Robert Bradshaw wrote:
> On Jun 9, 2009, at 9:47 AM, Jason Bandlow wrote:
>
>>> On Jun 9, 2009, at 8:29 AM, Jason Bandlow wrote:
>>>
>>>> Hi,
>>>>
>>>> I ran across the following behavior in sage-3.4.1 and sage-4.0 (I
>>>> don't
>>>> have 4.0.1 yet), and I find it fairly disturbing.
>>>>
>>>> sage: d = {'a': 1} # Create some object
>>>> sage: d
>>>> {'a': 1}
>>>> sage: type(d) = type({}) # Attempt to check the type but
>>>> # foolishly use = instead of ==
>>>>
>>>> TypeError: cannot coerce type '<type 'type'>' into a
>>>> SymbolicExpression. # An error is expected
>>>>
>>>> sage: d # But now my data is gone!
>>>> d
>>>> sage: type(d) # Replaced by a symbolic variable
>>>> <class 'sage.calculus.calculus.SymbolicVariable'>
>> Robert Bradshaw wrote:
>>> This isn't a coercion issue, it's because of how functions are
>>> defined in Sage using the "f(x) = expr" notation using the preparser.
>>> I think that's too useful to get rid of. We could special case a
>>> warning for "type(x) = ..." but I'm not sure if that's the best idea.
>> Interesting! This means the problem is worse than I thought.
>> Anytime we
>> have data 'd' and 'e' and a function 'f', doing
>>
>> sage: f(d) = e # Instead of f(d) == e
>>
>> will raise a TypeError and replace 'd' with a symbolic variable.
>> Fortunately, though,
>>
>> sage: if f(d) = e: # The usual reason to check equality
>>
>> raises a SyntaxError first, so maybe this problem isn't so serious.
>
> Yes, I think it's fairly rare to want to do this. Probably should be
> added to an FAQ somewhere at least, as it's a surprising and subtle
> side effect.
First of all, thanks very much for the explanations, Robert. Is there a
particular FAQ where I could add a brief description of this?
-Jason
>
>> Still, it seems like it should be possible for the preparser to check
>> whether f(d) evaluates to something (in which case, the assignment is
>> going to fail anyway) before it makes the argument(s) symbolic
>> variables.
>
> You don't want to dissallow
>
> sage: f(x) = x^2
> [do some stuff with f]
> sage: f(x) = x^3
> [do some stuff with the new f]
>
> - Robert
>
>
>
> >
>
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