add beginning, end

Safe versions of init and last

Utility/BadPrelude.hs

@@ -7,22 +7,44 @@
 
 module Utility.BadPrelude where
 
-{- head is a partial function; head [] is an error -}
-head :: [a] -> a
-head = Prelude.head
-
-{- tail is also partial -}
-tail :: [a] -> [a]
-tail = Prelude.tail
-
-{- init too -}
-init :: [a] -> [a]
-init = Prelude.init
-
-{- last too -}
-last :: [a] -> a
-last = Prelude.last
-
 {- read should be avoided, as it throws an error -}
 read :: Read a => String -> a
 read = Prelude.read
+
+{- head is a partial function; head [] is an error
+ - Instead, use: take 1 -}
+head :: [a] -> a
+head = Prelude.head
+
+{- tail is also partial
+ - Instead, use: drop 1 -}
+tail :: [a] -> [a]
+tail = Prelude.tail
+
+{- init too
+ - Instead, use: beginning -}
+init :: [a] -> [a]
+init = Prelude.init
+
+{- last too
+ - Instead, use: end -}
+last :: [a] -> a
+last = Prelude.last
+
+{- All but the last element of a list.
+ - (Like init, but no error on an empty list.) -}
+beginning :: [a] -> [a]
+beginning [] = []
+beginning (x:xs) =  beginning' x xs
+	where
+		beginning' _ [] = []
+		beginning' y (z:zs) = y : beginning' z zs
+
+{- Like last, but no error on an empty list. -}
+end :: [a] -> [a]
+end [] = []
+end (x:xs) = end' x xs
+	where
+		end' y [] = [y]
+		end' _ (y:ys) = end' y ys
+