a) modified this a bit because its more complicated than it needed

to be, in two ways. first, the map and filter situation was uneccesary.
list comprehension with a verify predicate is more easily followed.
b) the monadic bind thing could be cool in the predicate, as i could
create one mega computation for the verify, but its entirely unecessary,
and makes each individual check more complex than it needs to be.
c) this is simple enough that type annotations are unecessary.
d) stopped trying to do io or using the main function. it seems sensible
to treat this as a library of functions rather than a single program to be
executed.
e) the checks here are somewhat specific, it makes sense to make them
only visible to the passcodes function.

4.2/4.2.hs

@@ -0,0 +1,6 @@
+import Data.List
+import Data.List.Ordered
+
+passcodes = length [ x | x <- [254042 .. 789860], ascending x, adjacents x]
+  where ascending = isSorted . show
+        adjacents num = any (\digit -> length digit == 2) (group $ show num)

4.2/4.hs

@@ -1,15 +0,0 @@
-import Data.List
-import Data.List.Ordered
-import Data.Maybe
-
-ascending :: Int -> Maybe Int
-ascending num = case isSorted $ show num of True -> Just num
-                                            False -> Nothing
-
-adjacents :: Int -> Maybe Int
-adjacents num = case any (\digit -> length digit == 2) (group $ show num) of True -> Just num
-                                                                             False -> Nothing
-
-main = do
-  putStrLn $ show $ length $ filter (isJust) (map (verify) [254042 .. 789860])
-  where verify num = ascending num >>= adjacents

4/4.hs

@@ -1,15 +1,6 @@
 import Data.List
 import Data.List.Ordered
-import Data.Maybe
 
-ascending :: Int -> Maybe Int
-ascending num = case isSorted $ show num of True -> Just num
-                                            False -> Nothing
-
-adjacents :: Int -> Maybe Int
-adjacents num = case any (\digit -> length digit >= 2) (group $ show num) of True -> Just num
-                                                                             False -> Nothing
-
-main = do
-  putStrLn $ show $ length $ filter (isJust) (map (verify) [254042 .. 789860])
-  where verify num = ascending num >>= adjacents
+passcodes = length [ x | x <- [254042 .. 789860], ascending x, adjacents x]
+  where ascending = isSorted . show
+        adjacents num = any (\digit -> length digit >= 2) (group $ show num)