Summary

Scrutinizer seems to get confused about list length

Metadata

Description

Seen when compiling awful-salmonella-tar:

 (/usr/home/chicken/src/awful-salmonella-tar/ast-cache-manager.scm:86) in procedure call to `scheme#car', expected argument #1 of type `pair' but was given an argument of type `null'

Here's a stripped-down example that triggers it:

 (module ast-cache-manager ()
 
 (import scheme (chicken base) (chicken process-context))
 
 (define (usage #!optional exit-code)
   (when exit-code
     (exit exit-code)))
 
 (let ((args (command-line-arguments)))
 
   (when (or (member "-h" args)
             (member "-help" args)
             (member "--help" args))
     (usage 0))
 
   (when (< (length args) 3)
     (usage 1))
 
   (let ((cache-dir (car args))
         (awful-pid (string->number (cadr args)))
         (max-items (and (not (null? (cddr args)))
                         (string->number (caddr args)))))
     (assert awful-pid)))
 
 ) ;; end module

It seems that when you remove the `when`, the warning goes away.

Changes and comments

[2018-09-10 11:39:53 UTC] megane wrote:

There's at least two things going on here.

First is type smashing. The calls to `usage`, which has unknown type smashes the type of `args` to `(or pair null)`. This can be avoided by moving the `usage` calls to tail call positions.

In theory, defining `usage` like this should help:

 (define usage
   (the (#!optional fixnum -> noreturn)
         (lambda (#!optional exit-code)
           (when exit-code
             (exit exit-code)))))

But doesn't, for some reason.

Secondly, there's an issue with refinement:

 (import scheme (chicken base) (chicken type))
 
 (let ([a (the (or pair null) (cons 1 '()))])
   (length a) ; refine (or pair null) with list (= (or pair null))
   (compiler-typecase a ((not *) 1)))
 ;; 
 ;; Error: at toplevel:
 ;;   (hoi.scm:33) no clause applies in `compiler-typecase' for expression of type `null':
 ;;     (not *)

The type should stay `(or pair null)`, but is refined to `null`.

[2018-10-08 21:31:54 UTC] sjamaan changed status from new to closed

[2018-10-08 21:31:54 UTC] sjamaan set resolution to fixed

[2018-10-08 21:31:54 UTC] sjamaan wrote:

Fixed with b7e293696efe4f1bfdfdcd8239c0b76de0dd615f