Combinators for ReadP

[Daan Leijen]

John Meacham wrote:

> 2. give Fail a String argument and have it preserve the string passed to
> 'fail'.
> 
> I don't think either will adversly affect performance, 

I haven't studied the code for readP, but from experience I can tell 
that one needs to be very careful to not affect performance when adding
error messages. For example, if one wants to combine failure messages
from different alternatives, you could force too much evaluation.
i.e. When adding this, be very careful not to break the laziness of the
current implementation.

-- Daan.