I assumed it was something to do with mutability, but I don't understand what you mean when you say there is a two-way channel. The reference in typed racket ( https://docs.racket-lang.org/ts-reference/type-ref.html#%28form._%28%28lib._typed-racket%2Fbase-env%2Fbase-types..rkt%29._.Hash.Table%29%29) says this:
``` (HashTable <https://docs.racket-lang.org/ts-reference/type-ref.html#%28form._%28%28lib._typed-racket%2Fbase-env%2Fbase-types..rkt%29._.Hash.Table%29%29> k v) is the type of a mutable or immutable hash table <https://docs.racket-lang.org/reference/hashtables.html#%28tech._hash._table%29> with key type k and value type v. Example: > (make-hash <https://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._make-hash%29%29> '((a . 1) (b . 2))) - : (HashTable Symbol Integer) [more precisely: (Mutable-HashTable Symbol Integer)] '#hash((a . 1) (b . 2)) ``` That suggests to me that HashTable includes both Mutatable-HashTable and Immutable-HashTable. The example given even states that the HashTable Symbol Integer is more precisely of Mutable-HashTable Symbol Integer type - does that *not* mean that (Mutable-HashTable Symbol Integer) is a subtype of (HashTable Symbol Integer) in the reference example? I now tried to redefine DB as Mutable-HashTable to avoid this issue, but that doesn't work either. It still doesn't accept `(make-hash (list (cons 'study-type study-type)))` as something of type DB. How is (Mutable-HashTable Symbol Symbol) not a subtype of DB, which is (Mutable-HashTable DBKey (U DB DBValue)), and DBKey is (U Symbol ... ;other stuff) and same for DBValue? You wrote that make-hash is of type `(Listof (Pairof Symbol Symbol))`. Is there a way to expand and print the type of something in terms of primitive types (well, or maybe step through one layer of abstraction) so that I could play around with some toy examples to get a sense of what types are returned? I am clearly confused by what type `make-hash` returns and what `hash-ref` expects. Cheers, Marc On Tue, Nov 5, 2019 at 3:22 PM Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote: > The problem is that the `DB` type is _not_ a super-type of > `(Mutable-HashTable Symbol Symbol)`, because mutable data structures > are two way communications channels. If you used an immutable hash, > that's a one-way communication and you would have the expected result. > > However, the change you made fixed the problem because it changes the > type that `make-hash` picks to create the hash table. The argument to > `make-hash` has the type `(Listof (Pairof Symbol Symbol))` which _is_ > a sub-type of `(Listof (Pairof DBKey (U DB DBValue)))`. > > Sam > > > On Tue, Nov 5, 2019 at 6:39 AM Marc Kaufmann <marc.kaufman...@gmail.com> > wrote: > > > > Hi, > > > > in order to put some discipline on my code (and formalize to myself what > I'm passing around), I started using typed racket. I have definitely hit my > share of gotchas that make me scratch my head, but I kind of start to > understand how things work and adding `(ann this-is ThatType)` annotations. > > > > However, I have the following piece of code: > > > > ``` > > (: register-study-session! (-> Number Symbol Void)) > > (define (register-study-session! sid study-type) > > (unless (hash-has-key? (db-table 'studies) sid) > > (hash-set! (db-table 'studies) > > sid > > (make-hash (list (cons 'study-type study-type)))))) > > ``` > > > > then I get the following error: > > > > ``` > > exploration.rkt:139:4: Type Checker: Polymorphic function `hash-set!' > could not be applied to arguments: > > Argument 1: > > Expected: (HashTable a b) > > Given: DB > > Argument 2: > > Expected: a > > Given: Number > > Argument 3: > > Expected: b > > Given: (Mutable-HashTable Symbol Symbol) > > ``` > > > > The DB type is as follows: > > > > ``` > > (define-type DBKey (U Symbol Number)) > > (define-type DBValue (U Symbol Number Boolean Char)) > > (define-type DB (HashTable DBKey (U DB DBValue))) > > ``` > > > > However when I annotate this with an (ann ...) around the make-hash, it > works: > > > > ``` > > (: register-study-session! (-> Number Symbol Void)) > > (define (register-study-session! sid study-type) > > (unless (hash-has-key? (db-table 'studies) sid) > > (hash-set! (db-table 'studies) > > sid > > (ann (make-hash (list (cons 'study-type study-type))) > DB)))) ;; ONLY THIS LINE CHANGED > > ``` > > > > I don't understand why this leads to it passing, since it said that it > thought the DB earlier was a mutable hash of type Symbol to Symbol, which I > would have thought is of type DB (which is any hashtable). So why does the > type checker complain at first - or maybe why does it pass later? > > > > Cheers, > > Marc > > > > -- > > 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/6b08ac58-5c85-45ad-9fb7-dd367254e015%40googlegroups.com > . > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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