guillaume savaton - au mouvant sillage

My first domain-specific language with Racket

Step 6: Syntax analysis

Guillaume Savaton

This is the final step of my Racket DSL implementation roadmap. It consists in promoting Tiny-HDL as a standalone language with two flavors: a Lisp-like syntax and a custom syntax.

Two language flavors for Tiny-HDL

Since the beginning of this series, we have considered that the syntax of Tiny-HDL was based on S-expressions. This was natural and necessary because Tiny-HDL was only available as a Racket library. In step 6, we want to make Tiny-HDL a standalone language with two possible syntaxes:

  • Vanilla, for parentheses lovers,
  • Spicy, loosely inspired by VHDL and Verilog.

The vanilla description of the half adder and the full adder will look like this:

#lang tiny-hdl/vanilla

(entity half-adder ([input a] [input b] [output s] [output co]))

(architecture half-adder-arch half-adder
  (assign s  (xor a b))
  (assign co (and a b)))
#lang tiny-hdl/vanilla

(use "half-adder-step-06-vanilla.rkt")

(entity full-adder ([input a] [input b] [input ci] [output s] [output co]))

(architecture full-adder-arch full-adder
  (instance h1 half-adder-arch)
  (instance h2 half-adder-arch)
  (assign (h1 a) a)
  (assign (h1 b) b)
  (assign (h2 a) (h1 s))
  (assign (h2 b) ci)
  (assign s      (h2 s))
  (assign co     (or (h1 co) (h2 co))))

Compared to the same example in step 5, the noticeable changes are:

  • The #lang racket directives have been replaced by #lang tiny-hdl/vanilla.
  • The (require tiny-hdl) forms have been removed.
  • The Tiny-HDL code is no longer wrapped in (begin-tiny-hdl ...) forms.

And here is the spicy version of the same example:

#lang tiny-hdl/spicy

entity half-adder
    input a
    input b
    output s
    output co

architecture half-adder-arch of half-adder
  s  <= a xor b
  co <= a and b
#lang tiny-hdl/spicy

use "half-adder-step-06-spicy.rkt"

entity full-adder
  input a
  input b
  input ci
  output s
  output co

architecture full-adder-arch of full-adder
  h1 : half-adder-arch
  h2 : half-adder-arch
  h1.a <= a
  h1.b <= b
  h2.a <= h1.s
  h2.b <= ci
  s    <= h2.s
  co   <= or

In both cases, the #lang directive will have the following effect:

  1. Racket will look up, and run, a reader for the chosen language flavor. A reader is basically a parser that converts source text into a syntax object.
  2. The syntax object will come wrapped in a #%module-begin form that serves as a hook for macro expansion. Racket provides a default implementation of the #%module-begin macro, but we can provide our own to customize the expansion.

Implementing the vanilla flavor

Since Racket comes with a built-in S-expression parser, there is very little work to do. First, we create a module vanilla/lang/reader.rkt with the following content.

#lang s-exp syntax/module-reader

This file specifies that Tiny-HDL will use the default Racket reader, and the language implementation will be provided by the module tiny-hdl/vanilla/lang/expander. You can read more about the syntax/module-reader language in the Racket guide.

In the module vanilla/lang/expander.rkt, we implement a custom #%module-begin macro that wraps its body in a begin-tiny-hdl form. This macro has been named tiny-hdl-module-begin internally because we need to keep a reference to the original #%module-begin from Racket:

#lang racket

(require tiny-hdl)

  (all-from-out tiny-hdl)
  (rename-out [tiny-hdl-module-begin #%module-begin])
  xor and or not)

(define-syntax-rule (tiny-hdl-module-begin body ...)
    (begin-tiny-hdl body ...)))

This module also provides the boolean operators from Racket as part of Tiny-HDL.

And that’s it! The vanilla flavor of Tiny-HDL is complete.

Implementing the spicy flavor

There are several techniques to write a custom reader with Racket. You can either write a parser by hand, use one of the available parsing libraries like Megaparsack, or use a parser generator like Brag.

To implement the spicy flavor of Tiny-HDL, I have chosen to use Brag.

The reader module

This time, the spicy/lang/reader.rkt module defines custom versions of the read and read-syntax functions.

#lang s-exp syntax/module-reader
#:read                tiny-hdl-read
#:read-syntax         tiny-hdl-read-syntax
#:whole-body-readers? #t

(require "lexer.rkt" "grammar.rkt")

(define (tiny-hdl-read in)
  (syntax->datum (tiny-hdl-read-syntax #f in)))

(define (tiny-hdl-read-syntax src ip)
  (list (parse src (tokenize ip))))

As you can see, tiny-hdl-read-syntax uses two functions:

  • tokenize, imported from lexer.rkt, converts a Tiny-HDL source file into a token stream.
  • parse, imported from grammar.rkt, converts a token stream into a syntax object.

The lexical analyser

The lexical analyzer (lexer, or tokenizer) is implemented in spicy/lang/lexer.rkt. It uses the brag/support module that provides facilities for creating lexers:

#lang racket

(require brag/support)

(provide tokenize)

(define (tokenize ip)
  (port-count-lines! ip)
  (define tiny-hdl-lexer
      [(:or "use" "entity" "input" "output"
            "architecture" "of" "end"
            "and" "or" "not" "xor"
            "<=" ":" "." "(" ")")
       (token lexeme (string->symbol lexeme))]
      [(:or "false" "true")
       (token 'BOOLEAN (eq? lexeme "true"))]
      [(:seq alphabetic (:* (:or alphabetic numeric "_" "-")))
       (token 'ID (string->symbol lexeme))]
      [(from/to "\"" "\"")
       (token 'STRING (trim-ends "\"" lexeme "\""))]
       (token 'WHITESPACE lexeme #:skip? #t)]
  (λ () (tiny-hdl-lexer ip)))

tokenize returns a function that reads an input port ip and returns a new token each time it is called. Tokens are generated by matching the character sequence from the input port against regular expression patterns:

  • A keyword, operator or punctuation mark (use, entity, ...) is converted into a symbol token.
  • A boolean literal (false, true) is converted into a BOOLEAN token with a Racket boolean value (#f, #t).
  • An identifier is converted into an ID token with a symbol value.
  • A string literal is converted into a STRING token whose value is the string content.
  • Whitespace is matched against a built-in whitespace pattern and converted into a WHITESPACE token that will be ignored.
  • The end-of-file condition returns (void) as a termination indicator for the parser.

The parser

The parser is specified as a grammar in spicy/lang/grammar.rkt using the Brag language. Interestingly, Brag itself is implemented as a Racket DSL with a custom syntax.

The starting rule of the grammar is begin-tiny-hdl. It defines a Tiny-HDL module as a sequence of use clauses, entities and architectures:

#lang brag

begin-tiny-hdl: (use | entity | architecture)*

When this rule matches, the parser generates a syntax object with a root form begin-tiny-hdl whose body is a sequence of syntax objects for use, entity, and architecture forms.

A use clause is composed of the keyword use followed by a string literal. The / character below means that the keyword use will be discarded from the output to prevent a double use symbol in the result:

use: /"use" STRING

Similarly, we define the entity rule that generates an entity form. All keywords are discarded from the result. We define a separate rule port-list to force the port declarations to be kept in a list inside the entity form. However, we don’t want the names port-list and port to be part of the result:

entity: /"entity" ID port-list /"end"

/port-list: port*

/port: ("input" | "output") ID

The architecture rule behaves a little differently. In this case, we want the instance and assign forms to be direct children of the architecture form. For this reason, we do not define a separate statement-list rule, and the statement rule is marked as spliced using the @ indicator:

architecture: /"architecture" ID /"of" ID statement* /"end"

@statement: instance | assign

instance: ID /":" ID

assign: port-ref /"<=" expression

The following rules define the syntax of boolean expressions and the operator precedence:

@expression: or | or-term

or:       (or-term /"or")+ or-term
@or-term: xor | xor-term

xor:       xor-term /"xor" xor-term
@xor-term: and | and-term

and:       (and-term /"and")+ and-term
@and-term: not | not-term

not: "not" not-term
@not-term: BOOLEAN
         | port-ref
         | /"(" expression /")"

@port-ref: ID | instance-port-ref

/instance-port-ref: ID /"." ID

The language module

Finally, the spicy/lang/expander.rkt module exports the definitions needed to expand Tiny-HDL code. In this case, since the begin-tiny-hdl form is already part of the generated syntax object, we do not redefine the #%module-begin macro:

#lang racket

(require tiny-hdl)

  (all-from-out tiny-hdl)
  xor and or not

Getting the source code and running the examples

The source code for this step can be found in branch step-06 of the git repository for this project.

There are two new versions of the full adder example:

The implementations of both language flavors are available in folders vanilla/lang and spicy/lang of the source tree.

Getting the source code for step 6

Assuming you have already cloned the git repository, switch to branch step-06:

git checkout step-06

Running the examples

Run full-adder-step-06-vanilla-test.rkt or full-adder-step-06-spicy-test.rkt with Racket:

racket examples/full-adder-step-06-vanilla-test.rkt
racket examples/full-adder-step-06-spicy-test.rkt


When I started this project, I came to Racket with a very narrow conception of what language-oriented programming was. Using Racket was a refreshing experience, but also frustrating at times.

Using a language with an effective macro system was completely new to me. Before learning Racket, the word « macro » reminded me of C’s #define, and that did not sound very exciting. Now I have a completely different vision of what macros are and what they can offer, especially in the context of language-oriented programming.

I found the concept of creating a language using macros, and the ability to implement custom readers, very elegant. However, implementing name resolution and cross-module bindings felt particularly unintuitive at first, mainly due to my limited understanding of the macro expansion process, and my ignorance of the available APIs. This gave me the impression that Racket lacked maturity compared to other language frameworks that I had used before. Now I know that there is an ongoing effort to provide new APIs for DSL creation, and this is very encouraging.

Since those difficulties are behind me, I can fully enjoy the reward of getting a fully-working DSL, and I feel ready to start a new language project with Racket.