Thanks Matthew. I've now found my example, which I share bellow for the record.

Cheers!
Facundo

> module A where
>
> a :: Bool
> a = False
> 
> a2 :: Bool
> a2 = True

> module B where
>
> import A
>
> b :: Bool
> b = not a

Building I get

> $ ghc A.hs B.hs
> [1 of 2] Compiling A                ( A.hs, A.o )
> [2 of 2] Compiling B                ( B.hs, B.o ) [A changed]
>
> $ ghc --show-iface A.hi | grep "ABI hash"
>   ABI hash: 811ba0740c0157da5cd2ea747468b19f

Changing A.a2 to have a different type causes the ABI hash to change yet GHC doesn't recompile module B:

> - a2 :: Bool
> - a2 = True
> + a2 :: Int
> + a2 = 0

> $ ghc A.hs B.hs
> [1 of 2] Compiling A                ( A.hs, A.o )
>
> $ ghc --show-iface A.hi | grep "ABI hash"
>   ABI hash: 6d7a56ce0b9f5fbca71f48bf08c2db84

I was previously trying to change the value of a2 rather than the type, which doesn't change the ABI hash if one isn't compiling with optimizations :)

I also tried doing modifications to the export list of A, but this seems to trigger recompilation of module B in all cases.


On Thu, Aug 11, 2022 at 9:20 AM Matthew Pickering <matthewtpickering@gmail.com> wrote:
Facundo,

If the interface in question is from a home package module, then it's
required that the ABI of the specifically used function is modified
rather than the ABI hash of the whole module. If the interface is from
an external package then if the ABI of the entire module changes then
that is sufficient to trigger recompilation.

Matt

On Thu, Aug 11, 2022 at 12:11 PM Domínguez, Facundo
<facundo.dominguez@tweag.io> wrote:
>
> Dear devs,
>
> rules_haskell [1] recently earned the ability to skip recompiling modules when the ABI hashes of dependencies don't change in interface files.
>
> We have noticed since then that sometimes ghc --make does avoid rebuilding even when ABI hashes change. However, so far I've been unable to reproduce this in the small. The wiki page on recompilation avoidance does remark that a changing ABI hash is a necessary but not sufficient condition for recompilation [2].
>
> Does anyone have a small example showing how ghc avoids rebuilding even when the ABI hash of a dependency changes?
>
> Thanks in advance!
> Facundo
>
> [1] https://github.com/tweag/rules_haskell/issues/1760#issuecomment-1179102232
> [2] https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/compiler/recompilation-avoidance#interface-file-invariants
> _______________________________________________
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs