#14396: Hs-boot woes during family instance consistency checks
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Reporter: simonpj | Owner: (none)
Type: bug | Status: patch
Priority: normal | Milestone:
Component: Compiler | Version: 8.2.1
Resolution: | Keywords: hs-boot
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4154
Wiki Page: |
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Comment (by Edward Z. Yang ):
In [changeset:"6998772043a7f0b0360116eb5ffcbaa5630b21fb/ghc"
69987720/ghc]:
{{{
#!CommitTicketReference repository="ghc"
revision="6998772043a7f0b0360116eb5ffcbaa5630b21fb"
Make use of boot TyThings during typechecking.
Summary:
Suppose that you are typechecking A.hs, which transitively imports,
via B.hs, A.hs-boot. When we poke on B.hs and discover that it
has a reference to a type from A, what TyThing should we wire
it up with? Clearly, if we have already typechecked A, we
should use the most up-to-date TyThing: the one we freshly
generated when we typechecked A. But what if we haven't typechecked
it yet?
For the longest time, GHC adopted the policy that this was
*an error condition*; that you MUST NEVER poke on B.hs's reference
to a thing defined in A.hs until A.hs has gotten around to checking
this. However, actually ensuring this is the case has proven
to be a bug farm. The problem was especially poignant with
type family consistency checks, which eagerly happen before
any typechecking takes place.
This patch takes a different strategy: if we ever try to access
an entity from A which doesn't exist, we just fall back on the
definition of A from the hs-boot file. This means that you may
end up with a mix of A.hs and A.hs-boot TyThings during the
course of typechecking.
Signed-off-by: Edward Z. Yang
Test Plan: validate
Reviewers: simonpj, bgamari, austin, goldfire
Subscribers: thomie, rwbarton
GHC Trac Issues: #14396
Differential Revision: https://phabricator.haskell.org/D4154
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14396#comment:11
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