#008 World! in Haskell

Notes

Haskell is an advanced purely-functional programming language. So no surprise hello_world.hs looks a little different!

Even this simple “Hello World” exercises at least 3 interesting Haskell features:

main = getArgs >>= parse

The >>= monad sequencing operator with value passing causes the result of getArgs (an array of arguments) to be passed to the next function, parse.

parse [name] = hello name
parse [] = hello_world

Two patterns are bound to the function parse, handle the case of either 0 or 1 parameters. This effects flow control without the need for case or if statements.

hello name = putStrLn $ "Hello " ++ name

The $ operator causes anything appearing after it to take precedence over anything that comes before. As used here, it ensures that the string concatenation ++ - which is normally right-associative - is executed before putStrLn

Compiling and Running the Example..

$ ghc hello_world.hs
[1 of 1] Compiling Main             ( hello_world.hs, hello_world.o )
Linking hello_world ...
$ ./hello_world
Hello World
$ ./hello_world Paul
Hello Paul

Credits and References

• The Haskell Programming Language - main site
• 3.4 Operator Applications - Haskell report
• Haskell: difference between . (dot) and $ (dollar sign) - SO
• Haskell operator vs function precedence - SO