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on Sep 15, 2016
In the three previous posts in this series, we learned how to use Ohm to parse numbers, build an expression tree, and process blocks of code with conditionals. In this final post on the Ohm series, we…

In the three previous posts in this series, we learned how to use Ohm to parse numbers, build an expression tree, and process blocks of code with conditionals. In this final post on the Ohm series, we will finish up our complete programming language, Meow, with looping and real function calls. Thanks to the power of Ohm our final language will be implemented in only 180 lines of code.

Adding the While Loop

Now that we have conditionals and blocks, we could write a loop by hand with just an if statement and a counter variable, but most languages provide a proper while loop. Since we already implemented block last time, a while loop will be simple to build. It is the keyword while followed by a condition block that resolves to true or false (a boolean), followed by a body block which is executed until the condition block returns false. The while loop itself will resolve to the last value from the body.

This is a simple example of a while loop in action.

{
    x = 0
    while { x < 5 } {
        x = x + 1
    }
}

Let’s start by adding a While expression to the grammar in grammar.ohm.

    Expr =  WhileExpr | IfExpr | Block | Assign | MathOp | Group | Identifier | Number
    WhileExpr = "while" Block Block

Then add a new WhileLoop class for our expression tree in ast.js. This class executes the body block over and over until the condition resolves to false.

class WhileLoop {
    constructor(cond, body) {
        this.cond = cond;
        this.body = body;
    }
    resolve(scope) {
        var ret = new MNumber(null);
        while(true) {
            var condVal = this.cond.resolve(scope);
            if (condVal.jsEquals(false)) break;
            ret = this.body.resolve(scope);
        }
        return ret;
    }
}

Next add a new semantics action for the WhileExpr. This action just creates an instance of the WhileLoop class above whenever a while rule is found in the code.

        WhileExpr: (_, cond, body) => new AST.WhileLoop(cond.toAST(), body.toAST()),

And finally we need some unit tests.

//while loop
test('{ x=0  while { x < 5 } { x = x+1 } } ',5);
test('{ x=4  while { x < 5 } { x = x+1 } } ',5);
test('{ x=8  while { x < 5 } { x = x+1 } } ',null);

Notice that the last test resolves to null. This is because x is already greater than five when the loop starts, so the body is never executed at all. The while loop must resolve to the last value of the body, but since the body never executed either it returns null.

Line Based Comments

Now let’s add a few other missing parts of our language: comments and string literals.

You may remember from the first blog in this series that rules that begin with capital letters automatically handle whitespace. When you declare a rule like this:

Expr = WhileExpr | IfExpr

Ohm is really doing something like this:

Expr = space* WhileExpr space* | space* IfExpr space*

Ohm handles the whitespace for us using a built in rule called space. Normally space just handles common whitespace characters like space, tab, and newlines, (which is also why we don’t need the semicolon for the end of lines). However, we can override the built in space rule to make it also ignore comments. Just add a few lines to the grammar file like so:

    // override space to include slash slash comments
    space := "" | " " | "
" | comment comment = "//" (~"
" any)*

Now any text on a line after the double slash (//) will be ignored.

String Literals

So far we have only worked with numbers. Our original design was for a calculator, after all. But now that we want to turn Meow into a full language, we need string literals.
Let’s add string literals to the grammar with a new String rule. Strings must be between double quotes (“). Notice that the rule for String says it can be anything which isn’t qq (the double quote). That means it could even be a newline! Yes, multi-line string literals are valid in our language.

    Expr =  WhileExpr | IfExpr | Block | Assign | MathOp | Group | Identifier | Number | String
...
    Term = Group | Identifier | Number | String
...    
    qq = "\""
    String = qq (~qq any)+ qq

Now let’s add a new action to the semantics for String. This action creates a new instance of MNumber from the source string. Now that we are using the MNumber class for booleans and strings, we should probably rename it to something more general in the future, like MValue.

        String: (a, text, b) => new AST.MNumber(text.sourceString)

And of course a few more unit tests:

//string literals
test(' "foo" ',"foo");
test(' "foo" + "bar" ', "foobar");

Function Calls

Now let’s add the ability to call functions. Arguably function calls are the only thing required for a language to be a real programming language. We could have implemented only function calls and built everything on top of that (like LISP), but most real world programming languages use all of the other language features we built here. With those in place we can now build function calls.

A function call can take the place of any standard terminal expression (number, identifier, etc.) so let’s update Term to support FunCall. The FunCall itself is an identifier followed by its arguments in parenthesis.

    Term = Group | FunCall | Identifier | Number | String
     ...
    FunCall = Identifier "(" Arguments ")"
    Arguments = ListOf<Expr, ",">

Now let’s build an expression class object to represent function calls:

class FunctionCall {
    constructor(fun, args) {
        this.fun = fun;
        this.args = args;
    }
    resolve(scope) {
        //lookup the real function from the symbol
        var fun = scope.getSymbol(this.fun.name);
        //resolve the args
        var args = this.args.map((arg) => arg.resolve(scope));
        //execute the real javascript function
        return fun.apply(null,args);
    }
}

This code looks similar to what we’ve done for conditionals and loops. It is constructed from a symbol referring to the function and the arguments, that will be sent to the function. In the resolution phase the args are each resolved, then the function is applied with these args.

Finally we can add the new semantic action rules.

    FunCall: (funName,_1,args,_2) => new AST.FunctionCall(funName.toAST(), args.toAST()),
    Arguments: (a) => a.asIteration().toAST(),

Of course we don’t have any functions to call yet. We don’t have a syntax to define new functions yet, so let’s pre-fill the global scope with some hard coded functions written in plain Javascript.

var GLOBAL = new Scope(null);
GLOBAL.setSymbol(new MSymbol("print"),function(arg1){
    console.log("print:",arg1.val);
    return arg1;
});
GLOBAL.setSymbol(new MSymbol("max"), function(A,B) {
    if(A.val > B.val) return A;
    return B;
});

The print function will print it’s first argument and max will return the larger of its two arguments. Now we can write some test functions.

//native function calls
test("print(4)",4); //returns 4, prints 4
test("max(4,5)",5); // returns 5
test('print("foo") ', 'foo');
// compound tests
// function returns value to math expression
test('6*max(4,5)',30);
test('max(4,5)*6',30);
test('4*max(4,5)',20);
test('4*max(5,4)',20);
// function returns value to function
test('max(4,max(6,5))',6);
test('max(max(6,5),4)',6);

So far so good. Everything we’ve implemented has the same structure as previous language features. We extended the grammar, added an AST object (if necessary), added a semantic action, then put in more tests. User defined functions will be a little bit harder, however.

User Defined Functions

User defined functions are functions created in the Meow language itself rather than in native Javascript. In Meow, we will define a function with the fun keyword like this:

 
    fun plus1(z) { 
       1+z 
    } 
    
    plus1(9)

The body of a function is just a block. As we defined earlier, a block is a sequence of statements that returns the value of the last one. This implicit return means we don’t need to have a return keyword. The code above will return 1 plus the z argument. This is the grammar update for function definitions in grammar.ohm.

    Expr =  FunDef | WhileExpr | IfExpr | Block | Assign | MathOp | Group | Identifier | Number | String
    FunDef  = "fun" Identifier "(" Parameters ")" Block
    Parameters = ListOf<Identifier, ",">

Our two new grammar rules need new semantic actions of course:

        FunDef: (_1, name, _2, params, _3, block) => new AST.FunctionDef(name.toAST(), params.toAST(), block.toAST()),
        Parameters: (a) => a.asIteration().toAST(),

So far we have built the FunDef feature the same as previous features but here’s where it gets tricky: scope. A function has direct access to the variables passed to it as parameters, but it also has access to variables defined outside the function, unless one of the parameters has the same name. To build this properly we need to expand our definition of scope.
A Scope is a list of names which map to values using symbols. A scope also has a parent scope. If a symbol can’t be found in the scope then it will ask it’s parent for the value instead. If the parent can’t find the symbol then it will ask its parent, and so on up the chain until we get to the GLOBAL scope. To implement this we need to update the Scope class with new constructor, getSymbol, and makeSubScope functions.

class Scope {
    constructor(parent) {
        this.storage = {};
        this.parent = parent?parent:null;
    }
    setSymbol(sym, obj) {
        this.storage[sym.name] = obj;
        return this.storage[sym.name];
    }
    getSymbol(name) {
        if(this.storage[name]) return this.storage[name];
        if(this.parent) return this.parent.getSymbol(name);
        return null;
    }
    makeSubScope() {   return new Scope(this)  }
}

Now we can make the FunctionDef expression object:

class FunctionDef {
    constructor(sym, params, body) {
        this.sym = sym;
        this.params = params;
        this.body = body;
    }

When the function definition is resolved it will create a new symbol in its scope which points to the actual function block. This function block will create a sub-scope for the function call, and fill this scope with the values of the arguments. This is dynamic, so we need to do the argument resolution when the function is called not when it is declared. To do this we will use a nested function like this:

    resolve(scope) {
        var body = this.body;
        var params = this.params;
        return scope.setSymbol(new MSymbol(this.sym.name),function() {
            var scope2 = scope.makeSubScope();
            params.forEach((param,i) => scope2.setSymbol(new MSymbol(param.name),arguments[i]));
            return body.resolve(scope2);
        });
    }
}

Now we can finally test a user defined function like this:

x = 5
fun plus1(z) {
  1+z
}
plus1(x)

To really test it, add this to the test.js file:

test('{ x=5  fun plus1(z){ 1+z }   plus1(x)  }', 6);

Note that I had to put {} around the code. This is because we have three top level statements: the variable assignment to x, the function definition of plus1, and the function call to plus1. Everything in Meow is an expression so we can’t just have three statements hanging out there, they have to be inside of a block, so the extra brackets do that. In the future we could make the top level brackets implicit, but for now it’s better to be explicit.

Conclusion in 180 Lines

We now have a real language with conditionals, loops, and function calls. It has enough power to write and execute real code. Anything else we want our language to do should be possible just by creating more user defined functions. I hope through this blog series you have seen both how powerful Ohm is, and how easy it is to create your own language. Meow is currently 180 lines of code; and that’s without any crazy esoteric Javascript hacking. The implementation is very straight forward and understandable. The code for Meow, and this entire blog series, is available in the github repo.

From this base there are a lot of things you could add to Meow. Experiment with new syntax, numbers with units, or asynchronous calls. Embed it in a webpage or cross-generate code for an embedded processor. Almost anything you want to do is just a few parser rules away.

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