• can document the intended use of fields
  • see example 1
• provides functions to get the records content
  • see example 2

• example 1:

  • {-| to be able to print into a 'CharMatrix'
     
    * prefix: xc
    * instance of Formatting to print the stored characters
    -}
    data MatrixCursor = 
        MatrixCursor {
            -- | the row, starting at 0
            rnRow :: Integer, 
            -- | the column starting at 0
            rnCol :: Integer }
        deriving Show

• example 2:

  •     print $ rnCol xc

An algebraic sum type (see also https://en.wikipedia.org/wiki/Algebraic_data_type regarding sum types) is a type with different constructors seperated by |. Record syntax can generate errors, if combined with sum types.

• example:

  • module Main where
     
    main :: IO ()
    main = do
        print $ nDiameterInMM SomethingElse
     
    data Wood = Log { nDiameterInMM :: Float,  nLengthInMM :: Float } | SomethingElse
        deriving Show

• output:

  • Test2-exe.exe: No match in record selector nDiameterInMM

module Main where
 
main :: IO ()
main = do
    print $ nCol xc
 
{-| to be able to print into a 'CharMatrix'
 
* prefix: xc
* instance of Formatting to print the stored characters
-}
data MatrixCursor = 
    MatrixCursor {
        -- | the row, starting at 0
        nRow :: Integer, 
        -- | the column starting at 0
        nCol :: Integer }
    deriving Show
 
xc :: MatrixCursor
xc = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor
 
incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor
incMatrixCursorCol n (MatrixCursor nRow nCol) = (MatrixCursor nRow (nCol+n))
 
initMatrixCursor :: MatrixCursor
initMatrixCursor = (MatrixCursor 0 0)

• compiler warning:

  • app\Main.hs:25:36: warning: [-Wname-shadowing]
        This binding for `nRow' shadows the existing binding
          defined at app\Main.hs:16:9
       |
    25 | incMatrixCursorCol n (MatrixCursor nRow nCol) = (MatrixCursor nRow (nCol+n))
       |                                    ^^^^