id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
5720,Unit unboxed tuple behaving oddly,simonpj,,"I’ve just discovered something very peculiar with unboxed tuples in GHC.
{{{
	f2 x = (# True, False #)
	f1 x = (# True #)
	f0 x = (# #)
}}}
What types do these functions have?
{{{
	f2 :: a -> (# Bool, Bool #)
	f1 :: a -> (# Bool #)
BUT
	f0 :: a -> b -> (# b #)
}}}
I think this is stupid.  It should be
{{{
	f0 :: a -> (# #)
}}}
But in fact even that type isn’t what you expect (ie the analogue of  `f :: a -> ()` )
Here are the kinds of the type constructors:
{{{
	(,,) :: * -> * -> * -> *
	(,) :: * -> * -> *
	() :: *

	(# ,, #) :: * -> * -> * -> #
	(# , #) :: *  -> * -> #
BUT
	(#  #) :: * -> #
}}}
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.


Simon
",bug,closed,normal,,Compiler,7.2.1,fixed,,,Unknown/Multiple,Unknown/Multiple,None/Unknown,Unknown,,,,