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Family instance modules are not fingerprinted in ABI

    • Closed Issue created by Edward Z. Yang @ezyang

    This leads to the following delightful, five module bug:

    -- A.hs
{-# LANGUAGE TypeFamilies #-}
module A where
type family F a
type instance F Int = Bool

-- B.hs
module B (module A) where
import A

-- C.hs
module C where
import B

-- D.hs
module D where
import C

-- E.hs
module E where
import D
import B

    Build these modules. Then rename A to A2 (fixing B's import), and build them again. You'll get:

    ezyang@sabre:~/Dev/labs/T3871$ ghc --make E.hs 
[1 of 5] Compiling A2               ( A2.hs, A2.o )
[2 of 5] Compiling B                ( B.hs, B.o )
[3 of 5] Compiling C                ( C.hs, C.o ) [B changed]
[5 of 5] Compiling E                ( E.hs, E.o ) [B changed]
attempting to use module ‘A’ (./A.hs) which is not loaded

    The problem is clear: D was not recompiled, but it needs to be, because when the module gets renamed, we need to update its list of family instance modules to rename A to A2. When we don't do this, the subsequent family instance check chokes because it tries to load A. And why did D decide not to get recompiled? Because the ABI hash of C did not change. And that's WRONG.

    (Also, family instances are really awful, you really do have to rebuild everything when you change them. UGH.)

    This occasionally affected the Cabal project, see https://github.com/haskell/cabal/issues/3871

    Edited
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