Question

“Ugh,” you might think… “Another syntax question, here let me google that for you noob.” But alas! I have googled it, and I am still stumped!

Found in this code from the yesod blog

import System.IO
import Data.Enumerator
import Data.Enumerator.Binary

main =
    withFile "output.txt" WriteMode $ \output ->
    run_ $ enumFile "input.txt" $$ iterHandle output 

However the “$$” operator is new to me. The Haskell 2010 report only mentions it once as an operator symbol. What does it do?

Answer 1

In Haskell, operators like $$ are not part of the syntax, they are user-definable functions. Hence, you need to look up the API documenation for Yesod to see what $$ is. In particular, the function $$ from your example is documented here.