25 Jan 2002 08:00:24 +0100, Martin Norbäck <[EMAIL PROTECTED]> pisze:
> newtype T1 = C1 Bool
> data T2 = C2 !Bool
>
> the difference is that the constructor C1 does not exist, so only the
> following values exist for T1:
>
> C1 True (which is the represented as True)
> C1 False (which is the represented as False)
> C1 _|_ (which is represented as non-termination or error)
>
> however, for T2, another value exist, namely _|_.
But it's the same as C2 _|_. These types have the same number of
values and T2 could be represented isomorphically to Bool too.
The only difference between T1 and T2 is that
case x of
C2 _ -> ...
causes the evaluation of x (and thus the Bool inside - because it was
"strictly" tied to the constructor!). It's the same as x `seq` ...
or as the_Bool_inside `seq` ...
The side effect of using data with a strict argument instead of
newtype is that
f' (C2 x) (C2 y) = C2 (f x y)
unexpectedly becomes strict in both arguments, and we would have
to write
f' x y = C2 (f (case x of C2 x' -> x')
(case y of C2 y' -> y'))
in the general case where we want to preserve strictness and don't
want to analyze it ourselves. Newtypes allow to use the convenient
syntax of pattern matching on the lhs.
--
__("< Marcin Kowalczyk * [EMAIL PROTECTED] http://qrczak.ids.net.pl/
\__/
^^
QRCZAK
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