Haskell Get Started

Getting started means saving a Haskell program in a file and running it with the GHC compiler. GHC is the tool that turns Haskell code into a program the computer can run.

A saved program prints its message

Save this code in a file named Main.hs. Compile it and run it to see the message.

main :: IO ()
main = putStrLn "Ready to run"

main is where running begins

The built program looks for main first. It carries out the action written after the equals sign.

main :: IO ()
main = putStrLn "Starting here"

A compiled program can print a result

The same file, compile and run steps work for a calculation. The result appears when the program runs.

main :: IO ()
main = print (2 + 3)

Example

main :: IO ()
main = putStrLn "My first Haskell program"

Key points

File: Haskell code is saved in a file whose name ends with .hs.
Compiler: a tool that turns source code into a runnable program.
GHC: the standard compiler for Haskell programs.
Compile: running ghc Main.hs builds a runnable program from the file Main.hs.
Run: running the built program carries out its main action.
Signature: the line main :: IO () labels main as the program's starting point.

In this example, the code is a complete program ready to save in a .hs file. Compiling it with GHC and running it prints the text after putStrLn.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Haskell Playground

main :: IO ()
main = putStrLn "My first Haskell program"

Exercise

Which file name is right for a Haskell source file?

Main.txt Main.py Main.hs Main.html