Ah yes, thank you!
* Francesco Mazzoli <f...@mazzo.li> [2013-05-08 08:51:12+0100]
> 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
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
