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Introduction to Haskell, Part 3: Monads

by Adam Turoff
08/02/2007

In Part 1 of this series, I described how programming languages are grouped into two families, mechanical and mathematical. Programmers also fall into two general categories, those who think mechanically about what a computer does as it executes a program and those who think mathematically about what a program means. Some programmers are gifted and hop between these two worlds as needed.

In Part 2, I described how Haskell is a purely functional programming language and how it uses pure functions to express programs. Unfortunately, pure functions are limiting, because they prohibit side effects and basic I/O.

This article describes monads, the mechanism Haskell uses to structure computation, enable side effects, and communicate with the outside world.

The Language Divide Revisited

Haskell comes from a long and rich history of research in mathematics and functional programming. Naturally, many mathematically inclined programmers are quite comfortable with Haskell, while more mechanically minded programmers are generally befuddled with the language and the mathematical concepts it draws upon.

No concept is more befuddling than the monad.

Monads come from a branch of mathematics known as category theory (See more about category theory on Wikipedia and Haskell.org). Unfortunately, category theory is an obscure branch of mathematics that most people never come across, even when studying the mathematical foundations of computer science.

The news isn't all bad. For example, SQL is built on a mathematical foundation of relational algebra. A programmer may think of a SQL query in terms of what the relational operators mean or in terms of what the database engine does. Whether or not a programmer is mathematically or mechanically inclined, he can still formulate SQL queries to examine the data in a database.

Similarly, mechanically inclined programmers do not need to learn category theory to understand what monads mean in order to understand how they work. But before I can explain what monads do and how they work, I need to explain one of Haskell's most innovative features, its type system.

The Type System

Because Haskell is so deeply tied to mathematics, its designers and practitioners are concerned with understanding what a program means. Haskell compilers attempt to understand what your program means through type analysis and type inferencing.

Languages like C, Java, and C# offer a simple form of static typing. Static typing in these languages involves adding annotations to every variable, function, and method declaration to tell the compiler how to allocate memory and generate code. Unfortunately, this kind of typing is quite verbose and very error prone. In these languages, the type system can also be subverted by type casting and by ignoring type errors from the compiler. As a result, it's not difficult to write code with type mismatch errors that cause a program to crash.

Languages like Perl, Python, and Ruby offer some form of dynamic typing. In these languages, little to no type checking is done when a program is compiled. Instead, type issues are deferred until runtime, and a program is assumed to be meaningful and well typed. This means type annotations are unnecessary, and programs in these languages are generally more concise than their counterparts in C and C-like languages. Type errors, if they do occur, generate exceptions at runtime, and may cause a program to crash; or they may be ignored, leading to odd and unexpected runtime behavior.

Haskell takes a very different approach. It starts by performing type analysis with a type checker to understand your program at compile time. The type checker strictly prohibits type casting and does not allow type errors to be ignored. Because types are checked at compile time, and there is no escape from the type checker, Haskell is frequently described as both statically typed and strongly typed.

Any Haskell program with a type error simply does not compile. Haskell programs that do compile are free from type errors, and this guarantee eliminates a whole class of crashes and unexpected runtime behaviors. (Type errors are merely one kind of problem that creep into a program. There are still many ways for a Haskell program to compile and misbehave at runtime.)

Additionally, Haskell uses type inferencing to deduce the types of functions and values, eliminating the need for verbose type annotations in most cases. These features combine to provide more safety than C-like static typing and code at least as concise as that found in dynamic languages.

For example, here is the definition of length, a function that takes any list and returns the number of values contained within it:

length [] = 0
length (x:xs) = 1 + length xs

Here is a type annotation the compiler could infer for this function:

length :: [a] -> Int

This annotation can be read as follows:

The compiler can infer this type because:

Although type annotations aren't strictly required, it's a good idea to include them directly above a function definition. This helps the compiler check that your idea of a function's type matches the actual type for a function. More importantly, the type annotation also acts as documentation for others who are reading your code. A better way to define length would be:

length :: [a] -> Int
length [] = 0
length (x:xs) = 1 + length xs

Here is a definition of the higher order function map, which applies a function to every element in a list to create a new list:

map :: (a -> b) -> [a] -> [b]
map f (x:xs) = (f x):(map f xs)

The type annotation here means the following:

Here is an example of a function using map:

f t = map length (lines t)

The compiler can infer the type of this function as:

f :: String -> [Int]

This is because:

Therefore, the type of f must be String -> [Int] (sometimes phrased as [Char] -> [Int]).

Type Classes

So far, type inferencing seems useful for deducing the type of a function when either specific types or generic types are used. However, this leaves a huge gap in the middle—functions where any one of a group of related types may be used.

This is where type classes come in. A type class unifies a set of interchangeable, related types. Here is a typical example, the Num type class that defines a series of operations that can be used with any kind of number:

class Num a where
  (+) :: a -> a -> a
  (-) :: a -> a -> a
  (*) :: a -> a -> a
  ...

This class declaration means that any type a that is a member of type class Num must implement the three operations +, -, and * (and a few others not listed here). Furthermore, the implementation of + for any type that implements this interface must take two values and return a value. All three values must also be of the same numeric type.

Type class declarations define the interface a type must implement to be a member of that type class. An instance declaration ties together this interface with a specific implementation for a single type. Here are portions of the instance declarations that specify that Int and Double are both members of type class Num:

-- low level multiplication of 2 integers
mulInt :: Int -> Int -> Int
mulInt a b = ... 

instance Num Int where
    (*) = mulInt
    ...

-- low level multiplication of 2 doubles
mulDouble :: Double -> Double -> Double
mulDouble a b = ...

instance Num Double where
    (*) = mulDouble
    ...

The type checker knows how to infer when a type class is needed to describe a function's type. Here is the simple function from the previous article that squares numbers:

square x = x * x

The type inferencer deduces the following type for this function:

square :: (Num a) => a -> a

This type signature is a little different, because it includes an added constraint, namely the implication (Num a) to the left of the => symbol. This means that the function square is a function that takes any value of type a, and returns a value of the same type a, so long as that type is also a member of the type class Num.

For example, the value of square 3 is valid, because 3 is of type Int and Int is a member of type class Num. Evaluating this expression causes the Haskell compiler to look under the covers to see how multiplication is defined for Int values and replace the invocation of * with the actual implementation of multiplication for Int, or the function mulInt. Evaluating the expression square 2.5 also works, because the value 2.5 is of type Double, and Double is a member of type class Num. In this case, the compiler invokes the version of * for the Double type, or the function named mulDouble in this example.

The expression square "this" is invalid because square is defined only over numbers, and the value "this" is a string, and strings are not numbers. Attempting to evaluate this expression in the interpreter produces the following error:

$ ghci
...
Prelude> let square x = x * x
Prelude> :t square
square :: (Num a) => a -> a
Prelude> square "this"

<interactive>:1:0:
    No instance for (Num [Char])
      arising from use of `square' at <interactive>:1:0-12
    Possible fix: add an instance declaration for (Num [Char])
    In the expression: square "this"
    In the definition of `it': it = square "this"

Here is proof that this one definition of square works on different kinds of numbers:

Prelude> square 3
9
Prelude> square 2.5
6.25

Monads

Now that you've seen how type classes group together a series of related types, it's time to return to the topic of monads. Type classes are important, because, at the most fundamental level of implementation, monads are merely containers that are instances of the type class Monad:

class Monad m where
    (>>=)  :: m a -> (a -> m b) -> m b
    (>>)   :: m a -> m b -> m b
    return :: a -> m a
    fail   :: String -> m a

Here is what this type class declaration means:

These ideas come from category theory, which also defines a few important properties that monadic operators must provide:

These definitions are quite formal and may be somewhat confusing the first few times you read them, but the goal here is to define a series of simple, mechanical operations for connecting containers together. Monads provide a way to chain together a sequence of operations in a manner that works using just pure functions. And because they are defined in terms of pure functions, the Haskell compiler can understand them and handle the busywork for you.

Thinking about these operations in the abstract can be a little difficult. Here is a simple example. These two functions operate within the IO monad, the mechanism Haskell uses to communicate with the outside world.

getLine  :: IO String
putStrLn :: String -> IO ()

Because all IO operations are monadic, the IO monad acts as a container that isolates external side effects in a Haskell program. The function getLine reads a line of input from the console and returns that string inside the IO container. The function putStrLn accepts a plain string value, prints that string to the console (with a trailing newline), and returns an empty value, also contained within the IO monad.

Here is how these functions are connected together, using the monad operators:

echo :: IO ()
echo = getLine >>= putStrLn

In this example, the monadic action getLine is evaluated and produces an IO String result. The bind function (>>=) gets this result, extracts the string from its IO container, and sends it to the monadic action putStrLn. Finally, the action putStrLn consumes this plain string, sends it to the console, and returns the empty value within the IO container (the IO () type). This final result is returned by the echo function, so echo must have the type IO () as well.

Using the monad operators in this fashion may be powerful, but it is very cumbersome. Thankfully, Haskell allows for an alternate means to describe monadic operations, the "monadic do" notation. Here is an equivalent way of writing the echo function:

echo :: IO ()
echo = do str <- getLine
          putStrLn str

Here, a monadic function is a series of statements, aligned according to the layout rule. The function getLine returns a value, so that value can be captured in the symbol str and used later within this function. (Remember, str is not a variable, because its value cannot change.) Because this value is extracted from the monadic container, the value of str here is of type String, not IO String. As a result, this value can be passed to the function putStrLn, which is expecting a String as its first argument.

Because these actions are performed left-to-right (or top-to-bottom in "monadic do" notation), monads enforce an order of execution on their statements. With pure functions, sub-expressions may be evaluated in any order without changing their meaning. With monadic functions, the order of execution is very important. For example, it is meaningless to evaluate putStrLn in this function before receiving its input from getLine.

Conclusion

The IO monad is not the only monad in Haskell, but it is one of the most frequently used. Other monads offer different kinds of containers and code structures. For example, lists are simple data structures, but they may also be used as monads. The GHC standard library contains series of useful monads that are simple, generic, and useful in a variety of contexts. One of these is Parsec, a very convenient library for writing parsers.

A lot of exciting new work in Haskell is being done using monads. For example, transactional memory and nested data parallelism are two useful techniques that help when distributing a Haskell program across multiple CPUs. The portions of the program that use these features are strictly segregated from the portions of a program that are purely functional or deal with IO. This makes it easy for the compiler to isolate the portions of a program that are transactional or parallel from the parts that aren't. Furthermore, the type system enforces this separation and highlights bugs in code where monads are improperly used and don't make any sense.

All of these structures share one thing in common—they are container types that are declared to be instances of the Monad type class. Each monad type defines its version of the basic operations of bind (>>=), then (>>), return, and fail. The Haskell compiler can then compose a sequence of monadic actions into a single, larger monadic action. The compiler also replaces these generic operations with the specific implementations needed for a given monad, just as it replaces the generic numeric multiplication operation with the specific operation needed to multiply Int or Double values. Additionally, because all monads use the same basic operators, complicated monadic actions can be expressed using a simple "monadic do" notation, making these operations easier to understand.

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