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I've noticed a strange interaction of Backpack with GHCi. For readability I'm giving the test case as a .bkp, but note that as --backpack isn't supported by GHCi, the actual test case uses cabal.

unit p where
    signature H where
      data S1
      data S2
    module A where
      import H
      data T = T S1 S2

unit q where
    dependency p[H=<H2>] (A as A2)
    module L where
      data S1 = MkS1
    signature H2(S1(MkS1), S2) where
      import L
      data S2
    module B where
      import A2
      import H2
      t = T :: S1 -> S2 -> T

When processing q, we instantiate p with H = H2 and add it to the EPS, even though it refers to L and H2 which are in the HPT. This is a problem, as the EPS is supposed to only contain stable information, whereas the HPT can be updated; any change to the HPT will not be reflected in the EPS and thus will refer to out of date information.

To demonstrate this, we can load everything in GHCi, change H2 and reload. (Recall: using cabal, not --backpack mode which doesn't work with GHCi.)

[1 of 2] Compiling H[sig]           ( p\H.hsig, nothing )
[2 of 2] Compiling A                ( p\A.hs, nothing )
[1 of 1] Instantiating bkp10-0-inplace-p
[1 of 4] Compiling H2[sig]          ( q\H2.hsig, nothing )
[2 of 4] Instantiating bkp10-0-inplace-p
[3 of 4] Compiling B                ( q\B.hs, nothing )
[4 of 4] Compiling L                ( q\L.hs, nothing )
Ok, three modules loaded.

now I change H2 so that S1 is defined in H2 instead of imported from L

ghci> :r
[2 of 4] Compiling H2[sig]          ( q\H2.hsig, nothing ) [Source file changed]
[3 of 4] Instantiating bkp10-0-inplace-p
[4 of 4] Compiling B                ( q\B.hs, nothing ) [H2 changed]
q\B.hs:4:9: error:
    * Couldn't match type `{H2.S1}' with `S1'
      Expected: S1 -> S2 -> T
        Actual: {H2.S1} -> S2 -> T
      NB: `S1' is defined at q\L.hs:3:3-16
          `{H2.S1}' is defined at q\H2.hsig:3:5-11

if I quit and restart, it works:
[1 of 1] Instantiating bkp10-0-inplace-p
[3 of 4] Instantiating bkp10-0-inplace-p
[4 of 4] Compiling B                ( q\B.hs, nothing ) [H2 changed]
Ok, three modules loaded.