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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

Trac metadata

Trac field	Value
Version	7.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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