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.
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.
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,
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.
m :: * -> * for which we have such functions
<=< 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
(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
<=< 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
<=< have the properties we required: associativity and left/right identity. Expressed using
>>= 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 arise to so-called list monad. The composition of
f :: a -> [b] and
g :: b -> [c] works as follows:
f <=< g :: a -> [c] means to compute all possible results of type
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
(>>=) :: [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
State), but some don't, like
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.
- 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.
- 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