[Haskell-cafe] Derived Read instance for types with infix constructors (ghc 6.4.1)

Robert Dockins robdockins at fastmail.fm
Fri Aug 25 19:45:06 EDT 2006

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


And (And A A) A


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

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