[racket-dev] Catching the undefined value

On Sat, Apr 19, 2014 at 4:45 PM, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
>
> Morally TR ought to report the type of this x as empty set,
> which would inform you that the else branch is unreachable:
>
>   (letrec ([x : Integer (if #t 0 x)]) x)

I don't think that's right -- the else branch is unreachable, but
that's not a fact about the type of `x` in particular, any more than
it's a fact about the type of `+` if it was used in the else branch.

> One day we may wish to augment syntax check in TR so that
> programmers can read of the reconstructed types for interior
> expressions.

Definitely.

Sam

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> On Apr 19, 2014, at 3:26 PM, Robby Findler wrote:
>
>> Ah. There are other approaches to this problem that result in
>> compile-time errors for such programs. Maybe the TR guys will adopt
>> one or figure out a better one! But for regular ole Racket, we're
>> stuck with a runtime error, I'm afraid.
>>
>> Robby
>>
>> On Sat, Apr 19, 2014 at 2:04 PM, Gustavo Massaccesi <gustavo at oma.org.ar> wrote:
>>>>> (letrec ([x (if #t 8 x)]) x) ;==>8
>>>
>>> It was a mistake. I thought that the “x: undefined; ...” error was an
>>> expansion-time error, not a run-time error.
>>>
>>> (I expected an error, because the x in the else part is “undefined”,
>>> even if it’s never accessed.)
>>>
>>> Gustavo
>>>
>>>
>>> On Sat, Apr 19, 2014 at 1:02 PM, Robby Findler
>>> <robby at eecs.northwestern.edu> wrote:
>>>> These seem correct to me. What were you expecting (and why?).
>>>>
>>>> Robby
>>>>
>>>>
>>>> On Saturday, April 19, 2014, Gustavo Massaccesi <gustavo at oma.org.ar> wrote:
>>>>>
>>>>> I found another problem with the optimizer and the new undefined behavior.
>>>>>
>>>>> (letrec ([x (if #t 8 x)]) x) ;==>8
>>>>>
>>>>> I also consider this correct in a strange sense :).
>>>>>
>>>>> Gustavo
>>>>>
>>>>>
>>>>> Welcome to Racket v6.0.1.4.
>>>>>> (letrec ([x x]) x)
>>>>> x: undefined;
>>>>> cannot use before initialization
>>>>>  context...:
>>>>>   C:\Program Files\Racket-6.0.1.4\collects\racket\private\misc.rkt:87:7
>>>>>> (letrec ([x 5]) x)
>>>>> 5
>>>>>> (letrec ([x (if #t 8 x)]) x)
>>>>> 8
>>>>>> (letrec ([x (if #f 8 x)]) x)
>>>>> x: undefined;
>>>>> cannot use before initialization
>>>>>  context...:
>>>>>   C:\Program Files\Racket-6.0.1.4\collects\racket\private\misc.rkt:87:7
>>>>>>
>>>>>
>>>>>
>>>>> On Wed, Apr 16, 2014 at 10:09 AM, Matthias Felleisen
>>>>> <matthias at ccs.neu.edu> wrote:
>>>>>>
>>>>>> Ah, too bad:
>>>>>>
>>>>>>> pkgs/racket-pkgs/racket-test/tests/racket/optimize.rktl
>>>>>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>>> --- OLD/pkgs/racket-pkgs/racket-test/tests/racket/optimize.rktl
>>>>>>> +++ NEW/pkgs/racket-pkgs/racket-test/tests/racket/optimize.rktl
>>>>>>> @@ -3416,5 +3416,16 @@
>>>>>>>          (read (open-input-bytes (get-output-bytes o))))))
>>>>>>>
>>>>>>> ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>>>>>>> +;; Check that an unsufe opertion's argument is
>>>>>>> +;; not "optimized" away if it's a use of
>>>>>>> +;; a variable before definition:
>>>>>>> +
>>>>>>> +(err/rt-test (let ()
>>>>>>> +               (unsafe-fx+ x 1)
>>>>>>> +               (define x 3)
>>>>>>> +               x)
>>>>>>> +             exn:fail:contract:variable?)
>>>>>>> +
>>>>>>> +;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>>>>>>
>>>>>>
>>>>>> :-)
>>>>>>
>>>>>> On Apr 16, 2014, at 9:02 AM, Matthias Felleisen <matthias at ccs.neu.edu>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>> On Apr 15, 2014, at 9:29 PM, Asumu Takikawa <asumu at ccs.neu.edu> wrote:
>>>>>>>
>>>>>>>> On 2014-04-15 18:13:31 -0400, claire alvis wrote:
>>>>>>>>> The push below includes changes to letrec expressions, internal
>>>>>>>>> definitions, units, classes, and certain ill-formed shared
>>>>>>>>> expressions so
>>>>>>>>> that they no longer leak the `undefined' value.
>>>>>>>>
>>>>>>>> This is great! (especially happy that TR, even with classes, doesn't
>>>>>>>> have to worry about #<undefined> anymore)
>>>>>>>>
>>>>>>>> BTW, I found this weird behavior:
>>>>>>>>
>>>>>>>> Welcome to Racket v6.0.1.3.
>>>>>>>> -> (require racket/unsafe/ops)
>>>>>>>> -> (let () (+ x 3) (define x 3) 5)
>>>>>>>> ; x: variable used before its definition [,bt for context]
>>>>>>>> -> (let () (unsafe-fx+ x 3) (define x 3) 5)
>>>>>>>> 5
>>>>>>>
>>>>>>>
>>>>>>> I consider this correct in a strange sense.
>>>>>>>
>>>>>>> Interestingly enough,
>>>>>>>
>>>>>>>> (let () (displayln  (unsafe-fx+ x 3)) (define x 3) 5)
>>>>>>> x: variable used before its definition
>>>>>>> context...:
>>>>>>>  /Users/matthias/plt/racket/collects/racket/private/misc.rkt:87:7
>>>>>>>
>>>>>>> which is good too. I don't know how Claire and Matthew did this,
>>>>>>> but it's good :-)
>>>>>>> _________________________
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>>>>>>
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>
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