# Expression introspection?

**URL:** <https://www.ruby-forum.com/t/expression-introspection/141293>\
**Category:** Ruby\
**Created:** [June 24, 2008, 12:23pm UTC](https://www.ruby-forum.com/t/expression-introspection/141293 "2008-06-24T12:23:57Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [June 24, 2008, 12:23pm UTC](https://www.ruby-forum.com/t/expression-introspection/141293/1 "2008-06-24T12:23:57Z")

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I wonder if anyone has any suggestions for handling a design question  
I have. I think I want introspection of Ruby Expressions, but my ruby  
experience is limited and there may be an alternative approach that  
I’ve missed…

I am using Ruby expressions to gate state transitions in a kind of  
state machine. To prototype this I can use something like the  
following Evaluator class:

class Evaluator  
#add new data to evaluator  
def add(var, obj)  
# TODO?: I assume var is a string - enforce this?  
instance\_variable\_set “@#{var}”, obj  
# dynamically add reader method, if not already present  
if !methods.include?(var)  
instance\_eval(“def #{var}; @#{var};end”)  
end  
end

# test expression

def test(expression)  
#TODO: handle NameError: undefined local variable or method errors  
instance\_eval(expression)  
end  
end # Evaluator

so that I can load data into my evaluator at runtime (with the add  
method) and then test expressions based on that data with the test  
call. An example irb session might be:

irb(main):002:0\> e = Evaluator.new  
=\> #Evaluator:0xb7c840d0  
irb(main):003:0\> e.add(“age”, 20)  
=\> nil  
irb(main):004:0\> e.test(“age\>16”)  
=\> true  
irb(main):005:0\> e.test(“age\>70”)  
=\> false

In my program, the state machine will be defined declaratively and  
loaded at runtime so an extended Evaluator object will have access to  
a list of pre-defined expressions. As I will already know what the  
expressions are I would like to be able to assist my users by warning  
them when they present data that is incompatible with the current  
expression list, so that for instance, if I knew that “age\>16” was one  
of my expressions then when someone added a value for “age” I could  
test whether the presented object responded to the “\>” call. To do  
this I suppose I need to inspect the expressions and understand the  
methods each variable requires (and thus whether the presented data  
supports those methods) but I cannot see an easy way to do this. Can  
I do this without building my own Ruby expression parser?