Show how to use Haskell type symbols in Servant APIs to generate string URLs

Main.hs

#! /usr/bin/env nix-shell
#! nix-shell -i 'runghc -XOverloadedStrings -XDataKinds -XTypeOperators' -p 'ghc.withPackages (pkgs: [ pkgs.servant-server ])'

module Main where

import Data.List
import Data.Proxy
import GHC.TypeLits
import Servant

type ApiTerm     = "api"
type VersionTerm = "v1"
type FooTerm     = "foo"
type BarTerm     = "bar"

type Api = ApiTerm :> VersionTerm :> (
         FooTerm :> Get '[JSON] ()
    :<|> BarTerm :> Get '[JSON] ()
  )

main :: IO ()
main = do
  let a = symbolVal (Proxy :: Proxy ApiTerm)
      b = symbolVal (Proxy :: Proxy VersionTerm)
      c = symbolVal (Proxy :: Proxy FooTerm)
      f = "/" ++ intercalate "/" [a, b, c]
  print f