On 07/06/2012 09:11 AM, Sam Tobin-Hochstadt wrote:
On Fri, Jul 6, 2012 at 11:59 AM, Neil Toronto <neil.toro...@gmail.com> wrote:
Anticipating a bug fix, I've started converting my recent TR code so that it
doesn't define predicates for mutable container types. Instead of using
`define-predicate', I need to *copy* the mutable data structure, using
occurrence typing on each immutable containee. It's kind of a pain, but I
only have one type that needs this treatment.
Can you not do the checking *after* extracting the elements from the
container? That may well be faster.
I can't maintain my invariants that way, because the same user code that
sent the container might change the contained values.
(define-converter (make-vectorof T) (Vectorof T))
#;(: make-vectorof (All (A B) ((A -> Boolean : B)
(Vectorof A) -> (U False (Vectorof B)))))
;; Receives a predicate that indicates `B' instances and a vector
;; Returns #f when every `A' in the vector isn't also a `B'
[...]
I'm confused about what this does. When does it copy, and when does
it produce `#false`?
Here's a case where `make-vectorof' copies:
(define ds (vector 0 1 2 3))
(make-vectorof index? ds)
Here's a case where `make-vectorof' returns #f:
;; (expt 2 60) is not an Index on any platform:
(define ds (vector (expt 2 60)))
(make-vectorof index? ds)
Of course, this wouldn't leverage the contract system nicely like
define-predicate does.
And in general, implicitly copying mutable data seems like a bad idea
-- mutable data is for sharing.
I don't read `(make-vectorof index? ds)' as implicit.
It's generally true that mutable data is for sharing. But sometimes I
don't intend to share mutable data; for example, when using a mutable
data structure is a lot faster than using its functional counterpart. In
that case, there's no way to follow basic user-interface guidelines for
typing functions; i.e. to have general argument types and precise return
types.
It's kind of a moot point now, anyway. I've decided to receive (Listof
Integer) and return (Listof Index), and convert them to and from
vectors. Lists will subtype nicely for my library users.
Neil ⊥
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