Problem: Can't avoid a module cycle with class definitions in Haskell.

Foo.hs contains:

module Foo where

import {-# SOURCE #-} Bar

class FooClass a where 
  ...

myBar :: BarClass -> a -- use a class from Bar
...

Foo.hs-boot contains:

module Foo where

import {-# SOURCE #-} Bar

class FooClass a where 
  ...
myBar :: BarClass -> a -- use a class from Bar
...
----------------------

Bar.hs contains:

module Bar where

import Foo

class BarClass a where 
  ...

myFoo :: FooClass -> a -- use a class from Foo
...

Bar.hs-boot contains:

module Bar where

?? import Foo -- causes ghc to report a module cycle
?? import {-# SOURCE #-} Foo -- also causes ghc to report a module cycle

class BarClass a where 
  ...

myFoo :: FooClass -> a -- without import, FooClass is out of scope
...

Command run:

ghc Bar.hs Foo.hs

Removing Foo.hs-boot does NOT fix the problem.

What's the solution?