Bar.hs

module Bar where

bar = answer * 20

Foo.hs

import Language.Haskell.Interpreter

foo :: IO (Either InterpreterError String)
foo = runInterpreter $ do
  reset
  loadModules ["Bar", "Prelude"]
  setTopLevelModules ["Bar"]
  eval "bar"

Prelude.hs

{-# LANGUAGE PackageImports #-}

module Prelude (module P, answer) where

import "base" Prelude as P

answer = 42