What does the @ symbol do within an expression in Haskell?

Question

I'm trying to figure out what @ does in an expression like endpoint @"start". Is it part of a language extension perhaps?

I see the follow extensions enabled for the module the function is in.

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DeriveAnyClass             #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE DerivingStrategies         #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE LambdaCase                 #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE NoImplicitPrelude          #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE RecordWildCards            #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeApplications           #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE TypeOperators              #-}

The full function:

endpoints :: Contract () AuctionSchema Text ()
endpoints = (start' `select` bid' `select` close') >> endpoints
  where
    start' = endpoint @"start" >>= start
    bid'   = endpoint @"bid"   >>= bid
    close' = endpoint @"close" >>= close

This is `-XTypeApplications` combined with type-level string literals. — leftaroundabout, Apr 12 '21 at 13:58

score 10 · Accepted Answer · answered Apr 12 '21 at 13:59

There are two relevant extensions' documentation to read: TypeApplications and DataKinds. A snippet from the type applications documentation:

The TypeApplications extension allows you to use visible type application in expressions. Here is an example: show (read @Int "5"). The @Int is the visible type application; it specifies the value of the type variable in read's type.

And from the data kinds documentation:

With DataKinds, GHC automatically promotes every datatype to be a kind and its (value) constructors to be type constructors.

I guess you sort of also have to know about Symbol, a type-level representation of strings that is more efficient (but less featureful) than type-level [Char], but I couldn't find a good place in the official documentation to read about it. You can read about it some in the GHC.TypeLits haddocks.

Thanks, yes seems to be correct. Related answer to this is also here: https://stackoverflow.com/questions/61482257/besides-as-pattern-what-else-can-mean-in-haskell — Chris Stryczynski, Apr 12 '21 at 14:02

What does the @ symbol do within an expression in Haskell?

1 Answers1