I&#39;m currently working with a lot of very short arrays of fixed length and as a thought experiment I thought I would try to play with fast numeric field accessors<div><br></div><div>In particular, I&#39;d like to use something like foreign prim to do something like</div>
<div><br></div><div>&gt; foreign import prim &quot;cmm_getField&quot; unsafeField# :: a -&gt; Int# -&gt; b</div><div><br></div><div>&gt; unsafeField :: a -&gt; Int -&gt; b</div><div>&gt; unsafeField a (I# i) = a&#39; `pseq` unsafeField# a&#39; i -- the pseq could be moved into the prim, I suppose.</div>
<div>&gt;   where a&#39; = unsafeCoerce a</div><div><br></div><div>&gt; fst :: (a,b) -&gt; a</div><div>&gt; fst = unsafeField 0</div><div><br></div><div>&gt; snd :: (a,b) -&gt; b</div><div>&gt; snd = unsafeField 1</div><div>
<br></div><div>This becomes more reasonable to consider when you are forced to make something like</div><div><br></div><div><div>&gt; data V4 a = V4 a a a a</div></div><div><br></div><div>using</div><div><br></div><div>&gt; unsafeIndex (V4 a _ _ _) 0 = a</div>
<div>&gt; unsafeIndex (V4 _ b _ _) 1 = b</div><div>&gt; unsafeIndex (V4 _ _ c _) 2 = c</div><div>&gt; unsafeIndex (V4 _ _ _ d) 3 = d</div><div><br></div><div>rather than </div><div><br></div><div><div>&gt; unsafeIndex :: V4 a -&gt; Int -&gt; a</div>
<div>&gt; unsafeIndex = unsafeField</div></div><div><br></div><div><div>But I can only pass unboxed types to foreign prim.</div><div><br></div><div>Is this an intrinsic limitation or just an artifact of the use cases that have presented themselves to date?</div>
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