add beginning, end

Safe versions of init and last

1 files changed, 29 insertions, 7 deletions

diff --git a/Utility/BadPrelude.hs b/Utility/BadPrelude.hs
index 8e4105cee..49837b927 100644
--- a/Utility/BadPrelude.hs
+++ b/Utility/BadPrelude.hs
@@ -7,22 +7,44 @@
 
 module Utility.BadPrelude where
 
-{- head is a partial function; head [] is an error -}
+{- 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 -}
+{- tail is also partial
+ - Instead, use: drop 1 -}
 tail :: [a] -> [a]
 tail = Prelude.tail
 
-{- init too -}
+{- init too
+ - Instead, use: beginning -}
 init :: [a] -> [a]
 init = Prelude.init
 
-{- last too -}
+{- last too
+ - Instead, use: end -}
 last :: [a] -> a
 last = Prelude.last
 
-{- read should be avoided, as it throws an error -}
-read :: Read a => String -> a
-read = Prelude.read
+{- 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
+