[Haskell-beginners] How to write Read instance

[Baa]

Hmm, difference is that your example with "Foo" uses fixed (ident)
lexem. But when you need to return rest of the string which is not part
of your type - this leads to problem for me...

> The most common way is to just auto derive Read.  I'm not sure that
> that ever really fails.  Are you sure the problem isn't with the Show
> instance of the type?  People commonly write invalid Show instances
> to make them look pretty and they shouldn't.  read and show are
> supposed to be inverses of each other because when they aren't,
> problems like this occur.
> 
> The simple way to do a Read instance is to implement the reads
> function for it.  The confusion comes from its type.  readsPrec ::
> Int -> ReadS a. ReadS is defined as String -> [(a, String)], where a
> is the parsed result and String is the rest of the string that is
> being parsed, , which may look confusing, and the Int is precedence,
> which can usually be ignored.  It could have been Int -> String ->
> Maybe (a, String), but Read predates Maybe.  So instead it returns a
> list and if it fails to parse, it returns [] instead of Nothing.  So.
> 
> data MyFoo = MyFoo
> 
> instance Read MyFoo where
>   -- readsPrec :: Int -> String -> [(MyFoo, String)]
>   readsPrec _ = readFoo
> 
> readFoo :: String -> [(MyFoo, String)]
> readFoo str = case splitAt 5 str of
>   ("MyFoo", rest) -> [(MyFoo, rest)]
>   otherwise -> []
> 
> If you need something more complex, there are functions to do it in
> base that perform lexing and parsing.  I have never used them but you
> can go ahead and read some of the instances such as Ordering at
> https://hackage.haskell.org/package/base-4.10.0.0/docs/src/GHC.Read.html#line-398
> to try and learn how it might work for you.
> 
> But honestly I think you should look at fixing your Show instance
> first, if possible.
> 
> 
> On Thu, Nov 2, 2017 at 9:41 AM, Baa <aquagnu at gmail.com> wrote:
> 
> > Hello all!
> >
> > I found some errors in the reading of previously shown big and
> > complex record. Reason is: some of fields are reading wrongly. So,
> > is there any useful documents how to write `Read` instance correctly
> > (because I can't find good tutorial in the Web and often hit errors
> > with `Read` instance)? May be tutorial/examples/any good info...
> >
> >
> > ===
> > Best regards, Paul
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