id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	difficulty
1446	Implement contravariant type checking for procedure types	megane		"Consider list types `A = (list foo)` and `B = (list bar)`. In functions where list of type `A` is expected you can give list of type `B` if `bar` is more specific (or  of same type) than `foo`. For example `foo` could be `(or fixnum string)` and `bar` be `fixnum`.

Now consider procedure types `F = (t1 -> *)` and `H = (t2 -> *)`. In functions where a function parameter of type `F` is expected you can give function of type `H` if `t2` is more '''general''' than `t1`. For example, if `t2` is `(or fixnum string)` and `t1` is `string`.

Here is an example where the scrutinizer can't tell anything is wrong:
{{{
(define-type dog (vector string))
 (define-type cat (vector fixnum string))
 (define-type animal (or cat dog))
 
 (: make-dog (-> dog))
 (define (make-dog)
   (vector ""woof""))
 (define (dog? a)
   (= 1 (vector-length a)))
 
 (: make-cat (-> cat))
 (define (make-cat)
   (vector 0 ""meow""))
 (define (cat? a)
   (= 2 (vector-length a)))
 
 (: animal-print (animal -> *))
 (define (animal-print a)
   (cond
    [(dog? a) (dog-print a)]
    [(cat? a) (cat-print a)]))
 
 (: dog-print (dog -> *))
 (define (dog-print d)
   (print ""dog says "" (vector-ref d 0)))
 
 (: cat-print (cat -> *))
 (define (cat-print c)
   (print ""cat says "" (vector-ref c 1)))
 
 (: apply-printer (forall (T) ((T -> *)  T -> *)))
 (define (apply-printer pr e)
   (pr e))
 
 (: print-animals ((animal -> *) (list-of animal) -> *))
 (define (print-animals pr animals)
   (for-each pr animals))
 
 ;; (apply-printer cat-print (make-dog)) ; warns ok
 
 ;; (print-animals animal-print (list (make-dog) (make-cat))) ; works ok
 
 ;; doesn't warn, but fails at runtime
 (print-animals cat-print (list (make-cat) (make-dog)))
 
 ;; $ csc -O3 vector-contravariant.scm  && ./vector-contravariant
 ;; cat says meow
 ;; 
 ;; Error: (vector-ref) out of range
 ;; #(""woof"")
 ;; 1
 ;; 
 ;;  Call history:
 ;; 
 ;;  vector-contravariant.scm:44: print-animals    
 ;;  vector-contravariant.scm:36: g30      
 ;;  vector-contravariant.scm:29: print    
 ;;  vector-contravariant.scm:36: g30   
}}}"	enhancement	new	not urgent at all	someday	scrutinizer	5.0.0		contravariant types