#15481: TH fails to recover from reifyFixity with -fexternal-interpreter -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Template Haskell | Version: 8.4.3 Resolution: | Keywords: RemoteGHCi Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I now understand the underlying problem slightly better than before. One of the culprits appears to be whether `addErrTc` is used to accumulate errors (as opposed to `failWithTc`, which throws errors immediately). `reifyFixity` only calls `addErrTc` when used in the program above, whereas if you swap out `reifyFixity` for something like `reify` or `reifyConStrictness`, you'll end up reaching a code path that uses `failWithTc`. (Similarly, adding an explicit use of `fail` will also cause `failWithTc` to be invoked.) For some peculiar reason, `recover` is able to successfully recover from errors added via `failWithTc` when `-fexternal-interpreter` is enabled, but not errors added via `addErrTc`. I don't know why that is the case, however. There is a `Note [TH recover with -fexternal-interpreter]` [http://git.haskell.org/ghc.git/blob/4eebc8016f68719e1ccdf460754a97d1f4d6ef05... here], but I can't glean any nuggets of wisdom from that. Given that `failWithTc` appears to be recovered more reliably than `addErrTc`, one way to fix this bug is to just error more eagerly in `qReifyFixity`, like so: {{{#!diff diff --git a/compiler/typecheck/TcSplice.hs b/compiler/typecheck/TcSplice.hs index c26ba0d..000c84c 100644 --- a/compiler/typecheck/TcSplice.hs +++ b/compiler/typecheck/TcSplice.hs @@ -866,7 +866,7 @@ instance TH.Quasi TcM where qLookupName = lookupName qReify = reify - qReifyFixity nm = lookupThName nm >>= reifyFixity + qReifyFixity nm = checkNoErrs (lookupThName nm) >>= reifyFixity qReifyInstances = reifyInstances qReifyRoles = reifyRoles qReifyAnnotations = reifyAnnotations }}} This makes the original program in this ticket recover successfully, even with `-fexternal-interpreter`. That being said, I'm not sure if it's really the "right" way to fix this bug, since there appears to be some underlying issue in the way `-fexternal-interpreter` interacts with `addErrTc`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15481#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler