What manner of Haskell syntax is used in [$parseRoutes|/ Home GET|]?

Question

I found this code on the front page of the Yesod project:

import Yesod
data HelloWorld = HelloWorld
mkYesod "HelloWorld" [$parseRoutes|/ Home GET|]
instance Yesod HelloWorld where approot _ = ""
getHome = applyLayout [$hamlet|%h1 Hello World|]
main = toWaiApp HelloWorld >>= basicHandler 3000

What language features are used in [$parseRoutes|/ Home GET|] and [$hamlet|%h1 Hello World|] to get the $, |, / and % symbols working, and what do the symbols do?

Answer 1

Those would be a Quasiquotation. It's a way to use Template Haskell to embed another language into Haskell. The quasiquotes 'parseRoutes and hamlet define how to parse and interpret what is inside the bracket [$FOO| ... ]. Many more details at the link above.