[IronPython] Type analysis of expression

Vernon Cole vernondcole at gmail.com
Mon Oct 19 07:38:03 PDT 2009


Stephen said what I was trying to say.  It does not matter what the user
sends you, you must try to coerce his result into something the database
will accept. This is most easily done dynamically at run time.
Also forgive my typo of  "impert" for "import" ... I reread my answer three
times and still missed it.
--
Vernon Cole

On Mon, Oct 19, 2009 at 8:29 AM, Lepisto, Stephen P <
stephen.p.lepisto at intel.com> wrote:

> Jonathan,
>
> Even if the variables going into the expression are strongly typed, python
> will evaluate the expression however it can, with the result being some type
> based on the coercion python applied to each variable.  However, I read
> Christian's problem as one where he needed to get the value into a database
> and to me that was the deciding factor as to which type to coerce the result
> of the expression, regardless of the original types of the variables going
> into the expression.
>
>
> -----Original Message-----
> From: users-bounces at lists.ironpython.com [mailto:
> users-bounces at lists.ironpython.com] On Behalf Of Jonathan Hartley
> Sent: Monday, October 19, 2009 7:19 AM
> To: Discussion of IronPython
> Subject: Re: [IronPython] Type analysis of expression
>
> Hey all,
> Stephen, I infer that Christian can't evaluate the expression yet. (If
> he could, then deriving the type of the result would be trivial)
> Presumably the variables in the expression don't yet have known values.
>
> So he has an expression which cannot be evaluated, but he wants to guess
> what type the return value would be if the variables in the expression
> were known.
>
> The variables in the expression do have known types. So I wonder if you
> could pick a prototypical value for each variable, (eg. set all floats
> to 1.0, and all integers to 1) and then evaluate the expression to see
> what type the result is?
>
> Obviously this won't work for all expressions (eg. "x / (x - 1)"), but
> maybe it would work enough of the time, depending on what form your
> expressions take.
>
> Am I misunderstanding entirely, or only partially?
>
>    Jonathan
>
> Lepisto, Stephen P wrote:
> > What about treating the return type of the python expression as object
> then coerce the object into the type required by the database?  If the
> coercion fails, then that could be treated as a type error.
> >
> > For example, after using python to evaluate the expression '2+(4*5)', the
> returned object could be converted to an integer using
> System.Convert.ToInt32() or to a string with System.Convert.ToString().
>  System.Convert throws InvalidCastException for those cases that cannot be
> converted.
> >
> >
> > -----Original Message-----
> > From: users-bounces at lists.ironpython.com [mailto:
> users-bounces at lists.ironpython.com] On Behalf Of Michael Foord
> > Sent: Monday, October 19, 2009 4:21 AM
> > To: Discussion of IronPython
> > Subject: Re: [IronPython] Type analysis of expression
> >
> > Christian Schmidt wrote:
> >
> >> Hello,
> >>
> >> we are using IronPython embedded into our application to evaluate user
> >> defined expression. The "variables" used in the expressions are strongly
> >> typed and the results need to be written back into a database.
> >>
> >> I know that the return type of a dynamic expression cannot be determined
> >> in general. But what would be a pragmatic way to guess the return type
> >> of an expression?
> >>
> >
> > Hehe. If you're evaluating arbitrary functions provided by the user then
> > I don't know of any way of 'inferring' the return type - beyond parsing
> > it and modelling the type flow through the expression (which would be a
> > lot of work).
> >
> > All the best,
> >
> > Michael
> >
> >> Thanks,
> >> Christian
> >>
> >>
> >>
> >> _______________________________________________
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> >> Users at lists.ironpython.com
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> >>
> >
> >
> >
>
> --
> Jonathan Hartley      Made of meat.      http://tartley.com
> tartley at tartley.com   +44 7737 062 225   twitter/skype: tartley
>
>
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