Learn Haskell in 10 minutes

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

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

Prelude> "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
Prelude> truncate 6.59
Prelude> round 6.59
Prelude> sqrt 2
Prelude> not (5 < 3)
Prelude> gcd 21 14

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)
Prelude> print (1 < 2)

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" }

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) }

(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 declaration has been mentioned. That's because Haskell does type inference. You generally don't have to declare types unless you want to. If you do want to declare types, you use :: to do it.

Prelude> 5 :: Int
Prelude> 5 :: Double

You can also ask ghci what type it has chosen for something. This is useful because you don't generally have to declare your types.

Prelude> :t True
True :: Bool
Prelude> :t 'X'
'X' :: Char
Prelude> :t "Hello, Haskell"
"Hello, Haskell" :: [Char]

(In case you noticed, [Char] is another way of saying String. See the section on lists later.)

Things get more interesting for numbers.

Prelude> :t 42
42 :: (Num t) => t
Prelude> :t 42.0
42.0 :: (Fractional t) => t
Prelude> :t gcd 15 20
gcd 15 20 :: (Integral t) => t

These type signatures mean:

  • 42 can be used as any numeric type. (This is why I was able to declare 5<hask> as either an Int or a Double earlier.) * <hask>42.0 can be any fractional type, but not an integral type.
  • gcd 15 20 (which is a function call, incidentally) can be any integral type, but not a fractional type.

The feature used here is "type classes". We can say that types belong to the Num type class, or the Fractional type class, or the Integral type class.

There are five numeric types in the Haskell "prelude" (the part of the library you get without having to import anything):

  • Int is an integer with at least 30 bits of precision.
  • Integer is an integer with unlimited precision.
  • Float is a single precision floating point number.
  • Double is a double precision floating point number.
  • Rational<hask> is a fraction type, with no rounding error. All five of these belong to the <hask>Num type class. The first two belong to Integral, and the last two to Fractional.

Putting it all together,

Prelude> gcd 42 35 :: Int
Prelude> gcd 42 35 :: Double

    No instance for (Integral Double)

The final type worth mentioning here is (), pronounced "unit." It only has one value, also written as () and pronounced "unit." You can think of this as similar to the void keyword in C family languages. You can return () from a function or I/O action if you don't want to return anything.