[Haskell-cafe] Data creation pattern?

[Daniel Fischer]

On Thursday 13 May 2010 20:43:44, Eugeny N Dzhurinsky wrote:
> On Thu, May 13, 2010 at 07:14:25PM +0100, Stephen Tetley wrote:
> > Hi Eugene
> >
> > Is something like this close to what you want:
>
> Not really. First of all, there're many properties, not 3. So it may end
> up with plenty of support (boilerplate) code.
>
> Also, names of these parameters are not sortable. Of course I could make
> these names as another data type with deriving from Eq/Ord - but that
> increase complexity as well.
>
> Original idea was
>
> 1) create Array
> 2) if line "param_N_name=value" appear, then
> 2.1) try to take object N from array
> 2.2) if no object exists - then create one
> 2.3) set the property "name" to "value"
> 2.4) put resulting object back into array
> 3) take next line and go to 2
>
> so if it is possible to have partially initialized objects in Haskell,

If the fields aren't strict, there's no problem having

data MyObject = MyObject { param1, param2, param3 :: String }

emptyObject = MyObject undefined undefined undefined

-- Boilerplate code, but you can let that write the computer for you
update :: String -> MyObject -> String -> MyObject
update "param1" obj val = obj{param1 = val}
update "param2" obj val = obj{param2 = val}
update "param3" obj val = obj{param3 = val}
update _ obj _ = obj

and start from emptyObject, updateing the fields as they come (if the 
fields aren't all Strings, read or parse the value).

import qualified Data.Map as Map
import Data.List (foldl')

-- break all lines into:
-- object ID (N)
-- field to be set (name)
-- value
breakUpLine :: String -> (Int, String, String)

-- construct a map ID -> object from the lines of the file
construct :: [String] -> Map Int MyObject
construct = foldl' updateMap (Map.empty)
    where
        updateMap mp (i, fld, val) = 
            case Map.findWithDefault emptyObject i mp of
                obj -> Map.insert i (update fld obj val) mp


> then this approach should work. If now - well, then replace creation of
> object with adding name/value pair to an array. And then create objects
> from those arrays.