#14918: GHC 8.4.1 regression: derived Read instances with field names containing # no longer parse -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.4.2 Component: Compiler | Version: 8.4.1 Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: #5041, #14364 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: #14364 => #5041, #14364 Comment: Ah, I know what is happening here. #5041 is quite relevant, as is [http://git.haskell.org/ghc.git/blob/152055a19cf368439c8450040b68142f8e7d0346... this comment] from the GHC source: {{{#!hs -- For constructors and field labels ending in '#', we hackily -- let the lexer generate two tokens, and look for both in sequence -- Thus [Ident "I"; Symbol "#"]. See Trac #5041 }}} Now let's look at [http://git.haskell.org/ghc.git/blob/152055a19cf368439c8450040b68142f8e7d0346... what readField does]: {{{#!hs readField :: String -> ReadPrec a -> ReadPrec a readField fieldName readVal = do expectP (L.Ident fieldName) expectP (L.Punc "=") readVal }}} Alas, it attempts to treat the field name as a single `Ident`. But if that field name contains a `#` (e.g., `runT#`, as in the original example), then this will fail. It looks like we'll need a variant of `readField` that takes hashes into account. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14918#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler