Improve error recovery in the typechecker

Authored by simonpj on Mar 13 2019, 6:15 AM.


Improve error recovery in the typechecker

Issue Trac #16418 showed that we were carrying on too eagerly after a bogus
type signature was identified (a bad telescope in fact), leading to a
subsequent crash.

This led me in to a maze of twisty little passages in the typechecker's
error recovery, and I ended up doing some refactoring in TcRnMonad.
Some specfifics

  • TcRnMonad.try_m is now called attemptM.
  • I switched the order of the result pair in tryTc, to make it consistent with other similar functions.
  • The actual exception used in the Tc monad is irrelevant so, to avoid polluting type signatures, I made tcTryM, a simple wrapper around tryM, and used it.

The more important changes are in

  • TcSimplify.captureTopConstraints, where we should have been calling simplifyTop rather than reportUnsolved, so that levity defaulting takes place properly.
  • TcUnify.emitResidualTvConstraint, where we need to set the correct status for a new implication constraint. (Previously we ended up with an Insoluble constraint wrapped in an Unsolved implication, which meant that insolubleWC gave the wrong answer.

(cherry picked from commit 4927117cd6166a97455d788dbf7433c95441b57a)


bgamariNov 8 2019, 2:28 PM
rGHCbf5c4c194b68: Fix #17112