[Haskell-cafe] newtype is superfluous

[Lennart Augustsson]

Wolfgang Jeltsch wrote:
> Am Samstag, 15. Oktober 2005 08:31 schrieb Bulat Ziganshin:
> 
>>Hello Haskell,
>>
>>  number of type definition statements in Haskell (data, type, newtype) is
>>a bit too large. at least, newtype definition seems to be superfluous - it
>>can be replaced by the same `data` definition:
>>
>>newtype A = A Int
>>and
>>data A = A Int
>>
>>is equal, excluding for internal representation.
> 
> 
> This is not true.  With newtype, A _|_ is _|_, with data, A _|_ is not _|_.  
> But as far as I know, the above newtype declaration is equivalent to this:
> 
> 	data A = A !Int

Alas, Haskell is more subtle than that.  Which is why newtype exists.

Try
	case A _|_ of A _ -> 1
with the two versions of A to see the difference.

	-- Lennart