#9554: Pathological type family turns type error into runtime loop
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Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.8.2
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Blocked By: | Test Case:
Related Tickets: | Blocking:
| Differential Revisions:
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Comment (by Richard Eisenberg ):
In [changeset:"c1edbdfd9148ad9f74bfe41e76c524f3e775aaaa/ghc"]:
{{{
#!CommitTicketReference repository="ghc"
revision="c1edbdfd9148ad9f74bfe41e76c524f3e775aaaa"
Do proper depth checking in the flattener to avoid looping.
This implements (roughly) the plan put forward in comment:14:ticket:7788,
fixing #7788, #8550, #9554, #10139, and addressing concerns raised in
#10079.
There are some regressions w.r.t. GHC 7.8, but only with pathological type
families (like F a = F a). This also (hopefully -- don't have a test case)
fixes #10158. Unsolved problems include #10184 and #10185, which are both
known deficiencies of the approach used here.
As part of this change, the plumbing around detecting infinite loops has
changed. Instead of -fcontext-stack and -ftype-function-depth, we now have
one combined -freduction-depth parameter. Setting it to 0 disbales the
check, which is now the recommended way to get (terminating) code to
typecheck in releases. (The number of reduction steps may well change
between
minor GHC releases!)
This commit also introduces a new IntWithInf type in BasicTypes
that represents an integer+infinity. This type is used in a few
places throughout the code.
Tests in
indexed-types/should_fail/T7788
indexed-types/should_fail/T8550
indexed-types/should_fail/T9554
indexed-types/should_compile/T10079
indexed-types/should_compile/T10139
typecheck/should_compile/T10184 (expected broken)
typecheck/should_compile/T10185 (expected broken)
This commit also changes performance testsuite numbers, for the better.
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9554#comment:3
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