Re: [Haskell-beginners] How to write Read instance

2 Nov 2017


      David, your experience and intuition were absolutely right: solution is
in the right Show and Read pair. So, this seems to work in all cases:

  import qualified Text.ParserCombinators.ReadP as P
  import           Text.ParserCombinators.ReadPrec (lift)
  import qualified Data.Char           as C

  -- |Type of Git hashes
  newtype GitHash = GitHash { ghValue :: T.Text } deriving (Eq, Ord, Generic)

  instance Show GitHash where
    showsPrec p a = showParen (p > 10) (showString $ T.unpack $ ghValue a)

  hexDigits :: P.ReadP String
  hexDigits = P.munch1 C.isHexDigit

  instance Read GitHash where
    readPrec = parens $ do
      lift P.skipSpaces
      s::String <- lift hexDigits
      return $ GitHash $ T.pack s


Thanks a lot again!!
...
My own toy example works.  I don't know how yours differs.
On Thu, Nov 2, 2017 at 10:51 AM, Baa  wrote:
...
Hello, David!
`Show` instance is simple:
instance Show Hex where
    show = T.unpack . ghValue
so
...
show (Hex "1234ab")  
  1234ab  
read "1234ab"::Hex  
  1234ab  
read "Just 1234ab"::Maybe Hex -- fails like wrapped in the
record!!
Yes, I'm ignoring precedence usually too. And I return rest of the
string. Playing with `UTCTime`'s readsPrec shows me that behaviour
looks the same: UTCTime's reader returns the same rest. So, may be
trick is in the:
1. Precedence ?!
2. readListPrec = readListPrecDefault
   readList     = readListDefault   ?
But I get error: readListPrec is not visible method of class
Read... 3. readPrec ? I implemented readsPrec which is old-style,
but am I right that old-style and new-style are absolutely the same
from reading point of view and old-style can replace new-style and
vice versa?
PS. I don't import any special modules.
...
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  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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