Interactive code snippets not yet available for SoH 2.0, see our Status of of School of Haskell 2.0 blog post

I/O in Haskell for Java programmers

Hello, world!

Let's go right ahead and get started with the classic "Hello, world!" program in Java:

In Java

public static void main(String[] args) {
  System.out.println("Hello, world!");
}

In Haskell

The code has very much the same shape in Haskell, but with slightly different notation, types, and prodecure naming and behavior:

module Main where {
  main :: IO ()
  main = do {
    putStrLn "Hello, world!";
  }
}

Haskell can infer the type signature of main, and can also insert the default module declaration for us, so for future examples I will omit these.

main = do {
  putStrLn "Hello, world!";
}

Notation and types

If you've seen Haskell before, you may be scratching your head. Haskell has whitespace-sensitive syntax, but I've chosen to use Haskell's optional curly braces and semicolons for familiarity to Java programmers.

You'll notice, however, that function application looks different. While in Java, you write foo(bar, baz, qux), in Haskell you merely write foo bar baz qux. You can think of it as shifting the original paren to the left, and omitting the commas: (foo bar baz qux). While I could have written putStrLn("Hello, world!");, this would be misleading because the comma-separated notation does not work for Haskell functions with multiple arguments. On this I will not compromise and give a Java-esque presentation, since using pre-defined Haskell functions with multiple arguments will be necessary.

In Haskell, the type signature is written separately from the definition. Types are written identifier :: Type, which reads "identifier has type Type". The () type is like Haskell's version of void. The Haskell type signature of main also explicitly notes that IO effects can happen during main. You don't need to worry much about this for now, though we would certainly hope that IO effects can happen in main, given that this is the whole point of the tutorial!

You may notice that in Haskell, we write main = do { ... } while in Java we simply write main(...) { ... }, without the = do part. That's just some magic notation that I want you to ignore for right now.

The main method

There are a few differences between main in Haskell and main in Java.

Modules vs classes

In Java, the main method is a public, static member of the enclosing class. This means that you do not need to create an object of that type in order to invoke the main method. If you "run" a Java file, then by default, that file's outermost class's main function is invoked.

In Haskell, the main method must be exported by a module. When you "run" a Haskell file, it invokes the files' outermost module's main function. In this regard, modules and classes fulfill a similar role, but they are different in essentially every other way imaginable. I won't discuss the details here; we have access to a runnable main method and that's all we really need to know about classes or modules for this tutorial.

Program arguments

You'll notice that the Java version of main includes an array of Strings, typically named args, which are the textual arguments that the program is invoked with. Haskell's version of main does not mention args anywhere, but you can gain access to the program arguments by using getArgs.

import System.Environment (getArgs)

main = do {
  args <- getArgs;
  print args;
}

I'll explain imports and the left-facing arrow (<-) soon, though you probably already have an intuition for how they work.

Printing a String

In Haskell, you can use putStrLn in place of System.out.println. The only catch is that in Haskell, there is no implicit invocation of toString as there is in Java. The Haskell equivalent of toString is show. (Actually, show is more like Python's repr, because it produces a string which, when read, can typically be parsed back into the data it represents.)

main = do {
  putStrLn (show 3);
}

String concatenation in Java is done with +, which again implicitly calls toString on things. In Haskell, string concatenation is done with ++, and you must perform explicit string conversions.

main = do {
  putStrLn ("I have " ++ show 3 ++ " apples");
}

Now you try

Try out Haskell's string printing, string concatenation, and coersion to strings. Just don't try to use variables yet; I'll teach you those soon.

main = do {
  putStrLn ("edit me!");
}

Echoing and looping

Now let's move on to something a little more interesting. Again, we'll start with a Java example and transliterate to Haskell.

In Java

import java.util.Scanner
class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        System.out.println("Please enter a line of text");
        String line = scan.nextLine();
        while (!line.equals("end")) {
            System.out.println(line);
            line = scan.nextLine();
            System.out.println("Please enter a line of text");
        }
        System.out.println("The end.");
    }
}

This simple Java program scans standard input one line at a time, and echoes the line back to standard output.

Reading input in Haskell

Ok, let's start simple. We'll just read in one line and spit it back out.

main = do {
  putStrLn "Please enter a line of text";
  line <- getLine;
  putStrLn line;
}

Here I used getLine and the left-facing arrow (<-). getLine eats up a line from standard input, and the left-facing arrow binds the identifier line to the result of running getLine. If you click on getLine in the code above, you'll see that it has type IO String, meaning that it is an IO action that produces a String. In Haskell, you assign the result of actions using the left-facing arrow.

Aside

You can assign the result of a pure computation using let.

main = do {
  putStrLn "Please enter your name";
  name <- getLine;
  let {greeting = "Hello, " ++ name ++ "!"};
  putStrLn greeting;
}

Looping in Haskell

import Control.Monad (unless)

main = do {
  putStrLn "Please enter a line of text";
  line <- getLine;
  if (line /= "end")
  then do {
      putStrLn line;
      main;
  }
  else do {
      putStrLn "The end.";
  }
}

In Haskell, you can "loop" via primitive recursion. In this example, I just invoked main again. You might worry about blowing the call stack by doing this. Don't. Haskell's "stack" is very different from Java's. This has to do with lazy evaluation, among other things. It is beyond the scope of this tutorial to explain why this Haskell code is efficient, so just trust me for now.

Work in progress! Suggestions welcome: danburton.email AT gmail