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
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