Neko sources syntax is easy to read, though can sometimes be difficult to generate. Also, it does not permit embedding file and line number information. For example, if you generate from a file in your language MyFile.mylang to myfile.neko you would like to get error traces in terms of position in the original MyFile.mylang file.

For these reasons, an extension of the Neko syntax is allowed which is called NXML. This is not a different format in the sense that you easily mix NXML and Neko sources together. You can put some NXML expressions in Neko sources and some Neko sources into an NXML document. NXML is based on XML and is representing a Neko Abstract Syntax Tree (AST).

NXML Nodes

NXML is not a different Neko syntax, but a syntax extension. This means that you can put some NXML expressions inside of a Neko program and some Neko program inside of NXML as well.

In order to use the NXML syntax, you need to start with <nxml> and finish with </nxml>. All NXML nodes inside are Neko expressions. An NXML block is like a Neko block. For example, <nxml></nxml> is the equivalent of the empty Neko block { }.

Other nodes are the following :

  • <i v="3"/> the literal integer 3
  • <f v="1.5"/> the literal float 1.5
  • <s v="a string"/> the literal string a string
  • <v v="id"/> the identifier id (includes special identifiers such as null, true, false and this)
  • <b>e1 e2 e3...</b> a block having several subexpressions
  • <p>e</p> parentheses around a subexpression
  • <g v="field">e</g> field access of a subexpression (e).field
  • <c>e0 e1 e2 e3...</c> call of e0(e1,e2,e3...)
  • <a>e1 e2</a> array access e1[e2]
  • <var><v v="x">e</v><v v="y"/></var> local variable declaration, equivalent to var x = e, y
  • <while>e1 e2</while> while loop : while e1 e2
  • <do>e1 e2</do> do...while loop : do e1 while e2
  • <if>e0 e1</if> equivalent to if e0 e1
  • <if>e0 e1 e2</if> equivalent to if e0 e1 else e2
  • <o v="*">e1 e2</o> a binary operation such as e1 * e2
  • <try v="exc">e1 e2</try> a try..catch block try e1 catch exc e2
  • <function v="x:y:z">e</function> a function declaration such as function(x,y,z) e
  • <return/> the return statement without an expression
  • <return>e</return> return with an expression value
  • <break/> the break statement without an expression
  • <break>e</break> break with an expression value
  • <continue/> the continue statement
  • <next>e1 e2</next> a way to tie two expressions together (such as e1;e2)
  • <label v="here"/> the goto label here:
  • <switch>e0 <case>e1 e2</case> <case>e1 e2</case> <default>edef</default></switch> a switch with several cases and an optional default
  • <object><v v="f0"><i v="42"/></v><v v="f1"><s v="foo"/></v></object> an object literal, equivalent to the neko code { f0 => 42, f1 => "foo" }
  • <neko>....</neko> neko source can be embedded into a %%<!CDATA[[...]]%%> section

For example, if we want to represent the fibonacci function in NXML :

fib = function(n) {
    if( n <= 1 ) 1 else fib(n-1)+fib(n-2)
<o v="=">
    <v v="fib"/>
    <function v="n">
             <o v="<="><v v="n"/><i v="1"/></o>
             <i v="1"/>
             <o v="+">
                 <c><v v="fib"/><o v="-"><v v="n"/><i v="1"/></o></c>
                 <c><v v="fib"/><o v="-"><v v="n"/><i v="2"/></o></c>

File Position

The additional attribute p can be placed on every NXML node in order to specify the original file and line the expression is generated from. For example <i v="33" p="myfile.l:478"/> is the integer 33 referenced in myfile.l at line 478.

When encountered, such position is stored and remains valid for all NXML nodes. For example, <nxml><i v="33" p="myfile.l:478"/><i v="34"/></nxml> is listing two integers from myfile.l, both at line 478.

If you don't specify the filename in the p attribute, it's considered to be a number of lines skipped since the last p information. For example, <nxml><i v="33" p="myfile.l:478"/><i v="34" p="2"/></nxml> is listing two integers from myfile.l, the first 33 at line 478 and the second 34 at line 480 (478 + 2).

NXML to Neko

There is a NXML-to-Neko generator which is available using the nekoc compiler. Simply run nekoc myfile.neko containing Neko/NXML syntax and it will create a myfile2.neko that will only contain Neko source code.

There is not a Neko-to-NXML generator right now, although it should be possible to write one very easily.

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