[Haskell-cafe] Difficulties in accessing inner elements of data types

[Neil Mitchell]

Hi David,

What you are wanting to do is query and transform a reasonably large
AST. Fortunately there are solutions, generic programming should do
exactly what you want without too much hassle. I'd personally
recommend taking a look at the Uniplate library, starting by reading
the Haskell Workshop 2007 paper (its an academic paper, but for you
the  really useful stuff is in Section 2 which is more tutorial style)
- http://www-users.cs.york.ac.uk/~ndm/uniplate/

I'm sure I could solve your particular queries using Uniplate, but
I'll leave it for you to take a look and figure out. If you can't
figure out how to do it after reading section 2 of the paper mail back
and I'll take a closer look.

Thanks

Neil

2009/3/3 David Miani <davidmiani at gmail.com>:
> Hi,
> I'm working on a Haskell library for interacting with emacs org files. For
> those that do not know, an org file is a structured outline style file that
> has nested headings, text, tables and other elements. For example:
>
> * Heading 1
> Some text, more text. This is a subelement of Heading 1
> 1. You can also have list
> 1. and nested lists
> 2. more...
>
> ** Nested Heading (subelement of Heading 1)
> text... (subelement of Nested Heading)
>
> ** Another level 2 heading (subelement of Heading 1)
> | Desc | Value |
> |-----------+--------------------------------------|
> | Table | You can also have tables in the file |
> | another | row |
> | seperator | you can have seps as well, eg |
> |-----------+--------------------------------------|
>
> * Another top level heading
> There are many more features, see orgmode.org
>
> My library enables read and write access to a subset of this format (eg
> lists aren't parsed atm).
>
> The data structures used for writing are:
>
> data OrgFile = OrgFile [OrgFileElement]
> data OrgFileElement = Table OrgTable
> | Paragraph String
> | Heading OrgHeading
>
> -- heading level, title, subelements
> data OrgHeading = OrgHeading Int String [OrgFileElement]
>
> data OrgTable = OrgTable [OrgTableRow]
>
> data OrgTableRow = OrgTableRow [String] | OrgTableRowSep
>
>
> To write a file you contruct a OrgFile out of those elements, and pass it to
> a writeOrgFile func. Eg:
>
> writeOrg $ OrgFile [Heading (OrgHeading 1 "h1" [Paragraph "str"])]
> would produce:
> * h1
> str
>
> I was going to use the same data structures for reading an org file, but it
> quickly became apparent that this would not be suitable, as you needed the
> position of the file of an element to be able to report errors. Eg if you
> needed to report an error that a number was expected, the message "'cat' is
> not a number" is not very useful, but "Line 2031: 'cat' is not a number" is.
> So the data structures I used were:
>
> data FilePosition = FilePosition Line Column
>
> data WithPos a = WithPos {
> filePos :: FilePosition,
> innerValue :: a
> }
>
> data OrgTableP = OrgTableP [WithPos OrgTableRow]
>
> data OrgFileElementP = TableP OrgTableP
> | ParagraphP String
> | HeadingP OrgHeadingP
>
> data OrgHeadingP = OrgHeadingP Int String [WithPos OrgFileElementP]
>
> data OrgFileP = OrgFileP [WithPos OrgFileElementP]
>
>
> Finally there is a function readOrg, which takes a string, and returns an
> OrgTableP.
>
>
> Now, this all works as expected (files are correctly being parsed and
> written), however I am having a lot of trouble trying to come up with a
> decent API to work with this. While writing an OrgFile is fairly easy,
> reading (and accessing inner parts) of an org file is very tedious, and
> modifying them is horrendous.
>
> For example, to read the description line for the project named "Project14"
> in the file:
>
> * 2007 Projects
> ** Project 1
> Description: 1
> Tags: None
> ** Project 2
> Tags: asdf,fdsa
> Description: hello
> * 2008 Projects
> * 2009 Projects
> ** Project14
> Tags: RightProject
> Description: we want this
>
> requires the code:
>
> type ErrorS = String
> listToEither str [] = Left str
> listToEither _ (x:_) = Right x
>
> get14 :: OrgFileP -> Either ErrorS String
> get14 (OrgFileP elements) = getDesc =<< (getRightProject . concatProjects)
> elements where
> concatProjects :: [WithPos OrgFileElementP] -> [OrgHeadingP]
> concatProjects [] = []
> concatProjects ((WithPos _ (HeadingP h)) : rest) = h : concatProjects rest
> concatProjects (_ : rest) = concatProjects rest
>
> getRightProject :: [OrgHeadingP] -> Either ErrorS OrgHeadingP
> getRightProject = listToEither "Couldn't find project14" .
> filter (\(OrgHeadingP _ name _) -> name == "Project14")
>
> getDesc :: OrgHeadingP -> Either ErrorS String
> getDesc (OrgHeadingP _ _ children) =
> case filter paragraphWithDesc (map innerValue children) of
> [] -> Left $ show (filePos $ head children) ++
> ": Couldn't find desc in project"
> ((ParagraphP str):_) -> Right str
> _ -> error "should not be possible"
>
> paragraphWithDesc :: OrgFileElementP -> Bool
> paragraphWithDesc (ParagraphP str) = str =~ "Description"
> paragraphWithDesc _ = False
>
>
> If you think that is bad, try writing a function that adds the Tag "Hard" to
> Project2 :(
>
> What I really need is a DSL that would allow sql like queries on an
> OrgFileP. For example:
> select (anyHeading `next`
> headingWithName "Project14" `withFailMsg` "couldn't find p14" `next`
> paragraphMatchingRegex "Description" `withFailMsg` "no desc")
> org `output` paragraphText
>
> would return a String
> OR
> select (anyHeading `next` headingWithName "Project2" `next`
> paragraphMatchingRegex "Tag:") org `modify` paragraphText (++ ",Hard")
>
> would return an OrgFile, with the new Hard tag added.
>
> However, I don't know if this is even possible, how to do it, or if there is
> a better alternative to this. I would really apreciate any hints with
> regards to this. It would be useful to know if there are other libraries
> that also face this problem, and how they solved it.
>
> Finally, I would be grateful for any other advice regarding my code. One
> thing that has bugged me is my solution for having file position info - my
> solution never seemed very elegant.
>
> Thanks,
> David
>
>
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