Reduce repetition in derived Read instances

While looking at #7258 with tdammers, I noticed that 5000 of the 7000 terms that the W2 module simplifies to are attributed to $creadPrec. In fact, much of this is repetition of the form,

                  (GHC.Read.expectP
                     (Text.Read.Lex.Ident (GHC.CString.unpackCString# "field1"#)))
                  (>>
                     @ Text.ParserCombinators.ReadPrec.ReadPrec
                     Text.ParserCombinators.ReadPrec.$fMonadReadPrec
                     @ ()
                     @ DT
                     (GHC.Read.expectP
                        (Text.Read.Lex.Punc (GHC.CString.unpackCString# "="#)))
                     (>>=
                        @ Text.ParserCombinators.ReadPrec.ReadPrec
                        Text.ParserCombinators.ReadPrec.$fMonadReadPrec
                        @ Int
                        @ DT
                        (Text.ParserCombinators.ReadPrec.reset
                           @ Int (GHC.Read.readPrec @ Int GHC.Read.$fReadInt))
                        (\ (a1_a1oe :: Int) ->
                           >>
                             @ Text.ParserCombinators.ReadPrec.ReadPrec
                             Text.ParserCombinators.ReadPrec.$fMonadReadPrec
                             @ ()
                             @ DT
                             (GHC.Read.expectP
                                (Text.Read.Lex.Punc (GHC.CString.unpackCString# ","#)))

Let's factor this pattern out into a readField helper in GHC.Read,

readField :: String -> ReadPrec a -> ReadPrec a
readField fieldName readVal = do
    expectP  (Ident fieldName)
    expectP (Punc "=")
    readVal
{-# NOINLINE readField #-}

This will at least knock off a constant factor from the size of what should not be performance-critical code.

We could also try folding the terminal "," into this, although then we would need to somehow handle the last field specially. This might not be worth it. Perhaps instead just factor out the comma case as well,

readComma :: ReadPrec ()
readComma = expectP (Punc ",")
{-# NOINLINE readField #-}

It's unclear whether this is worth it, however.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	tdammers
Operating system
Architecture