Haskell Introduction
Haskell is a programming language for building programs from values and expressions. A Haskell program starts at a special entry point named main.
A program can print text
The entry point uses putStrLn to print a line of text. The text sits inside double quotes.
main :: IO ()
main = putStrLn "Hello, Haskell!"A program can print a number
The print action prints a number value. No quotes are needed around a number.
main :: IO ()
main = print 42A calculation prints its result
Haskell works out the calculation inside print first. It then prints the result.
main :: IO ()
main = print (3 + 4)Example
main :: IO ()
main = putStrLn "Welcome to Haskell"Key points
- Program: a set of instructions that the computer follows.
- Value: a piece of data, such as a number or a line of text.
- Expression: a piece of code that works out a value.
- main: the starting point where every Haskell program begins.
- putStrLn: an action that prints text and then moves to a new line.
- print: an action that prints a value and then moves to a new line.
In this example, the main signature marks the entry point. The main action uses putStrLn to print the welcome text.
Note
Haskell programs describe what a result is, rather than listing step-by-step changes.
Try it Yourself
Edit the code, then press Run.
Running is simulated Β· Haskell Playground
Exercise
What is the starting point of every Haskell program in this course?