Chapter 11: Syntax macros

Where science turns into magic once more.

You've now learned how to create your own macros to make your own custom syntax, and the features involved.

I would advice you to take a look at the many macros in the Lux Standard Library for inspiration as to what can be accomplished.

In the meantime, let's find out how to take our macro chops to the next level.

The library/lux/control/parser/code module houses some powerful tools.

For starters, it's the home of the (code) Parser type:

(type .public Parser
  (//.Parser (List Code)))

Which is based on the generic Parser type from the library/lux/control/parser module:

(type .public (Parser s a)
  (-> s (Try [s a])))
**Note**: This is also a functorial/monadic type.

Parser (from library/lux/control/parser/code) is the type of code-parsers: parsers which analyze Code nodes to extract arbitrary information.

The Parser type works with streams of inputs instead of single elements, and it often consumes some of those inputs, which is why the output involves an updated list of Codes.

There are many such code-parsers (and combinators) in the library/lux/control/parser/code module, and you should definitely take a look at what's available in the documentation.

Then, in the library/lux/macro/syntax module, there is a mechanism for defining macros: the syntax macro.

... A more advanced way to define macros than 'macro'.
... The inputs to the macro can be parsed in complex ways through the use of syntax parsers.
... The macro body is also (implicitly) run in the Meta monad, to save some typing.
... Also, the compiler state can be accessed through the *lux* binding.
(def .public object
  (syntax (_ [.let [imports (class_imports *lux*)
                    class_vars (list)]
                    super (opt (super_class_decl^ imports class_vars))
                    interfaces (tuple (some (super_class_decl^ imports class_vars)))
                    constructor_args (constructor_args^ imports class_vars)
                    methods (some (overriden_method_def^ imports))])
    (let [def_code ($_ text#composite "anon-class:"
                       (spaced (list (super_class_decl$ (maybe.else object_super_class super))
                                     (with_brackets (spaced (list#each super_class_decl$ interfaces)))
                                     (with_brackets (spaced (list#each constructor_arg$ constructor_args)))
                                     (with_brackets (spaced (list#each (method_def$ id) methods))))))]
      (in (list (` ((, (code.text def_code)))))))))
This example is a macro for making anonymous _JVM_ classes that lives in `lux/ffi`.

The difference between macro and syntax is that syntax allows you to parse, in a structured manner, the inputs to your macro, thereby reducing considerably the complexity necessary for making big macros.

Also, because you're using code-parsers for the hard work, you can write reusable parsers that you can share throughout your macros, if you want to have common syntax.

You can even compose your parsers, or use parsers from someone else's library.

There are already some small modules under `library/lux/macro/syntax/` which house some reusable code-parsers and code-generators.

Additionally, syntax binds the Lux value on a variable called *lux*, so you can use it during your parsing.

What do those code-parsers look like?

Here is an example:

... Taken from library/lux/math/infix.

   [lux (.except)
     [monad (.only do)]]
     ["<>" parser (.use "[1]#[0]" functor)]]
     ["[0]" product]
      ["[0]" list (.use "[1]#[0]" mix)]]]
     ["[0]" code
      ["<[1]>" \\parser (.only Parser)]]
      [syntax (.only syntax)]
      ["[0]" code]]]
      ["n" nat]
      ["i" int]]]]])

(type Infix
  (Rec Infix
     {#Const Code}
     {#Call (List Code)}
     {#Unary Code Infix}
     {#Binary Infix Code Infix})))

(def literal
  (Parser Code)
  ($_ <>.either
      (<>#each code.bit <code>.bit)
      (<>#each code.nat <code>.nat)
      (<>#each <code>.int)
      (<>#each code.rev <code>.rev)
      (<>#each code.frac <code>.frac)
      (<>#each code.text <code>.text)
      (<>#each code.symbol <code>.symbol)))

(def expression
  (Parser Infix)
  (<| <>.rec (function (_ expression))
      ($_ <>.or
          (<code>.form (<>.many <code>.any))
          (<code>.tuple (<>.and <code>.any expression))
          (<code>.tuple (do <>.monad
                          [init_subject expression
                           init_op <code>.any
                           init_param expression
                           steps (<>.some (<>.and <code>.any expression))]
                          (in (list#mix (function (_ [op param] [_subject _op _param])
                                          [{#Binary _subject _op _param} op param])
                                        [init_subject init_op init_param]

And here are some examples of syntax macros:

... Also from library/lux/math/infix.

(def (prefix infix)
  (-> Infix Code)
  (when infix
    {#Const value}
    {#Call parts}
    (code.form parts)

    {#Unary op subject}
    (` ((, op) (, (prefix subject))))
    {#Binary left op right}
    (` ((, op) (, (prefix right)) (, (prefix left))))))

(def .public infix
  (syntax (_ [expr ..expression])
    (in (list (..prefix expr)))))
(def .public ^stream&
  (syntax (_ [patterns (<code>.form (<>.many <code>.any))
              body <code>.any
              branches (<>.some <code>.any)])
    (with_symbols [g!stream]
      (let [body+ (` (let [(,* (|> patterns
                                   (list#each (function (_ pattern)
                                                (list (` [(, pattern) (, g!stream)])
                                                      (` ((,! //.result) (, g!stream))))))
                       (, body)))]
        (in (list& g!stream body+ branches))))))
(def .public cond>
  (syntax (_ [_ _reversed_
              prev <code>.any
              else body^
              _ _reversed_
              branches (<>.some (<>.and body^ body^))])
    (with_symbols [g!temp]
      (in (list (` (let [(, g!temp) (, prev)]
                     (cond (,* (do list.monad
                                 [[test then] branches]
                                 (list (` (|> (, g!temp) (,* test)))
                                       (` (|> (, g!temp) (,* then))))))
                           (|> (, g!temp) (,* else))))))))))

This may be a short chapter, but not because its subject is small.

The opportunities that code-parsers open are fantastic, as it puts within your reach macros which would otherwise be much harder to implement correctly.

Don't worry about complex inputs: your macros can implement entire new embedded programming languages if you want them to.

Code-parsers can generate any data-type you want, so you can easily translate the information in the input syntax to whatever data-model you need.

But, now that we've spent 3 chapters about metaprogramming in Lux, I think it's fair that we clear our minds a little by looking at other subjects.

You're going to learn how to go beyond Lux and interact with everything and everyone.

See you in the next chapter!