Problem on overlapping instances

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Magicloud Magiclouds

5 Jan 2011 5 Jan '11

8:24 a.m.

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

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Steffen Schuldenzucker

5 Jan 5 Jan

8:31 a.m.

Am 05.01.2011 09:24, schrieb Magicloud Magiclouds:

...

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works.

Just a guess: Have you enabled TypeSynonymInstances? (As String is a type synonym, at least this extension would be required)

Jasper Van der Jeugt

8:33 a.m.

Hello, {-# LANGUAGE OverlappingInstances, FlexibleInstances #-} import Data.Binary instance Binary [String] where get = undefined put = undefined works fine here on GHC 6.12.3. That being said, it would be safer perhaps to add a newtype around [String] so you can avoid the orphan instance as well, i.e. import Data.Binary newtype MyType = MyType [String] instance Binary MyType where get = undefined put = undefined Cheers, Jasper On Wed, Jan 5, 2011 at 9:24 AM, Magicloud Magiclouds wrote:

...

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

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Luke Palmer

8:34 a.m.

You can't. If you have special semantics for [String], then it is not really a [String], it is something else. So let the type system know that: newtype SomethingElse = SomethingElse [String] instance Binary SomethingElse where ... On Wed, Jan 5, 2011 at 1:24 AM, Magicloud Magiclouds wrote:

...

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

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Stephen Tetley

8:41 a.m.

You have two choices (other people have enumerated the first while I was typing): First choice: Wrap your Stringlist with a newtype: newtype StringList = StringList [String] The downside of this your code gets "polluted" with the newtype. Second choice: Write special putStringList and getStringList functions. Hand-code the binary instances where you are wanting [String] to be special and call putStringList and getStringList rather than put and get. Downside - cannot automatically derive Binary. That's if Binary can be automatically derived anyway?, the times when I need Binary I write the instances myself anyway.

Magicloud Magiclouds

12:25 p.m.

Steffen Schuldenzucker: Sure. GHC would prompt that. Jasper Van der Jeugt: Not working with ghc7. But there sure are some threads about this kind of things. I do not know if this is a bug of 6.* or 7, either. Luke Palmer: Sorry, by special, I meant, for example, ["a", "b"] will be "ab" by default, but I want it to be "a,b". So I'd like to overload for certain types. Stephen Tetley: I think that are the only choices. The first is simple, but the second saves some code writing. After all, thanks. On Wed, Jan 5, 2011 at 4:41 PM, Stephen Tetley wrote:

...

You have two choices (other people have enumerated the first while I was typing):

First choice:

Wrap your Stringlist with a newtype:

newtype StringList = StringList [String]

The downside of this your code gets "polluted" with the newtype.

Second choice:

Write special putStringList and getStringList functions. Hand-code the binary instances where you are wanting [String] to be special and call putStringList and getStringList rather than put and get.

Downside - cannot automatically derive Binary. That's if Binary can be automatically derived anyway?, the times when I need Binary I write the instances myself anyway.

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-- 竹密岂妨流水过山高哪阻野云飞

Henning Thielemann

7 Jan 7 Jan

12:47 a.m.

Magicloud Magiclouds schrieb:

...

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works.

http://www.haskell.org/haskellwiki/List_instance

Antoine Latter

3:32 a.m.

Hello, I only use the 'Binary' class when I don't care about the specifics of the serialization format, only that it be reasonably fast, compact and stable. When I need to comply with some particular format I use the functions in Data.Binary.Builder and Data.Binary.Get directly. Sometimes I make my own Serialize/Deserialize classes if I think it will be helpful. But I do look at the source for the instances of the 'Binary' class for inspiration! Take care, Antoine On Wed, Jan 5, 2011 at 2:24 AM, Magicloud Magiclouds wrote:

...

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

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Magicloud Magiclouds

8:36 a.m.

I agree, but with a slight difference. Since I am so lazy, if Binary fits, like 80% of my requirements, for example this case, only [String] is not OK. I'd like to reuse it with a little modification made by myself, rather than re-write almost the whole of it. But for Data.Binary, I think this problem is simple. Since it cannot be derivinged, making extra function for [String] won't make the further usage of my library complex. On Fri, Jan 7, 2011 at 11:32 AM, Antoine Latter wrote:

...

Hello,

I only use the 'Binary' class when I don't care about the specifics of the serialization format, only that it be reasonably fast, compact and stable.

When I need to comply with some particular format I use the functions in Data.Binary.Builder and Data.Binary.Get directly. Sometimes I make my own Serialize/Deserialize classes if I think it will be helpful.

But I do look at the source for the instances of the 'Binary' class for inspiration!

Take care, Antoine

On Wed, Jan 5, 2011 at 2:24 AM, Magicloud Magiclouds wrote:

...
Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

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-- 竹密岂妨流水过山高哪阻野云飞

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participants (7)

Antoine Latter
Henning Thielemann
Jasper Van der Jeugt
Luke Palmer
Magicloud Magiclouds
Steffen Schuldenzucker
Stephen Tetley