Yes, I’ll add this. It slightly increases the pain density, especially since I think students likely to mistakenly write
(list (? symbol? #{s : Symbol}) …) instead of the correct (list (? symbol? #{s : (Listof Symbol)}) …) … but it’s probably better than having to cast. Thanks! John > On Feb 15, 2020, at 10:33, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote: > > You can annotate variables like that in match patterns using #{s : Symbol}, > which ought to work in common cases. > > On Sat, Feb 15, 2020, 1:23 PM 'John Clements' via users-redirect > <us...@plt-scheme.org> wrote: > Yes, absolutely. One reason that students in my class wind up using cast > quite frequently in their parsers is that they use patterns like (list (? > symbol s) …) which (as I recall) expand into unannotated lambda’s, and always > require a cast. I write that up here: > > https://www.brinckerhoff.org/clements/2202-csc430/Assignments/tr-notes.html#%28part._.Predicate_.Patterns_with_.Dot-dot-dot%29 > > Beyond that, though, I tell them never to use cast: > > https://www.brinckerhoff.org/clements/2202-csc430/Assignments/tr-notes.html#%28part._.Other_.Casting%29 > > The text I came up with for mutable hash tables is here: > > https://www.brinckerhoff.org/clements/2202-csc430/Assignments/tr-notes.html#%28part._.Mutable_.Hash_.Table_.Construction%29 > > Many thanks to all of you! > > John > > > On Feb 14, 2020, at 12:32, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote: > > > > The advice I would really give is to give a name and a type annotation to > > any mutable data structure you create. > > > > Also, cast is almost never what you want. I see lots of people (maybe > > including this student) using it as a substitute for type annotation but it > > really isn't. > > > > Sam > > > > On Fri, Feb 14, 2020, 2:41 PM 'John Clements' via users-redirect > > <us...@plt-scheme.org> wrote: > > I think I may understand what’s going on here, but a student and I worked > > on this for quite a while today before I found the problem. > > > > Here’s a program: > > > > #lang typed/racket > > > > (define-type Store (Mutable-HashTable Integer Value)) > > (define-type Value (U Real Boolean String)) > > > > (define top-store (cast (make-hash (list > > (cons -1 14) > > (cons 1 #t) > > (cons 2 #f))) > > Store)) > > > > (hash-set! top-store 5 1234) > > > > It fails with this error: > > > > contract violation > > expected: (or/c (and/c byte? positive?) #t #f) > > given: 1234 > > in: the values of > > the 3rd conjunct of > > (and/c > > hash? > > hash-mutable? > > (hash/c > > exact-integer? > > (or/c (and/c byte? positive?) #t #f) > > #:immutable > > #f)) > > contract from: typed-world > > blaming: cast > > (assuming the contract is correct) > > at: unsaved-editor:6.18 > > > > If I understand what’s going on here, the basic issue is that the mutable > > hash table’s type is being inferred as (Immutable-HashTable Exact-Integer > > (U Boolean Positive-Byte)), and then as part of the cast, a contract is > > being inserted, which checks that all added values match the expected value > > type. The outer cast allows type checking to proceed, but then at runtime > > it fails because the given value doesn’t match the inferred value type. > > > > This error doesn’t occur with immutable hash tables, because it’s fine to > > extend an immutable hash table to a larger one that contains it; the > > original one’s contract isn’t violated. > > > > In this case, one easy error is to change the ‘cast’ into an ‘ann’, which > > works fine. > > > > This is the first time I’ve encouraged my students to use a mutable hash, > > which is presumably why I haven’t encountered this before. > > > > I’m trying to formulate a solution that I can put in a Hints for TR file, > > and I think the answer is probably this: > > > > - When you use “make-hash” in TR, you should always specify the types > > explicitly using an “inst”. > > or maybe > > - When you call “make-hash” in TR, the call should be immediately wrapped > > with an “ann” type annotation. > > > > In a perfect world, I think I would ask for a warning when casting a > > mutable hash table. It could go in that nice “warnings” box. Oh, wait… > > > > … joking aside, actually I just did turn on the “log” window and I don’t > > see any warning about this, which is not too surprising. Oh, wait, I see > > another bug… > > > > Thanks for reading, let me know if there’s any more obvious solution. > > > > John > > > > > > > > > > > > > > -- > > You received this message because you are subscribed to the Google Groups > > "Racket Users" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to racket-users+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/racket-users/f24851ca-32aa-40e1-9db4-691da815ebe7%40mtasv.net. > > > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/f1cdb734-cb79-4a1a-8bb8-3bf3d4a425b4%40mtasv.net. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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