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<p class="MsoNormal">Dear GHC users<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’ve just discovered something very peculiar with unboxed tuples in GHC.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> f2 x = (# True, False #)<o:p></o:p></p>
<p class="MsoNormal"> f1 x = (# True #)<o:p></o:p></p>
<p class="MsoNormal"> f0 x = (# #)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What types do these functions have?<o:p></o:p></p>
<p class="MsoNormal"> f2 :: a -> (# Bool, Bool #)<o:p></o:p></p>
<p class="MsoNormal"> f1 :: a -> (# Bool #)<o:p></o:p></p>
<p class="MsoNormal">BUT<o:p></o:p></p>
<p class="MsoNormal"> f0 :: a -> b -> (# b #)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I think this is stupid. It should be<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> f0 :: a -> (# #)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But in fact even that type isn’t what you expect (ie the analogue of f :: a -> () )<o:p></o:p></p>
<p class="MsoNormal">Here are the kinds of the type constructors:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> (,,) :: * -> * -> * -> *<o:p></o:p></p>
<p class="MsoNormal"> (,) :: * -> * -> *<o:p></o:p></p>
<p class="MsoNormal"> () :: *<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> (# ,, #) :: * -> * -> * -> #<o:p></o:p></p>
<p class="MsoNormal"> (# , #) :: * -> * -> #<o:p></o:p></p>
<p class="MsoNormal">BUT<o:p></o:p></p>
<p class="MsoNormal"> (# #) :: * -> #<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In both respects unboxed unit tuples are behaving differently to boxed ones. This seems bonkers. I propose to fix this, but I wanted to check if anyone is relying on the current odd behaviour.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
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