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POnarrowing type of a vinyl record based on the lens/index chosen
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18436386
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2013-08-26T03:58:04.647
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2013-09-11T22:24:20.027
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2013-08-26T04:30:10.843
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Body
Extensible records provided by <a href="http://hackage.haskell.org/package/vinyl" rel="nofollow">http://hackage.haskell.org/package/vinyl</a> are different from what other libraries (HList and probably others like grapefruit-records/records) provide. This is because the result of the lookup is based on the pair <code>(result type, label)</code>, while the other libraries have the lookup based on <code>label</code> only. An example of this happening for vinyl: <pre><code>{-# LANGUAGE DataKinds, TypeOperators #-} import Data.Vinyl r = fixRecord $ (Field :: "x" ::: a) =: (3::Double) <+> (Field :: "x" ::: a) =: (2::Int) main = do print $ rGet (Field :: "x" ::: Int) r print $ rGet (Field :: "x" ::: Double) r -- prints 2\n3 </code></pre> For my application <a href="http://hackage.haskell.org/package/Rlang-QQ" rel="nofollow">http://hackage.haskell.org/package/Rlang-QQ</a>, the type of the elements of the record will be determined by their usage later on, so it seems I need the "x" to uniquely determine the value that's stored there. I want to avoid "redundant" type signature in the expression <pre><code>(rGet (Field :: "z" ::: [Double]) (ans :: PlainRec ("z" ::: [Double] ': '[]))) </code></pre> and have it look more like: <pre><code>(rGet (Field :: "z" ::: [Double]) ans) </code></pre> The whole code is in <a href="http://code.haskell.org/~aavogt/Rlang-QQ/examples/test2.hs" rel="nofollow">http://code.haskell.org/~aavogt/Rlang-QQ/examples/test2.hs</a> Is there some way to achieve this using vinyl, or should I adapt my code to use another library?
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<haskell>
Title
narrowing type of a vinyl record based on the lens/index chosen
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