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 < matthewtpicker...@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.doming...@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 >
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