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--- codesnippets:patternmatching [2021/04/14 12:00] – created f2b216
+++ codesnippets:patternmatching [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 5: / Line 5: @@
   * list comprehensions
   * do notations
+  * case constructs
 Before we start with that, we provide more details about pattern matching, regarding:
@@ Line 10: / Line 11: @@
   * algebraic data types
   * tuples
+Additionally, even before that we explain the wildcard ''_''.
+===== Wildcard =====
+  * wildcard ''_''
+    * stands-in for any value
+  * example
+    * code:<code Hsakell>
+main :: IO ()
+main =
+    do
+        print (firstFromTuple3 ("A",'b','c'))
+        print (secondFromList "Hallo!")
+        print (getZipFromPerson (PersonNameZipAndEMail "Haskell" "Brooks" "Curry" 16802 "hbc@psu.edu"))
+firstFromTuple3 :: (a,b,c) -> a
+firstFromTuple3 (x,_,_) = x
+secondFromList :: [a] -> a
+secondFromList (_:x:_) = x
+data Person = PersonNameAndZip String String String Int | PersonNameZipAndEMail String String String Int String
+getZipFromPerson :: Person -> Int
+getZipFromPerson (PersonNameAndZip _ _ _ nZip) = nZip
+getZipFromPerson (PersonNameZipAndEMail _ _ _ nZip _) = nZip
+</code>
+    * compiles, with warning: Pattern match(es) are non-exhaustive
+    * executes, with output:<code>
+"A"
+'a'
+</code>
+  * NOTE: The function ''secondFromList'' will execute with runtine error when there is less than two elements in the list.
+  * example
+    * code:<code Hsakell>
+main :: IO ()
+main =
+    do
+        print (secondFromList "H")
+secondFromList :: [a] -> a
+secondFromList (_:x:_) = x
+</code>
+    * compiles, with warning: Pattern match(es) are non-exhaustive
+    * executes, with output:<code>
+Test3-exe.exe: app\Main.hs:7:1-26: Non-exhaustive patterns in function secondFromList
+</code>
 ===== Context: function definition =====
@@ Line 23: / Line 73: @@
 main =
     do
-        print (length s)
+        print (length s)             -- "caller perspective" for first parameter of function length
 s :: String
@@ Line 29: / Line 79: @@
 length :: [a] -> Integer
-length [] = 0
+length [] = 0                        -- "inner function perspective" - selected when list is empty
-length (_:lrx) = 1 + (length lrx)
+length (_:lrx) = 1 + (length lrx)    -- "inner function perspective" - selected when at least one element
+                                     -- using the rest (lrx) for recursive call of length (again "caller perspective")
 </code>
+===== ✎ =====
+~~DISCUSSION~~