"Simon Peyton-Jones" <simonpj@microsoft.com> writes: | And, with a lot of help from Koen, I'm about to fold | in a much more efficient implementation of Read, which | may help. On Thu, 4 Apr 2002, Jan-Willem Maessen wrote: | Any of the guilty parties want to give a quick | overview of how it's going to work? The basic idea is as follows. We have an efficient (abstract) parser type 'ReadP', which supports the following operations: readP_to_S :: ReadP a -> ReadS a readS_to_P :: ReadS a -> ReadP a -- might be slow (The reason for ReadP to be abstract is so that it can be changed to a more efficient implementation if one comes along.) Remember that ReadS is defined as: type ReadS a = String -> [(a,String)] Now, suppose the definition of the Read class were as follows: class Read a where reads :: ReadS a What we did, we just added an extra method to the Read class: class Read a where reads :: ReadS a readp :: ReadP a -- default definitions reads = readP_to_S readp readp = readS_to_P reads Whenever GHC does a "deriving Read", it will define the readp parser (which is efficient) rather than the reads parser. The reads parser will just be defined in terms of the readp parser. Whenever an ignorant user comes along and makes an old-style instance of the Read class: instance Read MyType where reads s = ... The readp parser will still be there, but it might possibly be slow. Of course up-to-date users can make efficient instances by actually defining readp rather than reads. This is the basic idea. Unfortunately, the Read class does not look like this, and we have to take care of precedences (rather easy) and the readList method (tricky). We will probably submit a paper to the Haskell workshop about this. /Koen. -- Koen Claessen http://www.cs.chalmers.se/~koen Chalmers University, Gothenburg, Sweden.