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codesnippets:recordsyntax

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codesnippets:recordsyntax [2021/03/20 20:13] – [Risk of namespace colisions without strong coding rules] f2b216codesnippets:recordsyntax [2025/10/08 00:48] (current) – external edit 127.0.0.1
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 ====== Record syntax ====== ====== Record syntax ======
  
 +  * algebraic sum type )<sup>1</sup>
 +  * provides access functions
 +  * fields in curly brackets, comma separated
 +
 +  * short example:
 +    * <code Haskell>
 +data MatrixCursor = MatrixCursor { rnRow :: Integer, rnCol :: Integer }
 +</code>
 +
 +----
 +
 +)<sup>1</sup> 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.
 ===== Advantages===== ===== Advantages=====
  
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 ==== With sum types combined: risk of runtime errors ==== ==== With sum types combined: risk of runtime errors ====
- 
-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:   * example:
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 </code> </code>
  
-  * output:+  * output of runtime error:
     * <code>     * <code>
 Test2-exe.exe: No match in record selector nDiameterInMM Test2-exe.exe: No match in record selector nDiameterInMM
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 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). 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).
 +
 +<WRAP center round info 62%>
 +Risk can be completely excluded by compiling with <wrap em>ghc parameter -Wall</wrap> and cleaning up all warnings.
 +</WRAP>
 +
 +<WRAP center round info 62%>
 +Risk can be mitigated by <wrap em>[[codesnippets:codingconventions#record_syntax|Coding rules]]</wrap>.
 +</WRAP>
 +
 +
  
   * example 1:   * example 1:
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 main :: IO () main :: IO ()
 main = do main = do
-    print $ nCol xc+    print $ nCol xcToPrint
  
 {-| to be able to print into a 'CharMatrix' {-| to be able to print into a 'CharMatrix'
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     deriving Show     deriving Show
  
-xc :: MatrixCursor +xcToPrint :: MatrixCursor 
-xc = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor+xcToPrint = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor
  
 incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor
-incMatrixCursorCol n (MatrixCursor nRow nCol) = (MatrixCursor nRow (nCol+n))+incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (nRow xc) (nCol+n))
  
 initMatrixCursor :: MatrixCursor initMatrixCursor :: MatrixCursor
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 </code> </code>
  
-  * compiler warning, using ''stack ghc -- -Wall ./app/Main.hs'': +  * compiler output (using ''stack ghc -- **-Wall** ./app/Main.hs''): 
-    * <code>+    * <code Haskell>
 [1 of 1] Compiling Main             ( app\Main.hs, app\Main.o ) [1 of 1] Compiling Main             ( app\Main.hs, app\Main.o )
  
-app\Main.hs:25:36: warning: [-Wname-shadowing] +app\Main.hs:24:45: 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)) +
-   |                                    ^^^^ +
- +
-app\Main.hs:25:41: warning: [-Wname-shadowing]+
     This binding for `nCol' shadows the existing binding     This binding for `nCol' shadows the existing binding
-      defined at app\Main.hs:18:9+      defined at app\Main.hs:17:13
    |    |
-25 | incMatrixCursorCol n (MatrixCursor nRow nCol) = (MatrixCursor nRow (nCol+n)) +24 |     incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (nRow xc) (nCol+n)) 
-   |                                         ^^^^+   |                                             ^^^^
 Linking app\Main.exe ... Linking app\Main.exe ...
 </code> </code>
 +
 +
  
   * example 2:   * example 2:
     * <code Haskell>     * <code Haskell>
 +module Main where
 +
 +main :: IO ()
 +main = do
 +    print $ nCol xcToPrint
 +
 {-| to be able to print into a 'CharMatrix' {-| to be able to print into a 'CharMatrix'
  
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     deriving Show     deriving Show
  
-xc :: MatrixCursor +xcToPrint :: MatrixCursor 
-xc = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor+xcToPrint = incMatrixCursorCol 5 $ incMatrixCursorCol 2 $ incMatrixCursorCol 3 $ initMatrixCursor
  
 incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor incMatrixCursorCol :: Integer -> MatrixCursor -> MatrixCursor
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 [2 of 2] Compiling Main [2 of 2] Compiling Main
  
-app\Main.hs:25:67: error:+app\Main.hs:24:71: error:
     * Couldn't match expected type `MatrixCursor -> Integer'     * Couldn't match expected type `MatrixCursor -> Integer'
                   with actual type `Integer'                   with actual type `Integer'
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       In the expression: (MatrixCursor (nRow xc) (nCol + n))       In the expression: (MatrixCursor (nRow xc) (nCol + n))
    |    |
-25 | incMatrixCursorCol n xc@(MatrixCursor nRow nCol) = (MatrixCursor (nRow xc) (nCol+n)) +24 |     incMatrixCursorCol n xc@(MatrixCursor nRow nCol) = (MatrixCursor (nRow xc) (nCol+n)) 
-   |                                                                   ^^^^^^^+   |                                                                       ^^^^^^^
 </code> </code>
 +
 +
 +
 +  * example 3 (warning free with ''stack ghc -- **-Wall** ./app/Main.hs''):
 +    * <code Haskell>
 +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)
 +</code>
 +
 +  * output:
 +    * <code>
 +10
 +</code>
 +
 +
 +===== ✎ =====
 +~~DISCUSSION~~
codesnippets/recordsyntax.1616267630.txt.gz · Last modified: (external edit)
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