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--- codesnippets:filesystemioandstreams [2021/04/20 13:01] – created f2b216
+++ codesnippets:filesystemioandstreams [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+~~DISCUSSION ~~
 ====== Filesystem IO and streams ======
   *example:
-    * code<code Haskell>
+    * code, module FileSystem<code Haskell>
 {-|
 Description : is to provides a generalised, abstract, and save interface to the files system.
@@ Line 17: / Line 18: @@
         * uniform types for identification of input and output streams ('Inp', 'Out')
-        * higher order functions (stream handler functions, '(SysIo.Handle -> IO s)', '(SysIo.Handle -> s -> IO ())') are applied on streams
+        * higher order functions encapsulated in data types ('DataReader', 'DataWriter') are applied on streams
     * can not cause error by "invalid characters", as long as the stream handler functions do not cause them
-* supports the following file system features:
+    * supports the following file system features:
-    * deletion of files
+        * deletion of files
-    * reading and writing streams
+        * processing (read and write at once) and writing streams
-* supports the following data types:
+    * supports the following data types:
-    * Octets
+        * Octets
+NOTE: There is no isolated read function like the write function, because this would lead to errors like "illegal operation (delayed read on closed handle)".
+Background: The function hGetContents needs an open file handle, that may be closed before the lazy String has been evaluated.
 * example
     @
-    import qualified OctetsVertable as Oct
+import qualified FileSystem as FS
-    import qualified FileSystem as FS
+import qualified System.IO as SysIo
-    main :: IO ()
+main :: IO ()
-    main =
+main =
-        do
+        FS.processData
-            o1 <- FS.readFromStream (FS.FileInp ".\\test\\Spec.hs") Oct.getOctets
+            (FS.DataReader (FS.FileInp ".\\test\\a.txt") SysIo.hGetContents)
-            FS.writeToStream (FS.FileOut ".\\test\\Spec-2.hs") Oct.putOctets o1
+            [
-            FS.writeToStream FS.StdOut Oct.putOctets o1
+                FS.DataWriter (FS.FileOut ".\\test\\b.txt") SysIo.hPutStr,
-            FS.writeToStream FS.StdErr Oct.putOctets o1
+                FS.DataWriter FS.StdOut SysIo.hPutStr,
+                FS.DataWriter FS.StdErr SysIo.hPutStr
+            ]
     @
-    * copies the file "Spec.hs" in ".\test" to "Spec-2.hs"
+    * copies the file "a.txt" in ".\test" to "b.txt"
-    * sends to octal representation to 'stdout', and 'stderr'
+    * sends the file content of "a.txt" also to 'stdout', and 'stderr'
     * will not cause uncaught exception caused by "invalid characters"
 -}
@@ Line 52: / Line 59: @@
         Inp(..),
         Out(..),
+        DataReader(..),
+        DataWriter(..),
         removeFileIfExists,
-        readFromStream,
+        processData,
-        writeToStream
+        writeToData
     ) where
@@ Line 83: / Line 92: @@
                        | otherwise = Excp.throwIO e
-readFromStream :: Inp -> (SysIo.Handle -> IO s) -> IO s
+data DataReader s =
-readFromStream StdInp f = f SysIo.stdin
+    DataReader {
-readFromStream (FileInp sFileName) f =
+        rinp :: Inp,
+        rfRead :: (SysIo.Handle -> IO s) }
+data DataWriter s =
+    DataWriter {
+        rout :: Out,
+        rfWrite :: (SysIo.Handle -> s -> IO ()) }
+processData :: (DataReader a) -> [DataWriter a] -> IO ()
+processData (DataReader StdInp f) lwrt =
+    do
+        dt <- f SysIo.stdin
+        processData' dt lwrt
+processData (DataReader (FileInp sFileName) f) lwrt =
     do
         hFile <- SysIo.openFile sFileName SysIo.ReadMode
-        x <- f hFile
+        dt <- f hFile
+        processData' dt lwrt
         SysIo.hClose hFile
-        return x
-writeToStream :: Out -> (SysIo.Handle -> s -> IO ()) -> s -> IO ()
+processData' :: a -> [DataWriter a] -> IO ()
-writeToStream NullOut _ _ = return ()
+processData' x [] = return ()
-writeToStream StdOut f x = f SysIo.stdout x
+processData' x (wrt:lrwrt) =
-writeToStream StdErr f x = f SysIo.stderr x
+    do
-writeToStream (FileOut sFileName) f x =
+        writeToData wrt x
+        processData' x lrwrt
+writeToData :: (DataWriter a) -> a -> IO ()
+writeToData (DataWriter NullOut _) _ = return ()
+writeToData (DataWriter StdOut f) x = f SysIo.stdout x
+writeToData (DataWriter StdErr f) x = f SysIo.stderr x
+writeToData (DataWriter (FileOut sFileName) f) x =
     do
         hFile <- SysIo.openFile sFileName SysIo.WriteMode
@@ Line 102: / Line 131: @@
         SysIo.hClose hFile
 </code>
+    * code, use case of files system<code Haskell>
+import qualified FileSystem as FS
+import qualified System.IO as SysIo
+main :: IO ()
+main =
+        FS.processData
+            (FS.DataReader (FS.FileInp ".\\test\\a.txt") SysIo.hGetContents)
+            [
+                FS.DataWriter (FS.FileOut ".\\test\\b.txt") SysIo.hPutStr,
+                FS.DataWriter FS.StdOut SysIo.hPutStr,
+                FS.DataWriter FS.StdErr SysIo.hPutStr
+            ]
+</code>
+    * compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
+    * executes with output as file copy of "a.txt" to "b.txt", ''stdout'', and ''stderr''
+  * example, with conversions
+    * code:<code Haskell>
+import qualified FileSystem as FS
+import qualified System.IO as SysIo
+import qualified Data.Char as Ch
+main :: IO ()
+main =
+        FS.processData
+            (FS.DataReader (FS.FileInp ".\\test\\a.txt") hGetToUpper)
+            [
+                FS.DataWriter (FS.FileOut ".\\test\\b.txt") SysIo.hPutStr,
+                FS.DataWriter FS.StdOut hPutToLower,
+                FS.DataWriter FS.StdErr SysIo.hPutStr
+            ]
+hGetToUpper :: SysIo.Handle -> IO String
+hGetToUpper h =
+    do
+        s <- SysIo.hGetContents h
+        return (fmap Ch.toUpper s)
+hPutToLower :: SysIo.Handle -> String -> IO ()
+hPutToLower h s = SysIo.hPutStr h (fmap Ch.toLower s)
+</code>
+    * input, file "a.txt"<code>
+The quick brown fox jumps over the lazy dog.
+</code>
+    * output, in terminal:<code>
+the quick brown fox jumps over the lazy dog.
+THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
+</code>
+    * output, file "b.txt":<code>
+THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
+</code>
+===== ✎ =====
+~~DISCUSSION~~