Simon Peyton-Jones simonpj@microsoft.com writes:

[.. following up to Peter Thiemann's post on the slowness of Read(S) ..]

I do agree with you that it woud be better for the Read class to use a Maybe result rather than a list of parses. But I'm not sure your problem can be solved simply by making the Char instance of Read better. ....

For what it's worth, I played around with changing ReadS to do just that a couple of years ago, i.e., defining it as type ReadS a = String -> Maybe (a,String) The reductions in space&time were decent (at least 2x in some cases, if I remember correctly), but I didn't take the idea any further. It's a simple change - you can enable it by compiling GHC's PrelRead.lhs with the extra option -DNEW_READS_REP -- there's no 'deriving' support for this rep. of ReadS in the compiler itself tho. --sigbjorn