Haskell/Understanding monads/Maybe

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We introduced monads using Maybe as an example. The Maybe monad represents computations which might "go wrong" by not returning a value. For reference, here are our definitions of return and (>>=) for Maybe as we saw in the last chapter:[1]

    return :: a -> Maybe a
    return x  = Just x
 
    (>>=)  :: Maybe a -> (a -> Maybe b) -> Maybe b
    (>>=) m g = case m of
                   Nothing -> Nothing
                   Just x  -> g x

Safe functions[edit]

The Maybe datatype provides a way to make a safety wrapper around functions which can fail to work for a range of arguments. For example, head and tail only work with non-empty lists. Another typical case, which we will explore in this section, are mathematical functions like sqrt and log; (as far as real numbers are concerned) these are only defined for non-negative arguments.

> log 1000
6.907755278982137
> log -1000
''ERROR'' -- runtime error

To avoid this crash, a "safe" implementation of log could be:

safeLog :: (Floating a, Ord a) => a -> Maybe a
safeLog x
    | x >= 0    = Just (log x)
    | otherwise = Nothing
> safeLog 1000
Just 6.907755278982137
> safeLog -1000
Nothing

We could write similar "safe functions" for all functions with limited domains such as division, square-root, and inverse trigonometric functions (safeDiv, safeSqrt, safeArcSin, etc. all of which would have the same type as safeLog but definitions specific to their constraints)

If we wanted to combine these monadic functions, the cleanest approach is with monadic composition (which was mentioned briefly near the end of the last chapter) and point-free style:

safeLogSqrt = safeLog <=< safeSqrt

Written in this way, safeLogSqrt resembles a lot its unsafe, non-monadic counterpart:

unsafeLogSqrt = log . sqrt

Lookup tables[edit]

A lookup table relates keys to values. You look up a value by knowing its key and using the lookup table. For example, you might have a phone book application with a lookup table where contact names are keys to corresponding phone numbers. An elementary way of implementing lookup tables in Haskell is to use a list of pairs: [(a, b)]. Here a is the type of the keys, and b the type of the values.[2] Here's how the phone book lookup table might look like:

phonebook :: [(String, String)]
phonebook = [ ("Bob",   "01788 665242"),
              ("Fred",  "01624 556442"),
              ("Alice", "01889 985333"),
              ("Jane",  "01732 187565") ]

The most common thing you might do with a lookup table is look up values. Everything is fine if we try to look up "Bob", "Fred", "Alice" or "Jane" in our phone book, but what if we were to look up "Zoe"? Zoe isn't in our phone book, so the lookup would fail. Hence, the Haskell function to look up a value from the table is a Maybe computation (it is available from Prelude):

lookup :: Eq a => a  -- a key
       -> [(a, b)]   -- the lookup table to use
       -> Maybe b    -- the result of the lookup

Let us explore some of the results from lookup:

Prelude> lookup "Bob" phonebook
Just "01788 665242"
Prelude> lookup "Jane" phonebook
Just "01732 187565"
Prelude> lookup "Zoe" phonebook
Nothing

Now let's expand this into using the full power of the monadic interface. Say, we're now working for the government, and once we have a phone number from our contact, we want to look up this phone number in a big, government-sized lookup table to find out the registration number of their car. This, of course, will be another Maybe-computation. But if the person we're looking for isn't in our phone book; we certainly won't be able to look up their registration number in the governmental database. What we need is a function that will take the results from the first computation and put it into the second lookup only if we get a successful value in the first lookup. Of course, our final result should be Nothing if we get Nothing from the either of the lookups.

getRegistrationNumber :: String       -- their name
                      -> Maybe String -- their registration number
getRegistrationNumber name = 
  lookup name phonebook >>=
    (\number -> lookup number governmentDatabase)

If we then wanted to use the result from the governmental database lookup in a third lookup (say we want to look up their registration number to see if they owe any car tax), then we could extend our getRegistrationNumber function:

getTaxOwed :: String       -- their name
           -> Maybe Double -- the amount of tax they owe
getTaxOwed name = 
  lookup name phonebook >>=
    (\number -> lookup number governmentDatabase) >>=
      (\registration -> lookup registration taxDatabase)

Or, using the do-block style:

getTaxOwed name = do
  number       <- lookup name phonebook
  registration <- lookup number governmentDatabase
  lookup registration taxDatabase

Let's just pause here and think about what would happen if we got a Nothing anywhere. By definition, when the first argument to >>= is Nothing, it just returns Nothing while ignoring whatever function it is given. Thus, a Nothing at any stage in the large computation will result in a Nothing overall, regardless of the other functions. After the first Nothing hits, all >>=s will just pass it to each other, skipping the other function arguments. The technical description says that the structure of the Maybe monad propagates failures.

Open monads[edit]

Another trait of the Maybe monad is that it is "open": if we have a Just value, we can see the contents and extract the associated values through pattern matching.

zeroAsDefault :: Maybe Int -> Int
zeroAsDefault mx = case mx of
    Nothing -> 0
    Just x -> x

This usage pattern of replacing Nothing with a default is captured by the fromMaybe function in Data.Maybe.

zeroAsDefault :: Maybe Int -> Int
zeroAsDefault mx = fromMaybe 0 mx

The maybe Prelude function allows us to do it in a more general way, by supplying a function to modify the extracted value.

displayResult :: Maybe Int -> String
displayResult mx = maybe "There was no result" (("The result was " ++) . show) mx
Prelude> :t maybe
maybe :: b -> (a -> b) -> Maybe a -> b
Prelude> displayResult (Just 10)
"The result was 10"
Prelude> displayResult Nothing
"There was no result"

This possibility makes sense for Maybe, as it allows us to recover from failures. Not all monads are open in this way; often, they are designed to hide unnecessary details. return and (>>=) alone do not allow us to extract the underlying value from a monadic computation, and so it is perfectly possible to make a "no-exit" monad, from which it is never possible to extract values. The most obvious example of that is the IO monad.

Maybe and safety[edit]

We have seen how Maybe can make code safer by providing a graceful way to deal with failure that does not involve runtime errors. Does that mean we should always use Maybe for everything? Not really.

When you write a function, you are able to tell whether it might fail to produce a result during normal operation of the program,[3] either because the functions you use might fail (as in the examples in this chapter) or because you know some of the argument or intermediate result values do not make sense (for instance, imagine a calculation that is only meaningful if its argument is less than 10). If that is the case, by all means use Maybe to signal failure; it is far better than returning an arbitrary default value or throwing an error.

Now, adding Maybe to a result type without a reason would only make the code more confusing and no safer. The type signature of a function with unnecessary Maybe would tell users of the code that the function could fail when it actually can't. Of course, that is not as bad a lie as the opposite one (that is, claiming that a function will not fail when it actually can), but we really want honest code in all cases. Furthermore, using Maybe forces us to propagate failure (with fmap or monadic code) and eventually handle the failure cases (using pattern matching, the maybe function, or fromMaybe from Data.Maybe). If the function cannot actually fail, coding for failure is an unnecessary complication.


Notes[edit]

  1. The definitions in the actual instance in Data.Maybe are written a little differently, but are fully equivalent to these.
  2. Check the chapter about maps in Haskell in Practice for a different, and potentially more useful, implementation.
  3. With "normal operation" we mean to exclude failure caused by uncontrollable circumstances in the real world, such as memory exhaustion or a dog chewing the printer cable.