diff --git a/Data/Either/Unwrap.hs b/Data/Either/Unwrap.hs
--- a/Data/Either/Unwrap.hs
+++ b/Data/Either/Unwrap.hs
@@ -17,6 +17,9 @@
     ,    isRight
     ,    fromLeft
     ,    fromRight
+    ,    mapBoth
+    ,    mapLeft
+    ,    mapRight
     ,    eitherM
     ,    whenLeft
     ,    whenRight
@@ -48,6 +51,22 @@
 fromRight           :: Either a b -> b
 fromRight (Left _)  = error "Either.Unwrap.fromRight: Argument takes form 'Left _'" -- yuck
 fromRight (Right x) = x
+
+-- | The 'mapBoth' function takes two functions and applies the first if iff the value
+-- takes the form 'Left _' and the second if the value takes the form 'Right _'.
+mapBoth :: (a -> c) -> (b -> d) -> Either a b -> Either c d
+mapBoth f _ (Left x) = Left (f x)
+mapBoth _ f (Right x) = Right (f x)
+
+-- | The 'mapLeft' function takes a function and applies it to an Either value
+-- iff the value takes the form 'Left _'.
+mapLeft :: (a -> c) -> Either a b -> Either c b
+mapLeft = (`mapBoth` id)
+
+-- | The 'mapLeft' function takes a function and applies it to an Either value
+-- iff the value takes the form 'Left _'.
+mapRight :: (b -> c) -> Either a b -> Either a c
+mapRight = (id `mapBoth`)
 
 -- | The 'eitherM' function takes an 'Either' value and two functions which return monads.
 -- If the argument takes the form @Left _@ then the element within is passed to the first
diff --git a/either-unwrap.cabal b/either-unwrap.cabal
--- a/either-unwrap.cabal
+++ b/either-unwrap.cabal
@@ -1,6 +1,6 @@
 Name:           either-unwrap
 Description:    Functions for probing and unwrapping values inside of Either.
-Version:        1.0
+Version:        1.1
 Category:       Data
 Cabal-Version:  >= 1.2
 License:        BSD3
@@ -12,7 +12,7 @@
 Build-Type:     Simple
 
 Library
-  Build-Depends:    base >= 3
+  Build-Depends:    base >= 3 && < 5
   Hs-Source-Dirs:   .
   Exposed-Modules:  Data.Either.Unwrap
   GHC-Options:      -Wall