Exceptions should be used for exceptional conditions. Throwing an exception is basically making the statement, "I can't handle this condition here; can someone higher up on the call stack catch this for me and handle it?"
Returning a value can be preferable, if it is clear that the caller will take that value and do something meaningful with it. This is especially true if throwing an exception has performance implications, i.e. it may occur in a tight loop.
Throwing an exception takes much longer than returning a value (by at least two orders of magnitude).
Exceptions should never be used to implement program logic. In other words, don't throw an exception to get something done; throw an exception to state that it couldn't be done.
Views #1 and #2 are incorrect in general.
- Any data-type of kind
* -> * can work as a label, monads are much more than that.
- (With the exception of the
IO monad) computations within a monad are not impure. They simply represent computations that we perceive as having side effects, but they're pure.
Both these misunderstandings come from focusing on the IO monad, which is actually a bit special.
I'll try to elaborate on #3 a bit, without getting into category theory if possible.
Standard computations
All computations in a functional programming language can be viewed as functions with a source type and a target type: f :: a -> b. If a function has more than one argument, we can convert it to an one-argument function by currying (see also Haskell wiki). And if we have just a value x :: a (a function with 0 arguments), we can convert it into a function that takes an argument of the unit type: (\_ -> x) :: () -> a.
We can build more complex programs form simpler ones by composing such functions using the . operator. For example, if we have f :: a -> b and g :: b -> c we get g . f :: a -> c. Note that this works for our converted values too: If we have x :: a and convert it into our representation, we get f . ((\_ -> x) :: () -> a) :: () -> b.
This representation has some very important properties, namely:
- We have a very special function - the identity function
id :: a -> a for each type a. It is an identity element with respect to .: f is equal both to f . id and to id . f.
- The function composition operator
. is associative.
Monadic computations
Suppose we want to select and work with some special category of computations, whose result contains something more than just the single return value. We don't want to specify what "something more" means, we want to keep things as general as possible. The most general way to represent "something more" is representing it as a type function - a type m of kind * -> * (i.e. it converts one type to another). So for each category of computations we want to work with, we'll have some type function m :: * -> *. (In Haskell, m is [], IO, Maybe, etc.) And the category will contains all functions of types a -> m b.
Now we would like to work with the functions in such a category in the same way as in the basic case. We want to be able to compose these functions, we want the composition to be associative, and we want to have an identity. We need:
- To have an operator (let's call it
<=<) that composes functions f :: a -> m b and g :: b -> m c into something as g <=< f :: a -> m c. And, it must be associative.
- To have some identity function for each type, let's call it
return. We also want that f <=< return is the same as f and the same as return <=< f.
Any m :: * -> * for which we have such functions return and <=< is called a monad. It allows us to create complex computations from simpler ones, just as in the basic case, but now the types of return values are tranformed by m.
(Actually, I slightly abused the term category here. In the category-theory sense we can call our construction a category only after we know it obeys these laws.)
Monads in Haskell
In Haskell (and other functional languages) we mostly work with values, not with functions of types () -> a. So instead of defining <=< for each monad, we define a function (>>=) :: m a -> (a -> m b) -> m b. Such an alternative definition is equivalent, we can express >>= using <=< and vice versa (try as an exercise, or see the sources). The principle is less obvious now, but it remains the same: Our results are always of types m a and we compose functions of types a -> m b.
For each monad we create, we must not forget to check that return and <=< have the properties we required: associativity and left/right identity. Expressed using return and >>= they are called the monad laws.
An example - lists
If we choose m to be [], we get a category of functions of types a -> [b]. Such functions represent non-deterministic computations, whose results could be one or more values, but also no values. This gives rise to the so-called list monad. The composition of f :: a -> [b] and g :: b -> [c] works as follows: g <=< f :: a -> [c] means to compute all possible results of type [b], apply g to each of them, and collect all the results in a single list. Expressed in Haskell
return :: a -> [a]
return x = [x]
(<=<) :: (b -> [c]) -> (a -> [b]) -> (a -> [c])
g (<=<) f = concat . map g . f
or using >>=
(>>=) :: [a] -> (a -> [b]) -> [b]
x >>= f = concat (map f x)
Note that in this example the return types were [a] so it was possible that they didn't contain any value of type a. Indeed, there is no such requirement for a monad that the return type should have such values. Some monads always have (like IO or State), but some don't, like [] or Maybe.
The IO monad
As I mentioned, the IO monad is somewhat special. A value of type IO a means a value of type a constructed by interacting with the program's environment. So (unlike all the other monads), we cannot describe a value of type IO a using some pure construction. Here IO is simply a tag or a label that distinguishes computations that interact with the environment. This is (the only case) where the views #1 and #2 are correct.
For the IO monad:
- Composition of
f :: a -> IO b and g :: b -> IO c means: Compute f that interacts with the environment, and then compute g that uses the value and computes the result interacting with the environment.
return just adds the IO "tag" to the value (we simply "compute" the result by keeping the environment intact).- The monad laws (associativity, identity) are guaranteed by the compiler.
Some notes:
- Since monadic computations always have the result type of
m a, there is no way how to "escape" from the IO monad. The meaning is: Once a computation interacts with the environment, you cannot construct a computation from it that doesn't.
- When a functional programmer doesn't know how to make something in a pure way, (s)he can (as the last resort) program the task by some stateful computation within the
IO monad. This is why IO is often called a programmer's sin bin.
- Notice that in an impure world (in the sense of functional programming) reading a value can change the environment too (like consume user's input). That's why functions like
getChar must have a result type of IO something.
Best Answer
Control.Exceptionis designed for use:For an example of the first, take division by 0 in the
Inttype (e.g.div 3 0). You can't give a reasonable answer in that case, but you don't want your divide operator to return aMaybe Int– it could make programmers miserable! So you could use an exception to stop the computation.In this situation, you could also use
errorfor a similar effect. The difference is thatControl.Exceptiongives you more flexibility for catching and reporting what happened. But such exceptions can only be caught in the IO monad, which could place a burden on your client. Therefore, I'd suggest you save these for truly exceptional, perhaps catastrophic situations.The stuff in
Control.Monad.Errorworks via a monad, and uses the monadic binding/sequencing machinery to pass the error along. It doesn't subvert the type system likethrowdoes, so it has the virtues of purity and relative transparency going for it. It also gives you more flexibility to handle and recover from errors, and can also make it easier to pass errors back to the client if you're writing a library (see Parsec and polyparse, which both transform some monadic error + result into an Either value when they're done, though polyparse uses something a bit different fromControl.Monad.Error). But of course you have to work within the error-handling monad, or wrap an error-handling monad transformer around whatever existing monads you might be working in. Both options can make your code considerably more complex.If you haven't already, see Real World Haskell, Chapter 19 for a rundown of these and other error-handling options, and see the end of Chapter 18 for some of the more general drawbacks of using monads and monad transformers.