You are right. But then I don't see your problem. By the way, your code should
probably be:
struct S
is
const i : int32
const next : nullable S
boxed struct Container
is
mutable s : nullable S; // <--- Otherwise you can't represent a zero-length
list.
def S_length(s : nullable S) // <--- Or else a type error, right?
in
case s in
null => 0:int32
otherwise => 1 + S_length(s.next);
def f()
in
let container = Container(S(5, null)) // container-->[*]-->[5, null]
chain = S(4, container.s) // chain-->[4, *]----^
len = S_length(chain) // returns 2 :
in
container.s := S(6,S(7,null)) // container-->[*]-->[6, *]-->[7,
null]
// chain-->[4, *]-->[5, null]
Notice, that the last assignment does not change the value of S(5, null). As
you mentioned, they are copy-compatible.
PKE
________________________________________
From: [email protected] [[email protected]] On Behalf Of
Jonathan S. Shapiro [[email protected]]
Sent: Wednesday, March 23, 2011 10:26 PM
To: Discussions about the BitC language
Subject: Re: [bitc-dev] Mutability, again
On Wed, Mar 23, 2011 at 4:22 PM, Pal Engstad
<[email protected]<mailto:[email protected]>> wrote:
I'm not clear on what exactly const and mutable means here...
Sorry. That seems to be an inherent hazard when discussion mutability.
..., but shouldn't that be two type errors?
I don't think so. I think it's one type error resulting from a typo. See below.
struct S
is
const i : int32
const next nullable S
boxed struct Container
is
mutable s : S;
def S_length(s : S)
in
case tmp = s in
null => 0:int32
otherwise => 1 + S_length(s.next);
def f()
in
let container = Container(S(5, null))
chain = S(4, container.s)
~~~~~~~
Type Error: container.s is mutable, but argument 2 in
the constructor is const.
Nope. The const-ness of the passed argument doesn't matter, because this is a
copy boundary. The actual parameter here ans the formal parameter are copy
compatible.
len = S_length(chain)
in
container.s.next = S(6,S(7,null))
~~~~~~~~~~~~~~~~~~~~
Type Error: Trying to mutate a const struct member.
This is the typo. The line should read "container.s" rather than
"container.s.next". My apologies!
shap
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