Haskell - guard inside case statement

Question

I am going through Learn you a haskell book, and in Chapter 8 there is a snippet of code which looks like this

data LockerState = Taken | Free deriving (Eq, Show)
type Code = String
type LockerMap = Map.Map Int (LockerState, Code)

lookup' :: Int -> LockerMap -> Either String Code
lookup' num_ map_ =
   case (Map.lookup num_ map_) of
      Nothing -> Left $ "LockerNumber doesn't exist!"
      Just (state, code) -> if state == Taken
                              then Left $ "LockerNumber already taken!"
                              else Right $ code

This works. However, I wanted to convert if/else block to guard statements like this:

lookup' :: Int -> LockerMap -> Either String Code
lookup' num_ map_ =
   case (Map.lookup num_ map_) of
      Nothing -> Left $ "LockerNumber doesn't exist!"
      Just (state, code) ->
         | state == Taken = Left $ "LockerNumber already taken!"
         | otherwise = Right $ Code

This doesn't compile. It seems that usage of guards in Haskell is very restrictive/non intuitive. SO Ex1 SO Ex2. Is there a definite source which I can read which tells at which places I can use guards?

Answer 1

There are two places guards are allowed: function definitions and case expressions. In both contexts, guards appear after a pattern and before the body, so you use = in functions and -> in case branches, as usual:

divide x y
  | y == 0 = Nothing
  --------
  | otherwise = Just (x / y)
  -----------

positively mx = case mx of
  Just x | x > 0 -> Just x
         -------
  _ -> Nothing

Guards are simply constraints for patterns, so Just x matches any non-Nothing value, but Just x | x > 0 only matches a Just whose wrapped value is also positive.

I suppose the definitive reference is the Haskell Report, specifically §3.13 Case Expressions and §4.4.3 Function and Pattern Bindings, which describe the syntax of guards and specify where they’re allowed.

In your code, you want:

Just (state, code)
  | state == Taken -> Left "LockerNumber already taken!"
  | otherwise -> Right code

This is also expressible with patterns alone:

Just (Taken, _) -> Left "LockerNumber already taken!"
Just (_, code) -> Right code