[Haskell-cafe] Mutually recursive modules

[Roman Cheplyaka]

I wonder whether it's always possible to break cycles using GHC's
.hs-boot files.

Consider the following schematic example:

  module A where

  import B

  data A

  f :: B -> A
  f = undefined B.g

  module B where

  import A

  data B

  g :: A -> B
  g = undefined A.f

A.hs-boot must give a type signature for f, and since the signature
contains 'B', it must import 'B'. Ditto for B.hs-boot — it must import
'A'.

Even if we treat all imports as {-# SOURCE #-}, there is still a cycle
between the hs-boot files.

So, am I right in understanding that these recursive modules cannot be
compiled by GHC at all?

Roman