#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: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by RyanGlScott: Old description:

(Originally noticed [here](https://github.com/ekmett/transformers- compat/issues/32).)

Consider the following program:

{{{#!hs {-# LANGUAGE MagicHash #-} module Bug where

data T a = MkT { runT# :: a } deriving (Read, Show)

t1, t2 :: T Int t1 = MkT 1 t2 = read $ show t1

main :: IO () main = print t2 }}}

In GHC 8.2.1, this runs without issue:

{{{ $ /opt/ghc/8.2.2/bin/runghc Bug.hs MkT {runT# = 1} }}}

In GHC 8.4.1, however, this produces a runtime error:

{{{ $ ~/Software/ghc-8.4.1/bin/runghc Bug.hs Bug.hs: Prelude.read: no parse }}}

New description: (Originally noticed [https://github.com/ekmett/transformers- compat/issues/32 here].) Consider the following program: {{{#!hs {-# LANGUAGE MagicHash #-} module Bug where data T a = MkT { runT# :: a } deriving (Read, Show) t1, t2 :: T Int t1 = MkT 1 t2 = read $ show t1 main :: IO () main = print t2 }}} In GHC 8.2.1, this runs without issue: {{{ $ /opt/ghc/8.2.2/bin/runghc Bug.hs MkT {runT# = 1} }}} In GHC 8.4.1, however, this produces a runtime error: {{{ $ ~/Software/ghc-8.4.1/bin/runghc Bug.hs Bug.hs: Prelude.read: no parse }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14918#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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

#14918: GHC 8.4.1 regression: derived Read instances with field names containing # no longer parse -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch 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): Phab:D4502 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott ): In [changeset:"d5577f44eaf3b9dfdfc77828038782bf818c176a/ghc" d5577f44/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="d5577f44eaf3b9dfdfc77828038782bf818c176a" Special-case record fields ending with hash when deriving Read Summary: In commit dbd81f7e86514498218572b9d978373b1699cc5b, a regression was inadvertently introduced which caused derived `Read` instances for record data types with fields ending in a `#` symbol (using `MagicHash`) would no longer parse on valid output. This is ultimately due to the same reasons as #5041, as we cannot parse a field name like `foo#` as a single identifier. We fix this issue by employing the same workaround as in #5041: first parse the identifier name `foo`, then then symbol `#`. This is accomplished by the new `readFieldHash` function in `GHC.Read`. This will likely warrant a `base-4.11.1.0` release. Test Plan: make test TEST=T14918 Reviewers: tdammers, hvr, bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie, carter GHC Trac Issues: #14918 Differential Revision: https://phabricator.haskell.org/D4502 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14918#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14918: GHC 8.4.1 regression: derived Read instances with field names containing # no longer parse -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: high | Milestone: 8.4.2 Component: Compiler | Version: 8.4.1 Resolution: fixed | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect result | Test Case: at runtime | deriving/should_run/T14918 Blocked By: | Blocking: Related Tickets: #5041, #14364 | Differential Rev(s): Phab:D4502 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: merge => closed * resolution: => fixed Comment: Merged to `master` and `ghc-8.4`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14918#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler