$ operator

This is hopefully something worth sharing about Haskell. The $ operator.

 simpleHTTP $ buildRequest req_text

 simpleHTTP ( buildRequest req_text )

It is an application operator, it takes a function and an argument, and ... applies the function to the argument. It's purpose is to save typing parentheses. It is all about operator precedence.

 head . words $ config_file_contents

 ( head . words ) config_file_contents

Application, f a (f applies to a), binds stronger than any operator. If it was an operator, think about multiplication operator which people often omit, it would have precedence set to 10. $ has precedence set to 0, which is the lowest value of precedence possible.

The . is my another favourite. (f . g) a == f (g a) it set to 9, and therefore binds almost as strong as application.

 listActions :: String -> [Action]
 listActions = filter notDone . map actionFromMap . parseJSON