Minimal example of frontend plugin vs. regular GHC API usage

A.hs

-- In subdirectory `target/A.hs`
module A where

build.sh

#!/usr/bin/env bash
cabal build

# compare:
cabal run sandbox
# with:
ghc --frontend Plugin -itarget -package-db dist-newstyle/packagedb/ghc-8.6.5 Plugin -plugin-package sandbox -hide-all-packages

Main.hs

module Main where
import GHC
import GhcPlugins
import DynFlags
import GHC.Paths ( libdir )
  
main :: IO ()
main = runGhc (Just libdir) $ do
  dflags <- getSessionDynFlags
  setSessionDynFlags $ dflags {
      importPaths = "target":(importPaths dflags)
    }
  target <- guessTarget "A" Nothing
  setTargets [target]
  () <$ load LoadAllTargets

Plugin.hs

module Plugin where
import GHC
import GhcPlugins
import DynFlags
  
frontendPlugin :: FrontendPlugin
frontendPlugin = defaultFrontendPlugin {
  frontend = fe
}

fe :: [String] -> [(String, Maybe Phase)] -> Ghc ()
fe _ _ = do
  target <- guessTarget "A" Nothing
  setTargets [target]
  () <$ load LoadAllTargets

sandbox.cabal

cabal-version:       >=1.10

name:                sandbox
version:             0.1.0.0
license-file:        LICENSE
author:              Derek Lam
maintainer:          derek@lam.io
build-type:          Simple
extra-source-files:  CHANGELOG.md

library
  exposed-modules:     Plugin
  build-depends:       base >=4.12 && <4.13, ghc
  default-language:    Haskell2010

executable sandbox
  main-is:             Main.hs
  build-depends:       base >=4.12 && <4.13, ghc-paths, ghc
  default-language:    Haskell2010