How to parse and translate DSL using Red or Rebol

Question

I'm trying to see if I can use Red (or Rebol) to implement a simple DSL. I want to compile my DSL to source code for another language, perhaps Red or C# or both - rather than directly interpreting and executing it.

The DSL has only a couple of simple statements, plus an if/else statement. Statements can be grouped into rules. A rule would get translated into a function definition, with each statement the equivalent statement in the target language.

The parse capability in Red/Rebol is great and lets me implement a parser very easily - in effect it's basically just the definition of the grammar itself.

However I haven't been able to find any examples of how to take the next steps, specifically handling an if statement and translating it to other source code. Translating an if statement seems a good example of something minimal but still slightly tricky - because in Red having an else means you need to change the if to an either, rather than just an extra optional else.

Traditionally, during parsing I would build an abstract syntax tree, and then have functions to operate on the AST and generate the new source code. Should I be following this same approach or is there some other more idiomatic way in Red ?

I've experimented with using collect/keep in my parse rules to return a block of nested blocks, which in effect forms the AST. Another approach would be to save data into specific objects representing the different statements etc.

I'm still getting to grips with collect/keep, as to when a new block will be created and what will be kept. I'd also like to keep my parser rules as "clean" as possible, with as little other code intertwined in it. So I'm still not sure how best to add in Red code in round brackets in the parse rules. Adding code too early can cause the Red code to get executed, even if the rule eventually fails. Adding code too late means the code may not be executed in the order you expect, especially when dealing with multi-level statements like if, which can contain other statements.

So, specifically, any help on how to translate my example DSL to Red source code would be appreciated. Also any links to implementing DSLs like this in Red or Rebol would be great ! :)

Here are my parse rules :-

Red [
    Purpose: example rules for parsing a simple language
]

SimpleLanguageParser: make object! [
    Expr: [string! | integer! | block!]
    Data: ['Person.AGE | 'Person.INCOME]
    WriteMessageToLog: ['write 'message 'to 'log Expr]
    SetData: ['set 'data  Data '= Expr]
    IfStatement: ['if Expr [any Statement] opt ['else [any Statement]] 'endif]
    Statement: [WriteMessageToLog | SetData | IfStatement]

    Rule: [
        'rule word!
        [any Statement]
        'endrule
    ]

    AnySimpLeLanguage: [Rule | [any Statement]]
]

SL: function [slInput] [
    parse slInput SimpleLanguageParser/AnySimpleLanguage
]

An example of some source in the DSL :-

RULE TooYoung
IF [Person.Age < 15]
   WRITE MESSAGE TO LOG "too young to earn an income"
   SET DATA Person.Income = 0
ELSE 
   WRITE MESSAGE TO LOG "old enough"
ENDIF
ENDRULE 

Translated to Red source code :-

TooYoung: function [] [
    either Person.Age < 15 [
        WriteMessageToLog "too young to earn an income"
        Person.Income: 0
    ] [
        WriteMessageToLog "old enough"
    ]
]

The data, ie Person.Age, Person.Income, and the function WriteMessageToLog are all things which would have been previously defined. Note, for simplicity I've left Expr as block! etc, rather than defining Expr in any more detail in the DSL itself. Also, setting Person.Income in the function doesn't work as coded as it sets a local - but that's ok for now :)

Answer 1

Always nice to see someone digging language-oriented programming, keep it up and welcome to Red! ;)

---

Specifying correct grammar rules is the trickiest part of the job, and you've already nailed that. What's left is to intersperse your PEG (parsing expression grammar) with set, copy, collect/keep combo and paren! expressions in the right places, and then either create an AST from that or, in simplier cases, emit code directly.

Example

Here's a quickly baked (and definitely buggy!) example of how I'd tackled your task. Basically, it's slightly reworked code of yours, where matched patterns are setted, copyed or collected, and then bounded to specific words, which then just pasted into "template" (compose function inside emit-rule) to produce a Red code.

It's not the only way, I believe. @rebolek might come up with more industrial-strength solution, as he has experience with sophisticated parsers, which I'm lacking :P

Followup

As for if/else dilemma, I followed the approach proposed above -- instead of using opt I wrapped rule for else-branch into block and added an alternative match, which just sets false-block to none.

What to use for AST -- anything that allow to express a hierarchical structure, which is either a block! (though for performance gain you might want to use hash! or map!) or an object!. The advantage of object! is that it provides a context to be bound to, but here we're approaching a realm of so-called Bindology ("scoping" rules of Red language), which is another beast :)

Emitting C# code would be harder, but doable -- you'll need to assemble a string instead of Red code. I think, however, that, as a newcomer, you should stick with parsing directly at block-level (the way you done in your example), because it a lot easier and much expressive.

Another interesting (but much hairy) approach would be to re-define all words used in your DSL-block to work as you want.

Resources

We have a wiki entry about Red/Rebol dialects on github, which you might find if not useful, but interesting to read.

Also, two articles (this and this) in Red blog, I think you skimmed over first one already (if not, you should!).

Last, but not least, an exhaustive review of Parse principles and keywords (which has a couple of wrong parts in it though, so, caveat emptor). It's written for Rebol, but you should adapt examples to Red rather easily.

As a relative newcomer to the language, I do agree that there's a lack of examples and tutorials about DSL development, but we're working on that (at least in our heads) :)