[GHC] #12358: Read1/Read2 don't have methods defined in terms of ReadPrec

2 Jul 2016

      ...
GHC 8.0 added the [http://hackage.haskell.org/package/base-4.9.0.0/docs
 /Data-Functor-Classes.html Data.Functor.Classes] module to `base`, and
 with it the `Read1` and `Read2` typeclasses. The current definition of
 `Read1` is this:
{{{#!hs
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 [http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-
 ParserCombinators-ReadP.html#t:ReadS admit as such]), 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:
{{{#!hs
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`:
{{{#!hs
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
 [https://gist.github.com/RyanGlScott/7cdd11d6aa878e4229acf1a682beb1fc full
 gist] with a sketch of what the new `Read1`/`Read2` definitions would look
#12358: Read1/Read2 don't have methods defined in terms of ReadPrec
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:                 |           Version:  8.0.1
  libraries/base                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Original Haskell libraries mailing list discussion:
 https://mail.haskell.org/pipermail/libraries/2016-June/027102.html

 like, including what the default definitions of the other methods would
 be.

 Diff coming soon.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12358
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler