At Wed, 8 May 2013 09:46:08 +0300, Roman Cheplyaka wrote:

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?

This configuration works for me: A.hs: module A where import B data A f :: B -> A f = undefined B.g A.hs-boot: module A where import {-# SOURCE #-} B data A f :: B -> A B.hs: module B where import {-# SOURCE #-} A data B g :: A -> B g = undefined A.f B.hs-boot: module B where data B Then I can compile them: bitonic@clay /tmp % ghc -c B.hs-boot bitonic@clay /tmp % ghc -c A.hs-boot bitonic@clay /tmp % ghc -c B.hs bitonic@clay /tmp % ghc -c A.hs Francesco