4

I'm working on defining a language which has casts and subtyping as follows:

(define-language base 
  (t ::= int any) 
  (e ::= number (cast t e)) 
  #| stuff ... |#)

I then define the following judgment form over it:

(define-judgment-form base
   #:mode (<: I I)
   #:contract (<: t t)
   [------ 
    (<: t t)]
   [------ 
    (<: int any)])

And now, in my reduction relation, I want to write something like 

(define b-> (reduction-relation base
    (--> (cast t v)
         v
         (where-judgment-holds (<: (typeof v) t)))))

Where we assume that (typeof v) returns the type of the value v. As far as I can tell, there isn't anything like where-judgment-holds in the Redex library, and while I can work around it with an unquote, it would be nice if there was something built into Redex.

Leif Andersen
  • 21,580
  • 20
  • 67
  • 100
ckfinite
  • 427
  • 2
  • 6

1 Answers1

3

What you are looking for actually is called judgment-holds, and it works in your example exactly where you put where-judgment-holds:

(define b-> (reduction-relation base
    (--> (cast t v)
         v
         (judgment-holds (<: (typeof v) t)))))
Leif Andersen
  • 21,580
  • 20
  • 67
  • 100