fre 2002-01-25 klockan 02.21 skrev Feuer:
> Please reply to [EMAIL PROTECTED]
>
>
> I don't remember if I answered this before, but...
>
> I don't see the relevance of there being no constructor to match on. That
> is the case for any tuple type. It seems that
>
> newtype T1 [a1 a2 ...] = C1 ...
> is the same as
> data T2 [a1 a2 ...] = C2 !... !... !...
>
> Except that pattern matching on C1 is like lazy pattern matching on C2.
> Since newtype is supposed to be about efficiency, I am trying to
> understand what makes this more efficient. I have not yet seen any
> explanation of this.
Those two constructs are not the same
Compare
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 _|_.
So, pattern matching on (T2 x) may fail it the value is _|_. You are
right that semantically newtype is the same as always matching lazily on
the constructor.
The case for a tuple type (a,b,c) for instance, is different. Here there
is a constructor, you can even type it by itself, it is (,,), try ":t
(,,)" in hugs or ghci.
Chapter 4.2.3 in the Haskell 98 report clarifies this.
Regards,
Martin
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