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Today's Topics:
1. How to write Read instance (Baa)
2. Re: How to write Read instance (David McBride)
3. Re: How to write Read instance (Baa)
4. Re: How to write Read instance (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Thu, 2 Nov 2017 15:41:21 +0200
From: Baa <aqua...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] How to write Read instance
Message-ID: <20171102154121.6535e6ac@Pavel>
Content-Type: text/plain; charset=US-ASCII
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
------------------------------
Message: 2
Date: Thu, 2 Nov 2017 10:27:12 -0400
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID:
<can+tr41uxx59uxnyuav5rz3fc_k6zcvxdhm6bnwxqkppym7...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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
>
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Message: 3
Date: Thu, 2 Nov 2017 16:51:06 +0200
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID: <20171102165106.4f61f6f0@Pavel>
Content-Type: text/plain; charset=US-ASCII
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
> >
------------------------------
Message: 4
Date: Thu, 2 Nov 2017 11:05:37 -0400
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] How to write Read instance
Message-ID:
<CAN+Tr41Rx_zsFvKy1epYk6sLL5roXph6vHT85K=6a111rbw...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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
>
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