I think I got this idea from JHC, but I&#39;m unable to find it documented in the JHC manual at the moment.  The idea is that when _ appears on the RHS of an equation it is another name for undefined.<div><br></div><div>Here is a hypothetical example, inspired by code I&#39;ve seen in FFI definitions:</div>
<div><br></div><div>\begin{code}</div><div>{-# LANGUAGE EmptyDataDecls #-}</div><div>data Bar</div><div>instance Storable Bar where</div><div>  sizeOf _ = sizeOf #{size struct Bar}</div><div>  alignment _ = alignment (_ :: CDouble) -- here we could have just as succinctly said alignment (1 :: CDouble)</div>
<div><br></div><div>data Foo -- the struct Foo on the C side contains a Bar</div><div><br></div><div>instance Storable Foo where</div><div>  sizeOf _ = sizeOf #{size struct Foo}</div><div>  alignment _ = alignment (_ :: Bar) -- we can&#39;t instantiate a Bar</div>
<div>\end{code}</div><div><br></div><div>Is this something people would be interested in having as an extension in GHC?  Or is it just too fluffy for anyone to really care?</div><div><br></div><div>Thanks,</div><div>Jason</div>