Haskell: What's my indent error

I'm getting what I think is an indent error with the following code

152     -- find the first one who can refute the scenario
153     let cs = map ($scenario) [ Suspect . getWho, Room . getWhere, Weapon . getHow ]
154     let (qs,rss) = break (not . null . intersect cs . hand) ps
155     unless (null rss) $ do
156       let refuter:ss= rss
157       let valid = intersect cs $ hand refuter
158 
159       (refuter, shown) <- if cheated refuter
160                             -- if the refuter is a cheater, just choose for him
161                             then return (refuter, head valid)
162                             else do
163                               (refuter, shown) <- runStateT $ reveal scenario
164                               -- make sure they don't cheat
165                        开发者_开发技巧       return if shown `elem` valid
166                                         then (refuter, shown)
167                                         else (refuter { cheated = True }, head valid)
168       putTail (qs ++ refuter:ss)
169     return ()

The error I'm getting is

Game.hs:159:6:
    The last statement in a 'do' construct must be an expression

When I delete or comment out lines 159-167, the error goes away.

What am I doing wrong?

You need to put parenthesis around the if-then-else in the value for return in line 165:

return (if shown `elem` valid
           then (refuter, shown)
           else (refuter { cheated = True }, head valid))

A pure if-then-else is not allowed by the Haskell grammar as a function argument, it has to be put in parenthesis to be used in that place. (It is a lexp while a function argument has to be an aexp.)

The error message generated is not very helpful in finding that issue.

return $ if ..

This should work. Matter of function application priority.