Proposal: add ReadPrec-based functions to Read1/Read2

[Carter Schonwald]

I'm always in favor of providing algorithmicly better interfaces, so
barring me learning that there's doe thing even better we should do , +1 :)

On Sunday, June 19, 2016, Ryan Scott <ryan.gl.scott at gmail.com> wrote:

> 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.
> -----
> [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
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