Send Beginners mailing list submissions to
beginners@haskell.org
To subscribe or unsubscribe via the World Wide Web, visit
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
beginners-requ...@haskell.org
You can reach the person managing the list at
beginners-ow...@haskell.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: How to write Read instance (Baa)
2. Re: How to write Read instance (Baa)
3. Re: How to write Read instance (Baa)
4. Re: How to write Read instance (Baa)
----------------------------------------------------------------------
Message: 1
Date: Thu, 2 Nov 2017 17:13:43 +0200
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID: <20171102171343.76a1c322@Pavel>
Content-Type: text/plain; charset=US-ASCII
Does it work with wrapping in records, in "Just" ?
Would you show this example, please?
I try this:
import qualified Data.Text as T
import GHC.Read
import qualified Data.Char as C
newtype GitHash = GitHash { ghValue :: T.Text } deriving (Eq, Ord, Generic)
instance Show GitHash where
show = T.unpack . ghValue
instance Read GitHash where
readsPrec p = readParen (p > 10) reader where
reader :: ReadS GitHash
reader s =
case span C.isHexDigit s of
([], _) -> []
(hex, nohex) -> [(GitHash $ T.pack hex, nohex)]
readListPrec = readListPrecDefault
readList = readListDefault
and this parses:
> read "1234ab"::GitHash
but not this:
> read "Just 1234"::Maybe GitHash
*** Exception: Prelude.read: no parse
> My own toy example works. I don't know how yours differs.
>
> On Thu, Nov 2, 2017 at 10:51 AM, Baa <aqua...@gmail.com> 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 <aqua...@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
> > > > _______________________________________________
> > > > Beginners mailing list
> > > > Beginners@haskell.org
> > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > > >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
------------------------------
Message: 2
Date: Thu, 2 Nov 2017 17:17:09 +0200
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID: <20171102171709.597c46b5@Pavel>
Content-Type: text/plain; charset=US-ASCII
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 <aqua...@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
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
------------------------------
Message: 3
Date: Thu, 2 Nov 2017 17:27:53 +0200
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID: <20171102172753.56a426f4@Pavel>
Content-Type: text/plain; charset=US-ASCII
David, it's very strange! My snippet works with "Just" and wrapping in
a record too, but only if I surround hex-number with ( and ):
"D {d = (123abc)}"
or
"Just (123abc)"
If I remove call of `readParen` then it can parse strings like:
"D {d = 123abc}"
but can not like
"Just 123abc"
> My own toy example works. I don't know how yours differs.
>
> On Thu, Nov 2, 2017 at 10:51 AM, Baa <aqua...@gmail.com> 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 <aqua...@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
> > > > _______________________________________________
> > > > Beginners mailing list
> > > > Beginners@haskell.org
> > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > > >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
------------------------------
Message: 4
Date: Thu, 2 Nov 2017 18:45:37 +0200
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID: <20171102184537.7d5c1006@Pavel>
Content-Type: text/plain; charset=US-ASCII
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 <aqua...@gmail.com> 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 <aqua...@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
> > > > _______________________________________________
> > > > Beginners mailing list
> > > > Beginners@haskell.org
> > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > > >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
------------------------------
Subject: Digest Footer
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
End of Beginners Digest, Vol 113, Issue 2
*****************************************
