 
      shape :: MechShape,
      angle :: GLfloat,
      posX :: GLfloat,
      posY :: GLfloat
    }
and then to use "Show", I was declaring this.
instance Show SimplePlayer where
  show a = "<Simple> posX [" ++ show a{posX} ++ "]"
Of course, this didn't compile.
How should I change this code to get the field label values of the type.
Tests/GLTests.hs:20:42: parse error on input `}'
Just derive Show. Types with records will be printed with their labels.
You can then also derive Read, and get serialisation for free.
data SimplePlayer = SimplePlayer
            { shape :: MechShape
            , angle :: GLfloat
            , posX  :: GLfloat
            , posY  :: GLfloat }
deriving (Read,Show)
*M> show $ SimplePlayer "square" pi 1.0 (exp 1)
"SimplePlayer {shape = \"square\"
                  , angle = 3.1415927
                  , posX = 1.0
                  , posY = 2.7182817}"
I did this too:

instance Show SimplePlayer where
	show simpleEntity = "<Simple> posX [" ++ show (posX simpleEntity) ++ 
						"] posY [" ++ show (posY 
simpleEntity) ++ "]"


--
Berlin Brown
[berlin dot brown at gmail dot com]
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