#10009: type inference regression when faking injective type families
-------------------------------------+-------------------------------------
Reporter: aavogt | Owner:
Type: bug | Status: new
Priority: high | Milestone: 7.10.2
Component: Compiler (Type | Version: 7.10.1-rc1
checker) | Keywords:
Resolution: | Architecture:
Operating System: Unknown/Multiple | Unknown/Multiple
Type of failure: GHC rejects | Test Case:
valid program | Blocking:
Blocked By: | Differential Revisions:
Related Tickets: #10226 |
-------------------------------------+-------------------------------------
Comment (by Simon Peyton Jones ):
In [changeset:"a1275a762ec04c1159ae37199b1c8f998a5c5499/ghc"]:
{{{
#!CommitTicketReference repository="ghc"
revision="a1275a762ec04c1159ae37199b1c8f998a5c5499"
Improve improvement in the constraint solver
This regrettably-big patch substantially improves the way in which
"improvement" happens in the constraint solver. It was triggered by
trying to crack Trac #10009, but it turned out to solve #10340 as
well.
The big picture, with several of the trickiest examples, is described
in Note [The improvement story] in TcInteract.
The major change is this:
* After solving we explicitly try "improvement", by
- making the unsolved Wanteds into Deriveds
- allowing Deriveds to rewrite Deriveds
This more aggressive rewriting "unlocks" some extra
guess-free unifications.
* The main loop is in TcInteract.solveSimpleWanteds, but I also ended
up refactoring TcSimplify.simpl_loop, and its surrounding code.
Notably, any insolubles from the Givens are pulled out
and treated separately, rather than staying in the inert set
during the solveSimpleWanteds loop.
There are a lot of follow-on changes
* Do not emit generate Derived improvements from Wanteds.
This saves work in the common case where they aren't needed.
* For improvement we should really do type-class reduction on Derived
constraints in doTopReactDict. That entailed changing the GenInst
constructor a bit; a local and minor change
* Some annoying faffing about with dropping derived constraints;
see dropDerivedWC, dropDerivedSimples, dropDerivedInsols,
and their Notes.
* Some substantial refactoring in TcErrors.reportWanteds.
This work wasn't strictly forced, but I got sucked into it.
All the changes are in TcErrors.
* Use TcS.unifyTyVar consistently, rather than setWantedTyBind,
so that unifications are properly tracked.
* Refactoring around solveWantedsTcM, solveWantedsAndDrop.
They previously guaranteed a zonked result, but it's more
straightforward for clients to zonk.
}}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10009#comment:22
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
