>From what I have read till now I got the impression that conde in
minikanren is somehow an OR condition and a list of expressions is somehow
an AND condition. But that seems to be wrong. Maybe anyone can point out
where I did a mistake?
I tried to implement the following boolean condition as a minikanren
program.
(define (host-bool h a l)
(and
;; application
(or (equal? a "browser")
(equal? a "server")
(equal? a "proxy"))
;; location
(or (equal? l "home")
(equal? l "work")
(equal? l "isp"))
;; host
(or (and (equal? h "notebook") (equal? a "browser") (equal? l "home"))
(and (equal? h "tower") (equal? a "server") (equal? l "work"))
(and (equal? h "cluster") (equal? a "proxy") (equal? l "isp")))))
The function checks if A is one of the three applications, and if L is one
of the three locations and checks if H matches the host per application and
location.
The function succeeds for valid input and fails for invalid input.
(host-bool "cluster" "proxy" "isp") ;; => #t
(host-bool "cluster" "proxy" "###") ;; => #f
My aim was to have a consistency check for application and location. If the
host definition is not consistent with the application and location
requirements the function fails. This would be an inconsistent definition,
because (and (equal? l "isp") (equal? l "###")) fails.
(define (buggy-host-bool h a l)
(and
;; application
(or (equal? a "browser")
(equal? a "server")
(equal? a "proxy"))
;; location
(or (equal? l "home")
(equal? l "work")
(equal? l "isp"))
;; host
(or (and (equal? h "notebook") (equal? a "browser") (equal? l "home"))
(and (equal? h "tower") (equal? a "server") (equal? l "work"))
(and (equal? h "cluster") (equal? a "proxy") (equal? l "###")))))
The function fails for location "isp" and "###":
(buggy-valid-host "cluster" "proxy" "isp") ;; => #f
(buggy-valid-host "cluster" "proxy" "###") ;; => #f
Now I tried to do the same in minikanren with lists and conde's:
(define (host h a l)
;; application
(conde
((== a "browser"))
((== a "server"))
((== a "proxy")))
;; location
(conde
((== l "home"))
((== l "work"))
((== l "isp")))
;; host
(conde
((== h "notebook") (== a "browser") (== l "home"))
((== h "tower") (== a "server") (== l "work"))
((== h "cluster") (== a "proxy") (== l "isp"))))
At first glance it works as expected. The program enumerates all valid
combinations:
(run* (h a l) (host h a l))
;; => (("notebook" "browser" "home") ("tower" "server" "work") ("cluster"
"proxy" "isp"))
But when I have an inconsistent definition the results differ.
(define (buggy-host h a l)
;; application
(conde
((== a "browser"))
((== a "server"))
((== a "proxy")))
;; location
(conde
((== l "home"))
((== l "work"))
((== l "isp")))
;; host
(conde
((== h "notebook") (== a "browser") (== l "home"))
((== h "tower") (== a "server") (== l "work"))
((== h "cluster") (== a "proxy") (== l "###"))))
Here the "isp" and "###" do not match. But instead of failing the
inconsistent values are returned:
(run* (h a l) (buggy-host h a l))
;; => (("notebook" "browser" "home") ("tower" "server" "work") ("cluster"
"proxy" "###"))
Why is it so? Where is the OR condition between (== l "isp") and (== l
"###")? Why isn't there an AND?
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