plutus pioneer week01 src code explain

Question

I'm a little confused about the following code（from EnglishAuction.hs）:

auctionInstance :: Scripts.ScriptInstance Auctioning
auctionInstance = Scripts.validator @Auctioning
    $$(PlutusTx.compile [|| mkAuctionValidator ||])
    $$(PlutusTx.compile [|| wrap ||])                                                                                                                         
  where
    wrap = Scripts.wrapValidator @AuctionDatum @AuctionAction

What does "@" means? and what does Scripts.validator do? Why did it use two $$ instead of one?

Answer 1

This is part of Template Haskell. It is the way to have Haskell compiled at runtime basically.

This link may help https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/template-haskell.html

$$ is considered a typed expression splice. It also talks about [|| ... ||] which is a typed expression quotation.

Answer 2

The purpose of the code is to produce compiled Plutus-Core code which can be used as a validator. It is (more-or-less) boilerplate, so nearly the same for every program.

@ means we are giving types, not values

$$ is completely different to $ and it is used here to "splice" code into the program i.e. it inserts the compiled code 'syntax' into the program at this point.

See docs here : https://plutus.readthedocs.io/en/latest/plutus/tutorials/plutus-tx.html