erik writes:
> [...]
> instance Read A where
> readsPrec _ s = case s of { "1" -> [(A,"")]; "2" -> [(AA,"")] ; _ ->
> []}
>
> [...]
>
> Test> read (show X{a=A}) :: X
>
> Program error: PreludeText.read: no parse
Try the following (one could write it even nicer, but I tried to change
the code as little as possible):
----------------------------------------------------------------------
instance Read A where
readsPrec _ s = case s of { '1':xs -> [(A,xs)]; '2':xs -> [(AA,xs)] ; _ -> []}
----------------------------------------------------------------------
and you will get:
----------------------------------------------------------------------
Test> read (show X{a=A}) :: X
X{a=1}
----------------------------------------------------------------------
Your definition of readsPrec can parse an A only at the very end of the
input string. And in "X{a=1}" the `1' is not at the very end.
Regards, Heribert.
PS: This problem is a good argument for Phil Wadlers suggestion to
replace Read (and Show) by something less awkward.
(http://www.cs.chalmers.se/~rjmh/Haskell/Messages/Display.cgi?id=5)
