Records in Haskell
ml at isaac.cedarswampstudios.org
Wed Jan 11 08:42:48 CET 2012
On 01/10/2012 05:06 AM, Greg Weber wrote:
> Some of your comments seem to not fully recognize the name-spacing (plus
> simple type resolution) aspect of this proposal that I probably didn't
> explain well enough. Or maybe I don't understand your comments.
> For record.field, field is under the record's namespace. A namespace (from
> a module or under the new system a record), cannot export conflicting
> names, and there this system prevents the importer from every having a
> conflict with a record field name because the field is still under the
> record's namespace when imported. The type system must resolve the type of
> the record, and generally the field cannot contribute to this effort.
(I have only used Haskell for several years, not implemented Haskell
several times; apologies for my amateurish understanding of the type
Type inference proceeds assuming that "record.field" is something
equivalent to "undefined record" (using "undefined" as a function type),
and the program is only correct if the type of "record" resolves to a
concrete type? I don't know if "concrete type" is at all the right
terminology; I mean a type-variable doesn't count (whether
class-constrained, "Num a => a", or not, "a", or even "m Int" is not
concrete). Is "forall a. Maybe a" okay (if Maybe were a record)?
"forall a. Num a => Maybe a"? I'm guessing "yes".
Does it get harder in the presence of the type-system extensions?
"(a ~ SomeRecord) => a": presumably that works with record syntax? Can
the compiler always figure out whether or not it can find a
type-variable's concrete type?
My concept of Haskell is that (except for Template Haskell), scope
resolution happens before type inference happens, no exceptions. So
you're deliberately breaking that pattern. Right?
Does this order of stages (regular scope selection, then type inference,
then record scope) make as high a fraction of code work as Frege's
left-to-right model (which I am guessing interleaves type inference and
record scope selection as it proceeds left-to-right through the program)?
Correct me if I got something wrong,
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