Proposal: add ReadPrec-based functions to Read1/Read2

[Ryan Scott]

GHC 8.0 added the Data.Functor.Classes module [1] to base, and with it
the Read1 and Read2 typeclasses. The current definition of Read1 is
this:

    class Read1 f where
        liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (f a)
        liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [f a]

There's a pretty big problem with this definition: it uses ReadS (a
synonym for String -> [(a, String)]). This sort of parser is very slow
(the docs even admit as such [2]), and moreover, the actual Read
typeclass on which Read1 is based tries to avoid using it whenever
possible.

The Read typeclass has this definition currently:

    class Read a where
        readsPrec    :: Int -> ReadS a
        readList     :: ReadS [a]
        readPrec     :: ReadPrec a
        readListPrec :: ReadPrec [a]

Where ReadPrec is a much more efficient parser datatype. When deriving
Read instances, GHC defines them in terms of readPrec, and gives the
other methods default definitions that leverage readPrec.

For the sake of consistency, I propose adding analogous methods to
Read1 and Read2 that use the ReadPrec datatype. For example, here is
how I would change Read1:

    class Read1 f where
        liftReadsPrec    :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (f a)
        liftReadList     :: (Int -> ReadS a) -> ReadS [a] -> ReadS [f a]
        liftReadPrec     :: ReadPrec a -> ReadPrec [a] -> ReadPrec (f a)
        liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [f a]

And similarly for Read2. Here is a full gist [3] with a sketch of what
the new Read1/Read2 definitions would look like, including what the
default definitions of the other methods would be.

Ryan S.
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[1] http://hackage.haskell.org/package/base-4.9.0.0/docs/Data-Functor-Classes.html
[2] http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-ParserCombinators-ReadP.html#t:ReadS
[3] https://gist.github.com/RyanGlScott/7cdd11d6aa878e4229acf1a682beb1fc