Simple, Embeddable, Lisp-Like Language Parser in C++
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There have been several times that I needed to have a very simple, but flexible, embedded language for projects I was working on, and didn't really have what I wanted. Embedding something as complex as PHP is possible, but it's an enormous overhead for most projects, and adds in a ton of complexity that you have to factor into the cost of the project. After all, who is going to maintain this code base going forward? Do we really need the complexity of PHP?

What I arrived at was a very simple lisp-like language that has a very limited scope of data types for the variables, and consequently, is a pretty clean and efficient implementation. This is the point of LKit - to make this simple language easy to include in applications so that I (and others) don't have to re-write this over again when I need it.

Library Dependencies

The focus of this library is not to be a general threading library. Nor is it to attempt to write code that's already written. As such, we're going to be using boost for several components. This will include things only used in the test applications. Since boost is available on virtually all platforms, and even included in the next C++ standard, this should not be a significant burden to the user.

There are even instructions to build it on Mac OS X, if you don't wish to use the Homebrew version that's freely available.

The Value

LKit's base variable is lkit::value and is a typical variant-style value that can hold, at present, the following data types:

  • bool - the classic, and correct, C/C++ boolean value. When this is cast into any other values, a true is 1, and a false is 0.
  • int - the simple integer, typically a 32-bit integer, but the size is not the greatest factor, it's the lack of a decimal portion that is key. It makes math and equality so much easier to test.
  • double - the typical value. This is primarily for data manipulation, and in that, the double is by far the most often used data type.
  • uint64_t or timestamp - this is for the fact that a lot of the data is timeseries, and a lot of the manipulations are based on dealing with time, so we need to have something that can represent time. This is currently based as microseconds since epoch, in case you needed to understand the base and offset.

The lkit::value can be assigned values, used in standard mathematical expressions in C++, has all the equality and inequality operators defined for it - the works. This appears to C++ as just another data type, which is the point: make it fit into the standard C++ lexicon, and that will make dealing with the more advanced classes a lot easier.

The Function

An lkit::function is defined to be an operation, or series of operations, on zero or more arguments, in the form of value instances, that produces a value result. The basic API for a function is simple: implement a few standard constructors and destructor, and then one key method:

 *                       Evaluation Methods
 * This is the main evaluation point for the function. It takes a
 * vector of values and returns a value - simple. How it does this
 * is entirely up to the developer of that function.
virtual value eval( std::vector<value *> & anArg ) = 0;

While it's nice to implement the other utility methods like toString() and the equality and inequality operators, it's not strictly necessary.

The function can be stateful, if necessary, but then it's the responsibility of the author to make sure that the state is preserved properly across all calls to the eval() method. Most functions will be stateless because they won't need to be stateful, and it makes their implementation so much easier.

The Expression

The lkit::expression is a subclass of lkit::value, and as such, can appear as an argument to an lkit::function. The lkit::expression is basically an lkit::function and a vector of lkit::value instances as arguments. Since these are all pointers, polymorphism will work for us, and we can use value or expression instances interchangeably.

A simple example of using value, function, and expression is the summing of a few numbers:

lkit::value	a, b, c, d;
a = 10.1;
b = 5.5;
c = 3.14;
d = 6.2;
lkit::func::sum		f;
lkit::expression	y(&f, &a, &b, &c, &d);
std::cout << "The sum is: " << y.evalAsDouble() << std::endl;

Order of Arguments and Type Coercion

It's important to note that you can make use of the type conercion in the arguments to a function, and ultimately, an expression. The default for lkit::value operations is to retain the type of the target, and operate in the argument. The following test will explain:

lkit::value	a, b, c, d;
// note that 'a' will be an (int)
a = 10;
// ...and the rest are (double)
b = 5.5;
c = 3.14;
d = 6.2;
lkit::func::sum		f;
lkit::expression	y(&f, &a, &b, &c, &d);
std::cout << "The sum is: " << y.evalAsDouble() << std::endl;

This will return:

The sum is: 24

because the first argument is an integer and therefore, the other arguments will be adding their integer component to the summing integer. This can be very useful when you want to sum up the number of boolean values - each true will be 1, each false a 0 - so long as the first argument is an integer.

Contrast this to the following:

lkit::value	a, b, c, d;
// note that 'a' will be an (int)
a = 10;
// ...and the rest are (double)
b = 5.5;
c = 3.14;
d = 6.2;
lkit::func::sum		f;
lkit::expression	y(&f, &b, &a, &c, &d);
std::cout << "The sum is: " << y.evalAsDouble() << std::endl;

This will return:

The sum is: 24.84

Note that the first argument is b as opposed to a, and so the sum is a double value.

The Language Syntax

While it's possible to use these classes in any kind of language, there exists in LKit, the lkit::parser class to convert source code into a Langueage Tree built up of these class components. The description of the language is pretty simple, as it's meant to be a simple, transparent, mapping of these components into a human-readable grammar.

The Basic Expression

The basic expression is a function followed by a white-space, and then zero or more arguments to the function, each separated by a white-space, all surrounded by parentheses. Here is an example where the user wants to arithmetically sum three integers:

(+ 5 3 2)

The above example is equivalent to the infix-notation:

5 + 3 + 2

and all evaluate to:


For functions (i.e. operators) where order doesn't matter, the evaluated value is independent of the order of the arguments. However, for functions like subtraction and division, the order matters quite a bit, and the way the evaluation is done is based on the idea of placing the operator between each argument and then calculating left to right.

So the following expression:

(- 5 3 2)

is equivalent to the infix notation of:

5 - 3 - 2

which evaluates to:


The evaluation is similar for division:

(/ 10.0 2.0 5.0)

is equivalent to the infix notation:

10.0 / 2.0 / 5.0

and evaluates to:


Multiple Expressions

The language allows for multiple expressions to be in a single source. For example, the following is a simple way to define the variable x and then use it in the subsequent expression:

(set x (+ 1 2 3))
(* x 3 (* x 2))

The parser will make both language trees, and evaluate them in the order in which they appeared in the code. The resulting value of the evaluation will be the last expression evaluated.

Expressions as Values in Expressions

Of course this can all be included as a value (a.k.a. argument) in another expression:

(+ (/ 10.0 2.5) (* (+ 1.5 2 6) 2.0))

in infix notation:

(10.0 / 2.5) + (1.5 + 2 + 6) * 2.0

which evaluates to:


Equalities, Inequalities, and Logical Functions

In addition to the obvious arithmetic functions, there are the complete suite of equality and inequality functions where for == all elements have to be equal to one another, and for != the first argument is the 'test' condition, and all remaining elements have to be not equal to this 'test' value. Again, it's the same as distributing the operator in between the arguments and processing left to right.

The following table has several examples of the functions and their respective results:

Expression Evaluates to
(== 1 1.0 (* 2.0 0.5)) true
(!= 1 1.0 (* 2.0 0.5) (/ 2.0 1.0)) false
(< 1 3 5 6 10) true
(> 10 9 8 5 5 2) false
(or 1 0 0 1) true
(and 1 0 0 1) false
(not 1) false

Variable Definition and Assignment

Variables can be referenced in the code anywhere a constant can be used. The variables are typically the output of an expression, but they can be simple constants as well. There are a few default variables defined in the system at this time:

e = 2.71828183
pi = 3.14159265

but setting a variable is very simple. The code looks like this:

(set x 14.5)

And the result is the value of the variable, in this case, the double 14.5. This can work in many ways, for instance, the following defines the variable x to be 6, and then uses that in another calculation in the code:

(* (set x (+ 1 2 3)) 3 (* x 2))

Since the processing of the set function is done at compile time, and the lookup of the value of x in the last multiplication is done at evaluation time, you don't have to worry about variables not being defined in time.

Additionally, as long as the lkit::parser is active, the variables defined in one source code can be used in another. For instance, the following first defines the variable x and then uses it in the next code segment:

lkit::parser	p;
p.setSource("(+ (set x 5) 3 (* x 2))");
p.setSource("(* x 7 14)");
value	v = p.eval();

i.e. the parser retains all variable and function definitions across invocations until reset() or clear() is called.

Additionally, the parser allows for variables to have their values set as many times as needed. Meaning they will not be immutable variables. But they will be set at compile time, so the user is going to have to be careful about the order that the variable assignments are being made.


Function Definition

I also need to add in the function definition to the parser. A simple function that adds two numbers might look like this:

(defun add [a, b] (+ a b))

I'm not sure about the syntax for the variables, but we'll see.

One thing for sure - the user-defined functions will have to be in the code before they are referenced in the code itself. I'm not going to mess with a two-pass compilation process at this point.


Past the basics, I need to start looking at speed and efficient updating of the expressions and therefore the resulting parser tree. I don't want to recalculate things that haven't changed, and constants don't change, so it's going to make a partial update scheme possible.

Time-Series Data

Beyond that, I need to think about the syntax for time-series data. Specifically, how do I want to short-hand the processing of time-series data: filters, sliding windows, etc. There needs to be something other than a simple looping construct. There's a lot to be gained in the efficiency of the processing that way.

Quick List of Ideas

  • Put in detection of change in the lkit::expression values. This will then allow us to skip the calculation of an expression if the arguments haven't changed since the last calculation. This is a big speed boost.
  • Put in "delta" driven updates for the expressions. Allow that those functions not supporting delta-updates will still work, but they won't work as efficiently.
  • If we need to have simple arithmetic processing, pull in muParser from CKit and retrofit the lkit::value for the CKVariant in the code. This gives us a completely new way to work on the expressions without any additional complication in the code.


Copyright 2012 by Robert E. Beaty

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