[Haskell-beginners] Re: Advice wanted on parallel processing

18 Mar 2009

      Am Mittwoch, 18. März 2009 16:49 schrieb Colin Paul Adams:
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"Daniel" == Daniel Fischer  writes:
Daniel> If e.g.
Daniel> data Move = Move {from :: Position, to :: Position}
Daniel> , the instance would be
Daniel> instance NFData Move where rnf (Move f t) = rnf f `seq`
    Daniel> rnf t `seq` ()
Daniel> That might require NFData instances for Position and its
    Daniel> components, but specifying these should be automatic.
<switched to beginners list>
Move is somewhat more complicated than that, but it comes down to
specifying instances for
data Piece_colour
    = Black
| White
and
data Piece_type
    = Lance
| Reverse_chariot
    | Side_mover
    | Vertical_mover
    | White_horse
    | Rook
... many more
which I'm not sure how to do. I tried added a deriving clause, but the
compiler complains:
Can't make a derived instance of `NFData Piece_colour'
      (`NFData' is not a derivable class)
    In the data type declaration for `Piece_colour'
For a newtype-wrapper of a type which is an instance of NFData, you can do

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype Thing = T nfdata
	deriving NFData

, but for 

data Thingummy = ...

you have to write the instance yourself.
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here I don't have and fields to force.
For an enumeration like Piece_colour or Piece_type,

instance NFData Piece_colour where
	rnf = rwhnf

is all you need, but you don't even necessarily need that, for

data Move = Move {colour :: Piece_colour, typ :: Piece_type, start, end :: 
Position}

instance NFData Move where
	rnf (Move c t s e) = c `seq` t `seq` rnf s `seq` rnf e

is good.