some work on avoiding partial functions

There are still hundreds of places that use partial functions head, tail,
init, and last.

2 files changed, 37 insertions, 0 deletions

diff --git a/Utility/BadPrelude.hs b/Utility/BadPrelude.hs
new file mode 100644
index 000000000..1bb70adfb
--- /dev/null
+++ b/Utility/BadPrelude.hs
@@ -0,0 +1,24 @@
+{- Some stuff from Prelude should not be used, as it tends to be a source
+ - of bugs.
+ -
+ - This exports functions that conflict with the prelude, which avoids
+ - them being accidentially used.
+ -}
+
+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
diff --git a/Utility/Misc.hs b/Utility/Misc.hs
index 728598723..541e150b7 100644
--- a/Utility/Misc.hs
+++ b/Utility/Misc.hs
@@ -27,6 +27,19 @@ readMaybe s = case reads s of
 	((x,_):_) -> Just x
 	_ -> Nothing
 
+{- Like break, but the character matching the condition is not included
+ - in the second result list.
+ -
+ - separate (== ':') "foo:bar" = ("foo", "bar")
+ - separate (== ':') "foobar" = ("foo, "")
+ -}
+separate :: (a -> Bool) -> [a] -> ([a], [a])
+separate c l = unbreak $ break c l
+	where
+		unbreak r@(a, b)
+			| null b = r
+			| otherwise = (a, tail b)
+
 {- Catches IO errors and returns a Bool -}
 catchBoolIO :: IO Bool -> IO Bool
 catchBoolIO a = catchDefaultIO a False