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

1 files changed, 17 insertions, 0 deletions

diff --git a/libraries/base/GHC/Read.hs b/libraries/base/GHC/Read.hs
index ad51c46ec5..f7870a2df1 100644
--- a/libraries/base/GHC/Read.hs
+++ b/libraries/base/GHC/Read.hs
@@ -37,6 +37,7 @@ module GHC.Read
   , readListDefault, readListPrecDefault
   , readNumber
   , readField
+  , readFieldHash
   , readSymField
 
   -- Temporary
@@ -376,6 +377,22 @@ readField fieldName readVal = do
 
 -- See Note [Why readField]
 
+-- | 'Read' parser for a record field, of the form @fieldName#=value@. That is,
+-- an alphanumeric identifier @fieldName@ followed by the symbol @#@. The
+-- second argument is a parser for the field value.
+--
+-- Note that 'readField' does not suffice for this purpose due to
+-- <https://ghc.haskell.org/trac/ghc/ticket/5041 Trac #5041>.
+readFieldHash :: String -> ReadPrec a -> ReadPrec a
+readFieldHash fieldName readVal = do
+        expectP (L.Ident fieldName)
+        expectP (L.Symbol "#")
+        expectP (L.Punc "=")
+        readVal
+{-# NOINLINE readFieldHash #-}
+
+-- See Note [Why readField]
+
 -- | 'Read' parser for a symbol record field, of the form @(###)=value@ (where
 -- @###@ is the field name). The field name must be a symbol (operator-style),
 -- e.g. @(#)@. For regular (alphanumeric) field names, use 'readField'. The