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An AST data structure and pretty printer for "JavaScript: The Good Parts" in Haskell
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JavaScript: The Good Parts -- an AST and Pretty Printer


In Chapter 2 of "JavaScript: The Good Parts", Douglas Crockford presents a concrete grammar for "the good parts" of JavaScript.

This package provides an abstract grammar and pretty printer for those good parts. We will abbreviate this language to JS:TGP.

Crockford presents the grammar as a series of railroad diagrams. The correspondence between the concrete grammar and the abstract grammar in this module is NOT one-to-one. However, the following property does hold: the pretty printing of an abstract syntax tree will be parseable by the concrete grammar. i.e. For each valid program produced by the concrete grammar there is a corresponding abstract syntax tree that when pretty printed will produce that program (modulo whitespace).

The abstract grammar:

  • removes unnecessary characters such as parentheses (normal, curly and square)
  • represents JavaScript's string, name and number literals directly in Haskell as 'String', 'String' and 'Double' respectively.

The data structure ensures no incorrect JS:TGP programs

This library was designed so that it would be impossible, save for name, string literals to construct an incorrect JS:TGP program.

To this end some of the data structures may look like they contain redundancy. For instance, consider the 'JSESDelete' constructor which is defined

JSESDelete JSExpression JSInvocation

Why not just define it as JSESDelete JSExpression since type JSExpression has a constructor defined as JSExpressionInvocation JSExpression JSInvocation? The reason is that this would allow incorrect programs. A JSExpression is not necessarily an invocation.

A note on precedence of JavaScript operators

Interestingly, the precedence of JavaScript operators is not defined in the ECMAScript standard. The precedence used in this library comes from the Mozilla Developer's Network pages. (

I have not used the precise precedence numbers from that page since in this module a lower precedence means the operator binds more tightly (as opposed to the page where a higher precedence does the same). Also, we have need for less precedence values so they have been normalised to what we are using in JS:TGP

You will also note that we don't even consider the associativity/precedence of "=", "+=", "-=" etc. In JS:TGP the notion of expression statements is quite different to that of expressions. It simply isn't legal to write an expression statement like

(a += 2) -= 3


 a = (b = c) = (c = d)

although it is perfectly legal to write

 a = b = c = d += 2

which if we add brackets to disambiguate is really

a = (b = (c = (d += 2)))

Interesting aspects of "the good parts"

A JS:TGP program is a collection of statements. You'll note that there is no statement to declare a function in JS:TGP. However you can assign a function literal to a variable.

e.g. var fun = function(x) { return x + 1;}

What about recursive functions then? There is the option to give the function a name which is local to the literal.


var factorial = function f(n) {
                       if ( n > 0 ) {
                         return n * f(n - 1);
                       } else {
                         return 1;

f is local. It will not be in scope outside of the function body.

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