Learn Haskell in 10 minutes

Overview

Haskell is a functional (that is, everything is done with function calls), statically, implicitly typed (types are checked by the compiler, but you don't have to declare them), lazy (nothing is done until it needs to be) language. It's closest popular relative is probably the ML family of languages.

The most common Haskell compiler is GHC. You can download GHC from http://www.haskell.org/ghc/download_ghc_661.html. GHC binaries are available for Linux, FreeBSD, MacOS, Windows, and Solaris. Once you've installed GHC, you get two programs you're interested in right now: ghc, and ghci. The first compiles Haskell libraries or applications to binary code. The second is an interpreter that lets you write Haskell code and get feedback right away.

Simple Expressions

You can type most math expressions directly into ghci and get an answer.

Prelude> 3 * 5
15
Prelude> 4 ^ 2 - 1
15
Prelude> (1 - 5)^(3 * 2 - 4)
16

Strings are in "double quotes." You can concatenate them with ++.

Prelude> "Hello"
"Hello"
Prelude> "Hello" ++ ", Haskell"
"Hello, Haskell"

Calling functions is done by putting the arguments directly after the function. There are no parentheses as part of the function call:

Prelude> succ 5
6
Prelude> truncate 6.59
6
Prelude> round 6.59
7
Prelude> sqrt 2
1.4142135623730951
Prelude> not (5 < 3)
True
Prelude> gcd 21 14
7

The Console

I/O actions can be used to read from and write to the console. Some common ones include:

Prelude> putStrLn "Hello, Haskell"
Hello, Haskell
Prelude> putStr "No newline"
No newlinePrelude> print (5 + 4)
9
Prelude> print (1 < 2)
True

The putStr and putStrLn functions output strings. The print function outputs any type of value. (If you print a string, it will have quotes around it.)

If you need multiple I/O actions in one expression, you can use a do block. Actions are separated by semicolons.

Prelude> do { putStr "2 + 2 = " ; print (2 + 2) }
2 + 2 = 4
Prelude> do { putStrLn "ABCDE" ; putStrLn "12345" }
ABCDE
12345

Reading can be done with getLine (which gives back a String) or readLn (which gives back whatever type of value you want). The <- symbol is used to assign a value to the result of an I/O action.

Prelude> do { n <- readLn ; print (n^2) }
4
16

(The 4 was input. The 16 was a result.)

There is actually another way to write do blocks. If you leave off the braces and semicolons, then indentation becomes significant. This doesn't work so well in ghci, but try putting the file in a source file (say, Test.hs) and build it.

main = do putStrLn "What is 2 + 2?"
          x <- readLn
          if x == 4
              then putStrLn "You're right!"
              else putStrLn "You're wrong!"

You can build with ghc --make Test.hs, and the result will be called Test. (On Windows, Test.exe) You get an if statement as a bonus.

Every line that starts in the same column as the first putStrLn is part of the do block. This is called "layout", and Haskell uses it to avoid making you put in statement terminators and braces all the time. (The then and else phrases have to be indented for this reason: if they started in the same column, they'd be separate statements, which is wrong.)

Simple Types

So far, not a single type has been mentioned