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POInferred type of function that zips HLists
 

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  1. POInferred type of function that zips HLists
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    <p>Thanks to <a href="https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0" rel="nofollow">https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0</a> I understand how to zip shapeless HLists:</p> <p>Import some stuff from Shapeless 2.0.0-M1:</p> <pre><code>import shapeless._ import shapeless.ops.hlist._ import syntax.std.tuple._ import Zipper._ </code></pre> <p>Create two HLists:</p> <pre><code>scala&gt; val h1 = 5 :: "a" :: HNil h1: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 5 :: a :: HNil scala&gt; val h2 = 6 :: "b" :: HNil h2: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 6 :: b :: HNil </code></pre> <p>Zip them:</p> <pre><code>scala&gt; (h1, h2).zip res52: ((Int, Int), (String, String)) = ((5,6),(a,b)) </code></pre> <p>Now try to define a function that does the same thing:</p> <pre><code>scala&gt; def f[HL &lt;: HList](h1: HL, h2: HL) = (h1, h2).zip f: [HL &lt;: shapeless.HList](h1: HL, h2: HL)Unit </code></pre> <p>The inferred return type is Unit, and indeed applying f to h1 and h2 does just that:</p> <pre><code>scala&gt; f(h1, h2) scala&gt; </code></pre> <p>Is there a way to define f such that I get ((5,6),(a,b)) back in this case?</p> <p>Ultimately what I'm trying to do is define a function that zips the two HLists and then maps over them, choosing either _1 or _2 based a coin toss, which would yield another HL.</p> <pre><code>object mix extends Poly1 { implicit def caseTuple[T] = at[(T, T)](t =&gt; if (util.Random.nextBoolean) t._2 else t._1) } </code></pre> <p>Which works fine in the REPL:</p> <pre><code>scala&gt; (h1, h2).zip.map(mix) res2: (Int, String) = (5,b) </code></pre> <p>But I'm getting tripped up on the above issue when trying to pull this into a function.</p> <p>Thanks!</p>
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    1. USAdam Pingel
    1. USAdam Pingel
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      1. USMiles Sabin
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    1. COI'm not around a computer but the `Unit` issue is [a bug that's been fixed since the first milestone](https://github.com/milessabin/shapeless/pull/56). Now those examples won't compile, and the error message should tell you the `Zip` instance you need to add to your method definition.
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      1. USTravis Brown
    2. COThanks, Travis. I am seeing an error now when using 2.0.0-SNAPSHOT. I may still need a hint or two to accomplish my goal, but this new error is helpful.
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      1. USAdam Pingel
    3. COSpecifically, that error after defining f as shown above is: <console>:24: error: could not find implicit value for parameter transpose: shapeless.ops.tuple.Transposer[(HL, HL)] def f[HL <: HList](h1: HL, h2: HL) = (h1, h2).zip
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