[Haskell-cafe] a really juvenile question .. hehehehe ;^)
arnarbi at gmail.com
Mon Oct 6 09:19:30 EDT 2008
On Mon, Oct 6, 2008 at 14:58, Cale Gibbard <cgibbard at gmail.com> wrote:
> 2008/10/6 Don Stewart <dons at galois.com>:
>>> data and newtype vary in one more subtle way, and that's how/when they
>>> evaluate to bottom. Most of the time they behave identically, but in the
>>> right cases they act sightly differently. newtype is usually regarded as
>>> more efficient than data. This is because the compiler can choose to
>>> optimize away the newtype so that it only exists at type check time. I
>>> think this is also possible with data in some, but not all, uses.
>> The compiler *must* optimise away the use. They're sort of 'virtual'
>> data, guaranteed to have no runtime cost.
> I'm not sure that I'd want to be that emphatic about what an
> implementation *must* do regarding something so operational.
> The informal semantics of pattern matching in the Report says:
> Matching the pattern con pat against a value, where con is a
> constructor defined by newtype, depends on the value:
> * If the value is of the form con v, then pat is matched against v.
> * If the value is _|_, then pat is matched against _|_.
> That is, constructors associated with newtype serve only to change the
> type of a value.
> This clearly has an implementation which introduces no overhead, which
> one can expect from good implementations of the language. There are
> obviously implementations of these semantics which do introduce
> overhead as well though, so I would be hesitant to make any
> requirement like that.
And this requirement is there why? Is it specifically put in so that
one is able to create this overhead-less implementation?
data A = A Int
newtype B = B Int
ta (A x) = True
tb (B x) = True
This happens (not surprisingly given your above comments):
*Main GOA> :load test.hs
[1 of 1] Compiling Main ( test.hs, interpreted )
Ok, modules loaded: Main.
*Main GOA> ta undefined
*** Exception: Prelude.undefined
*Main GOA> tb undefined
Why is the x evaluated in ta?
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