Re: [Haskell-cafe] Parsing binary 'hierachical' objects for lazy developers

On Wed, Apr 27, 2011 at 10:18:47PM +0100, Stephen Tetley wrote:

On 27 April 2011 21:28, Alexander Solla wrote:

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On Wed, Apr 27, 2011 at 11:16 AM, John Obbele wrote:

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Second issue, I would like to find a way to dispatch parsers. I'm not very good at expressing my problem in english, so I will use another code example:

This sounds very hard in the general case.  Others have shown you how to dispatch on two types.  But there is no general data type which combines all (or even arbitrarily many) types.  Somehow, "Read" is able to do this, but I don't know what kind of magic it uses.

Read always "demands its type" so it doesn't use any magic - if the input string doesn't conform it will throw an error.

Any sensible binary format will have a scheme such as tag byte prefixes to control choice in parsing (binary parsing generally avoids all backtracking). If your binary data doesn't have a proper scheme it will be hard to parse for any language (or cast-to in the case of C), so the most sensible answer is to revise the format.

Oki, so far the use of the Control.Applicative magic, the syntax sugar for monadic operations and manually written it/then/else or 'case of' branching statements have helped me considerably in the parsing task. I have not try DrIFT since I prefer to avoid pre-processors for now. So the only quirk that is still upsetting me is the 'deriving' issue: if I know that what I am parsing could only result in ObjectA or ObjectB, every thing would be simple. But when someone decides to add an extension to the binary format, let's say add a new tag identifier and a new ObjectC with a different size and new attributes, I will have to re-write part of my Haskell parser. I think, I will just have to rewrite the abstract type to 'data AbstractObject = ObjectA | ObjectB | ObjectC' let my parser still have the type signature: 'parser :: B.ByteString -> AbstractObject' and modify the branching inside to add the C tag identifier: 'case tag identifier of A -> ... B -> ... C -> ...'. It's not straightforward but it should be manageable. regards, /john