overloading for ladder logic

[Paul McGuire]

On Nov 7, 7:48 am, jim... at gmail.com wrote:
> I am trying to simulate the execution of some PLC ladder logic in
> python.
>
> I manually modified the rungs and executed this within python as a
> proof of concept, but I'd like to be able to skip the  modification
> step.  My thought was that this might be able to be completed via
> overloading, but I am not sure if (or how) it could be done.
>
> overloadings:
>     + ==> OR
>     * ==> AND
>     / ==> NOT
>
> Example original code:
>      A=/B+C*D
> translates to:
>     A=not B or C and D
>
> I tried
>     def __add__ (a,b):
>         return (a or b)
>
> which gives me this:
>
>     >>> x=False
>     >>> y=True
>     >>> x+y
>         1
>     >>> x=True
>     >>> x+y
>         2
>
> How can this be done?

This reminds me of a little project I wrote a long time ago to use
operator overloading to compute the overall resistance of a network of
resistors - I used - for series connections and | for parallel.  You
can see the code here: http://pastebin.com/m1e89aae9

If you are going to design a mini-DSL using overloading, you'll first
have to choose which operators correspond to your syntax, working
within those offered by Python.  For NOT, you have only have two unary
operators to choose from, ~ (__invert__) or - (__neg__).  / is not
supported as a unary operator, only as binary division (__div__ or
__truediv__).  For AND and OR, you have a wealth of binary operators
from which to pick.  But first, think about any precedence of
operations.  As I recall from my ladder diagramming days, logic was
strictly left-to-right, with no precedence for one operation over the
other (as opposed to common arithmetic operator precedence, in which
in 4+2*3 evaluates as 4+(2*3), as opposed to (4+2)*3, which would be
strict left-to-right evaluation).  When you implement your DSL, you
will still be subject to Python's definitions for operator
precedence.  So if you want strict left-to-right evaluation, then
choose two operators at the same level of precedence, such as + and -,
or * and /.  On the other hand, if you want AND evaluated before OR
(which is typical precedence in programming), then pick operators from
two different levels of precedence.

-- Paul