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--- codesnippets:recordsyntax [2021/03/20 19:57] – created f2b216
+++ codesnippets:recordsyntax [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== 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=====
@@ Line 32: / Line 44: @@
 ==== 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:
@@ Line 47: / Line 57: @@
 </code>
-  * output:
+  * output of runtime error:
     * <code>
 Test2-exe.exe: No match in record selector nDiameterInMM
@@ Line 54: / Line 64: @@
 ==== Risk of namespace colisions without strong coding rules ====
+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).
-<code Haskell>
+<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:
+    * <code Haskell>
 module Main where
 main :: IO ()
 main = do
-    print $ nCol xc
+    print $ nCol xcToPrint
 {-| to be able to print into a 'CharMatrix'
@@ Line 76: / Line 97: @@
     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 n (MatrixCursor nRow nCol) = (MatrixCursor nRow (nCol+n))
+incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (nRow xc) (nCol+n))
 initMatrixCursor :: MatrixCursor
@@ Line 86: / Line 107: @@
 </code>
-  * compiler warning:
+  * compiler output (using ''stack ghc -- **-Wall** ./app/Main.hs''):
-    * <code>
+    * <code Haskell>
-app\Main.hs:25:36: warning: [-Wname-shadowing]
+[1 of 1] Compiling Main             ( app\Main.hs, app\Main.o )
-    This binding for `nRow' shadows the existing binding
-      defined at app\Main.hs:16:9
+app\Main.hs:24:45: warning: [-Wname-shadowing]
+    This binding for `nCol' shadows the existing binding
+      defined at app\Main.hs:17:13
    |
- | incMatrixCursorCol n (MatrixCursor nRow nCol) = (MatrixCursor nRow (nCol+n))
+ |     incMatrixCursorCol n xc@(MatrixCursor _ nCol) = (MatrixCursor (nRow xc) (nCol+n))
-   |                                    ^^^^
+   |                                             ^^^^
+Linking app\Main.exe ...
+</code>
+  * example 2:
+    * <code Haskell>
+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)
+</code>
+  * compiler output:
+    * <code Haskell>
+[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))
+   |
+|     incMatrixCursorCol n xc@(MatrixCursor nRow nCol) = (MatrixCursor (nRow xc) (nCol+n))
+   |                                                                       ^^^^^^^
+</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>
 </code>
+===== ✎ =====
+~~DISCUSSION~~