[Haskell-cafe] Problem with HXT `when`

[Vlatko Basic]

Hi Albert,

thanks for your answer. It helped in some other issues I have.
I did experiment with parenthesis, but obviously not in the right place.


I found the filterA arrow to do the thing I want, like this:

getRowWithHeading caption =
	filterA (deep (hasName "th" /> hasText (==caption) )) />
         hasName "td" >>>
         getChildren


I like to ask you if you might know the answer to the following:

In a structure
    +---XTag "a"
    |   +---XText "A text 1"
    |
    +---XText "Plain text 1"
    |
    +---XTag "a"
    |   +---XText "A text 2"
    |
    +---XText "Plain text 2"
    ...

I must combine "A text 1" with "Plain text 1", etc., but they are in sequence on 
the same level.

Is it possible, in a simple way, to do it in a single arrow flow?
(Or only outside the arrow by pairing the list items?)

Something like:
   hasName "a" /> getText  &&& (getNextItem??? >>> getText)


I assume not, but I'm still new to Haskell and would like to check.


br,

vlatko

-------- Original Message  --------
Subject: Re: [Haskell-cafe] Problem with HXT `when`
From: Albert Y. C. Lai <trebla at vex.net>
To: haskell-cafe at haskell.org
Date: 25.09.2013 03:22

> On 13-09-21 05:13 AM, Vlatko Basic wrote:
>> I'd like to extract A texts from row with header "Caption", and have
>> come up with this
>>
>> runX $ doc
>>      >>> (deep (hasName "tr")                                       --
>> filter only TRs
>>                 >>> withTraceLevel 5 traceTree                   --
>> shows correct TR
>>                 `when`
>>               deep (
>>                  hasName "th" >>>
>> -- filter THs with specified text
>>                  getChildren >>> hasText (=="Caption")
>>               ) -- inner deep
>>               >>> getChildren >>> hasName "td" -- shouldn't here be only
>> one TR?
>>               >>> getChildren
>>            )
>>           >>> getName &&& (getChildren >>> getText)  -- list has TDs
>> from all three TRs
>
> Operator precedences:
>    >>> infixr 1
>    `when` infixl 9 (default)
>
> Therefore, this expression redundantly parenthesized and systematically indented
> to ensure that you are on the same page with the computer is:
>
> runX $
>      doc
>      >>>
>      ( deep (hasName "tr")
>        >>>
> -- begin{conditionally prints but otherwise is arr id}
>        ( withTraceLevel 5 traceTree
>          `when`
>          deep ( hasName "th"
>                 >>>
>                 getChildren
>                 >>>
>                 hasText (=="Caption")
>               ) -- inner deep
>        )
> -- end{conditionally prints but otherwise is arr id}
>        >>>
>        getChildren
>        >>>
>        hasName "td"
>        >>>
>        getChildren
>      )
>      >>>
>      ( getName &&& (getChildren >>> getText) )
>
> The condition on <th>Caption</th> ends up controlling trace messages only; it is
> not used to limit real processing.
>
> "when" doesn't help even when used correctly: it doesn't ban data. "guards" and
> "containing" ban data, but you have to put them at the right place, i.e.,
> parenthesize correctly.
>
> runX $
>      doc
>      >>>
>      ( deep ( hasName "tr"
>               `containing`
>               deep ( hasName "th"
>                      >>>
>                      getChildren
>                      >>>
>                      hasText (=="Caption")
>                    )
>             )
>        >>>
>        getChildren
>        >>>
>        hasName "td"
>        >>>
>        getChildren
>      )
>      >>>
>      ( getName
>        &&&
>        (getChildren >>> getText)
>      )
>
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