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POlists searches in SYB or uniplate haskell
 

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  1. POlists searches in SYB or uniplate haskell
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    <p>I have been using uniplate and SYB and I am trying to transform a list</p> <p>For instance</p> <pre><code> type Tree = [DataA] data DataA = DataA1 [DataB] | DataA2 String | DataA3 String [DataA] deriving Show data DataB = DataB1 [DataA] | DataB2 String | DataB3 String [DataB] deriving Show </code></pre> <p>For instance, I would like to traverse my tree and append a value to all [DataB]</p> <p>So my first thought was to do this:</p> <pre><code>changeDataB:: Tree -&gt; Tree changeDataB = everywhere(mkT changeDataB') chanegDataB'::[DataB] -&gt; [DataB] changeDataB' &lt;add changes here&gt; </code></pre> <p>or if I was using uniplate</p> <pre><code> changeDataB:: Tree -&gt; Tree changeDataB = transformBi changeDataB' chanegDataB'::[DataB] -&gt; [DataB] changeDataB' &lt;add changes here&gt; </code></pre> <p>The problem is that I only want to search on the full list. Doing either of these searches will cause a search on the full list and all of the sub-lists (including the empty list)</p> <p>The other problem is that a value in [DataB] may generate a [DataB], so I don't know if this is the same kind of solution as not searching chars in a string.</p> <p>I could pattern match on DataA1 and DataB3, but in my real application there are a bunch of [DataB]. Pattern matching on the parents would be extensive.</p> <p>The other thought that I had was to create a </p> <pre><code>data DataBs = [DataB] </code></pre> <p>and use that to transform on. That seems kind of lame, there must be a better solution.</p> <p>Update: The main reason that I need to do this is that I need to</p> <ol> <li>change the order of [DataB]</li> <li>add something to [DataB]</li> </ol> <p>So if you all know a cool way to create a mkT that would match </p> <p>B1:B2:B3:B4:[] (which would be per say the full list of [DataB]</p> <p>and not</p> <p>B2:B3:B4:[]<br> or any other derivations of that.</p> <p>I am siding to just biting the bullet and createing the "DataBs", data type and doing a simple mkT match on that. </p>
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    1. USDon Stewart
    1. USChris
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    1. COWhat exactly is the "full list" you are speaking about? Could you provide samples of some "full lists" before and after the desired transformation?
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      1. USADEpt
    2. COLets say that I wanted to replace all Strings with "foo". Doing a transformBi strToFoo strToFoo:: String -> String strToFoo x = "foo" Would not work because String = [Char] So lets say that x = DataA2 "hi" That actually means x = DataA2 h:i:[] I would get matches on h:i:[] i:[] and [] So the result of my transform would be x = Foo:Foo:Foo:Foo (or something like that) I just bit the bullet and created my on transformation function which solves it. The more that I think of it, I dont think that there is a slick way of doing this without looking at the parent Constructor.
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      1. USChris
 

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