#15321: Typed holes in Template Haskell splices produce bewildering error messages -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Template Haskell | Version: 8.4.3 Resolution: | Keywords: TypedHoles Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by mgsloan): Ah, that makes sense Simon, thanks for the input! I have a concern with using `tryTcDiscardingErrors` here - it could hide problems. Errant omission of output is hard to notice in a mechanism like hole fitting - you would need to somehow notice a pattern of omission. Limiting the number of results also contributes to this. Based on this reasoning, here's what I'm thinking should be done: * First, the implementation you suggest. I'm not sure where things are in the release process, but having holes be so broken in GHCi isn't so good (https://ghc.haskell.org/trac/ghc/ticket/15202), and a change related to your suggestion would fix it. * In a subsequent change, using `tryTc` and recording the errors that are encountered, while still getting results for hole fitting. Then, including one of the exceptions as a "Please report this as a GHC bug" type of message. This way, we get hole fitting results, but errors like this get noticed and resolved. Breaking it up into these 2 steps is mainly motivated by the potential for the first portion to be included in the release. Coincidentally, before I read your comment I implemented a change that catches errors for the entire hole fitting process, rather than just portions - https://phabricator.haskell.org/D4908 . I'm favoring abandoning that approach and doing this more fine-grained catching. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15321#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler