On 10/06/2012 11:30 AM, Patrick Walton wrote:
I like this. The only concern, as a comment pointed out, is that "*"
might be slightly confusing; maybe "ref" is better.
I like it too. Though I wonder if the ambiguity between
&-as-a-reference-taker and &-as-a-pattern is actually problematic.
Consider two cases (assuming we use & here):
#1:
let foo = {1,2};
let {&a, &b} = foo;
#2:
let x = 1;
let y = 2;
let foo = {&x, &y};
let {&a, &b} = foo;
It seems to me that in both cases you're introducing two variables, a
and b, of type &int. In #1 they point into foo, using the & to "take
references" to the record components; in #2 they point to x and y
respectively, using the & to "match against" the existing &-types inside
the record.
I think to a user these are "intuitively" compatible interpretations,
and supporting both depending on the type being matched against would
not lead to any deep surprises; one could always force the "take a
reference" form for #2 by writing it with a double-ampersand (something
the user might even guess to do):
let x = 1;
let y = 2;
let foo = {&x, &y};
let {&&a, &&b} = foo;
Here a and b are references-to-references, type &&int. Which is,
conveniently, just how &&int reads anyway. They could also force the
_opposite_ interpretation (that is, "deref the & values") using a type
annotation (at least in let-patterns):
let x = 1;
let y = 2;
let foo = {&x, &y};
let {a, b} : {int,int} = foo;
The only practical (as in "maybe hard to implement") difference is that
the &-symbol _would_ have a syntactically ambiguous denotation in
patterns, that would be resolved one way or another during typechecking.
But patterns often have type-indeterminate parts they need to pick up
from the other variables at play in inference, and it seems to me a
plausible special-case in the unification rule on &-patterns, and this
might work ok.
Maybe I'm being naive though. I don't really know that code. Comments
from anyone who does?
-Graydon
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