Record syntax

• algebraic sum type )¹
• provides access functions
• fields in curly brackets, comma separated

• short example:

  • data MatrixCursor = MatrixCursor { rnRow :: Integer, rnCol :: Integer }

)¹ 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.

• 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

• example:

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

• output of runtime error:

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

Fields from records can be extrated by the accessor functions that have been declared within the record. Alternatively, records can be extrated by pattern matching, as well. If you use both with the same name then the pattern matching will shadow the accessor function(s).

• example 1:

  • module Main where
     
    main :: IO ()
    main = do
        print $ nCol xcToPrint
     
    {-| 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
     
    xcToPrint :: MatrixCursor
    xcToPrint = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor
     
    incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor
    incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (nRow xc) (nCol+n))
     
    initMatrixCursor :: MatrixCursor
    initMatrixCursor = (MatrixCursor 0 0)

• compiler output (using stack ghc – -Wall ./app/Main.hs):

  • [1 of 1] Compiling Main             ( app\Main.hs, app\Main.o )
     
    app\Main.hs:24:45: warning: [-Wname-shadowing]
        This binding for `nCol' shadows the existing binding
          defined at app\Main.hs:17:13
       |
    24 |     incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (nRow xc) (nCol+n))
       |                                             ^^^^
    Linking app\Main.exe ...

• example 2:

  • module Main where
     
    main :: IO ()
    main = do
        print $ nCol xcToPrint
     
    {-| 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
     
    xcToPrint :: MatrixCursor
    xcToPrint = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor
     
    incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor
    incMatrixCursorCol n xc@(MatrixCursor nRow nCol) = (MatrixCursor (nRow xc) (nCol+n))
     
    initMatrixCursor :: MatrixCursor
    initMatrixCursor = (MatrixCursor 0 0)

• compiler output:

  • [2 of 2] Compiling Main
     
    app\Main.hs:24:71: error:
        * Couldn't match expected type `MatrixCursor -> Integer'
                      with actual type `Integer'
        * The function `nRow' is applied to one argument,
          but its type `Integer' has none
          In the first argument of `MatrixCursor', namely `(nRow xc)'
          In the expression: (MatrixCursor (nRow xc) (nCol + n))
       |
    24 |     incMatrixCursorCol n xc@(MatrixCursor nRow nCol) = (MatrixCursor (nRow xc) (nCol+n))
       |                                                                       ^^^^^^^

• example 3 (warning free with stack ghc – -Wall ./app/Main.hs):

  • module Main where
     
    main :: IO ()
    main = do
        print $ rnCol xcToPrint
     
    {-| 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
     
    xcToPrint :: MatrixCursor
    xcToPrint = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor
     
    incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor
    incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (rnRow xc) (nCol+n))
     
    initMatrixCursor :: MatrixCursor
    initMatrixCursor = (MatrixCursor 0 0)

• output:

  • 10