What does @ prefix means?

Question

like this:

auctionInstance = Scripts.validator @Auctioning

and Hello-world app from https://playground.plutus.iohkdev.io/tutorial/tutorials/plutus-playground.html#running-the-hello-world-app

hello = logInfo @String "Hello, world"

Answer 1

It means type application.

In this case, the function is polymorphic with the following type:

logInfo :: forall a effs. Member (LogMsg a) effs => a -> Eff effs ()

by using TypeApplications extension, you can use @SomeType hint for the compiler, so the logInfo call is not ambiguous.

logInfo @String "Hello world" means apply type String to functions type variable a (the order depends on the order of type variables in explicit forall section - forall a effs - sometimes you may want to add multiple hints e.g. convert @String @Text someArg otherArg).

Ambiguity here stems from multiple string types in Haskell so you need to tell the compiler which type you are using (String, Text orByteString).