On Aug 25, 2006, at 6:50 PM, Misha Aizatulin wrote:
hi,
the Haskell Report 10.4 says that
"The result of show is readable by read if all component types are readable"
however if I define a type like
data T = A | T `And` T deriving (Read, Show)
then
*Main> show $ A `And` A "A And A" *Main> (read "A And A") :: T *** Exception: Prelude.read: no parse *Main>
In fact, I wasn't able to guess, what I should type so that the value (A `And` A) gets parsed.
I have ghc 6.4.1. Looking into the code of the derived instance I see that it expects Text.Read.Lex.lex to return (Symbol "And") for the constructor. If I understand the code for lex correctly, then it parses things as Symbol if they consist only of "!@#$%&*+./<=>?\\^|:-~"
How then do I read values of type T defined above? Thanks in advance for any directions.
In general, derived Read instances are designed to be inverses for Show. The easy thing to do is to print values of type T and see what you get. I expect that it will be in prefix form, eg: And A A or And (And A A) A etc. That is, I think the Show and Read instances are going to ignore the backticks in the definition.
Cheers, Misha
Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG