Question

I’ve seen quite a few expression parsers / tokenizers that can take a certain string and evaluate the result. For example, you could pump the string:

4+4

into the following code:

MSScriptControl.ScriptControl sc = new MSScriptControl.ScriptControl();
//' You always need to initialize a language engine
sc.Language = "VBScript";
//' this is the expression - in a real program it will probably be
//' read from a textbox control 
string  expr  = "4+4";
double res  = sc.Eval(expr);

and get 8. But, is there a parsing tool out there that can evaluate the string:

4 = 4 = 4

? So far, all examples fail with an error of not being able to compare a double and boolean (which makes sense from a compilers perspective, but from a human perspective, we can see that this is true). Anyone come across something that can achieve this?

Answer 1

From a human perspective, this is only true if we think of x = y = z as a special operator (with three operands), where it implies x = y, y = z, x = z. That is a specific syntactical interpretation of the expression. A human (particularly a programmer) could also interpret it the same way most compilers do, which is to choose the left-most grouping ( x = y ) and then compare the result of that comparison (a boolean value) to z. Even to a human, this doesn’t make sense under this syntax. It only seems obvious from a human perspective because humans are notoriously fuzzy when it comes to choosing a syntax that ‘makes sense’ for a given context.

If you really want that level of ‘fuzziness’, you’ll need to look into something like Wolfram Alpha, which performs contextual analysis to try to find a best guess for the meaning of the expression. If you enter ‘4 = 4 = 4’ there, it will reply True.