Custom Languages in Gerbil

Here we explore language extensibility in Gerbil by defining custom preludes.


The source code for all the examples is available at src/tutorial/lang. You can build the tutorial code using the build script:

$ cd gerbil/src/tutorial/lang
$ gerbil build

Custom Languages in the REPL

Before we go into the examples, it should be noted that if your language uses an s-expression syntax, you can also use its prelude for custom initial bindings in the repl by starting gxi with the --lang option. Simply set it to the module path of your custom language, and your repl will obtain initial bindings by importing the custom prelude.

For instance, to use our dot-app language in the repl:

$ gerbil env gxi --lang :tutorial/lang/dot-app
> _

Custom Module Expansion

Custom References and Procedure Application

The first example is the dot-app language, which illustrates reference and application indirection through custom expanders.

More specifically, we write a language where dots in identifiers are significant and are considered method calls, similar to how ruby treats dots.

We want to be able to write

(a.some-method arg ...)

and instead of invoking the procedure a.some-method, we want to invoke the method some-method on the object a.

Similarly for references, we want to be able to write


and obtain a reference to the bound method some-method of object a instead of the procedure a.some-method.

For chained dotted references we want to invoke the dotted parts as methods:

(a.b.c arg ...) => {{b a} c arg ...}

The dot-app prelude accomplishes this by redefining the %%app and %%ref expander hooks:

(defsyntax (my-%%app stx)
  (syntax-case stx ()
    ((_ rator . rands)
     (dotted-identifier? #'rator)
     (with ([hd . chain] (split-dotted-identifier #'rator))
       (let lp ((rest chain) (obj (datum->syntax #'rator hd)))
         (match rest
            (with-syntax ((obj obj) (method method))
              #'{method obj . rands}))
           ([method . rest]
            (with-syntax ((obj obj) (method method))
              (lp rest #'{method obj})))))))
    ((_ rator . rands)
     ;; default -- up the chain to core %%app
     #'(%%app rator . rands))))

(defsyntax (my-%%ref stx)
  (syntax-case stx ()
    ((_ id)
     (dotted-identifier? #'id)
     (with ([hd . chain] (split-dotted-identifier #'id))
       (let lp ((rest chain) (obj (datum->syntax #'id hd)))
         (match rest
            (with-syntax ((obj obj) (method method))
              #'(lambda args (apply call-method obj 'method args))))
           ([method . rest]
            (with-syntax ((obj obj) (method method))
              (lp rest #'{method obj})))))))
    ((_ id)
     ;; default -- up the chain to core %%ref
     #'(%%ref id))))

The two macros are defined with different names in local scope, to avoid shadowing the root %%app and %%ref and causing an expansion loop. The module exports them renamed, together with all the exports of the core prelude to define a language that is just like core Gerbil but with special semantics for the dot in identifiers:

(import :gerbil/core) ;; so that we can re-export, prelude imports are weak
(export (import: :gerbil/core)
        (rename: my-%%app %%app)
        (rename: my-%%ref %%ref))

The macros also need a couple of helper functions, defined for syntax:

  (def (dotted-identifier? stx)
    (and (identifier? stx)
         (let (str (symbol->string (stx-e stx)))
           (string-index str #\.))))

  (def (split-dotted-identifier stx)
    (map string->symbol (string-split (symbol->string (stx-e stx)) #\.))))


For a contrived example, consider the code in example/

$ cat example/
prelude: :tutorial/lang/dot-app

(export #t)

(defstruct A (x))
(defmethod {add-x A}
  (lambda (self y)
    (with ((A x) self)
      (+ x y))))

;; use the dotted %%app from dot-app prelude
(def (add-x a y)
  (a.add-x y))

(def (a-add-x a)

We can see the result in the interpreter:

$ gerbil env gxi
> (import "example/my-app")
> (def a (make-A 1))
> (add-x a 1)
> (def a.add-x (a-add-x a)) ; here the dot is a normal identifier
> a.add-x
#<procedure #5 a.add-x>
> (a.add-x 1)

Custom Body Expansion

The second example is the auto-export language, which illustrates the very basics of custom module body expansion.

The premise is very simple: we want a language that automatically exports all bindings, similar to how Schemes without modules work. The auto-export prelude accomplishes this by redefining the %%begin-module expander:

(import :gerbil/core) ;; so that we can re-export
(export (import: :gerbil/core)
        (rename: my-%%begin-module %%begin-module))

(defrules my-%%begin-module ()
  ((_ . body)
   (%%begin-module (export #t) . body)))

The macro is actually trivial and doesn't do any processing of the body; all it does is plaster an (export #t) and expand up the chain through the root %%begin-module.


The example is a module with a single definition greet and no export directive in sight:

$ cat example/
prelude: :tutorial/lang/auto-export

;; everything is automagically exported, so no export directive needed
(def (greet x)
  (displayln "hello " x))

We can verify that greet is indeed exported in the interpreter:

$ gerbil env gxi
> (import "example/my-auto-export")
> (greet "world")
hello world

Custom Readers

The next example is the sexp language, which illustrates the mechanics of custom module readers and #lang preludes.

The sexp prelude is a minimal module with a custom reader:

(import :gerbil/core) ;; so that we can re-export
(export (import: :gerbil/core)
        (phi: +1 read-module-body))

  (def (read-module-body port)
    (let lp ((body []))
      (let (next (read-syntax port))
        (if (eof-object? next)
          (reverse body)
          (lp (cons next body)))))))

The reader itself is quite uninteresting: it uses the builtin read-syntax primitive to read s-expressions, so this language is identical to Gerbil core. However, it can be used as a #lang prelude, in contrast to the gerbil core prelude.

The example demonstrates just that:

$ cat example/
#lang :tutorial/lang/sexp
(export #t)

(def (greet x)
  (displayln "hello " x))

$ gerbil env gxi
> (import "example/my-sexp")
> (greet "world")
hello world

Arbitrary Surface Syntax

We have so far illustrated the basics of custom module expansion and have built up our machinery up to the #lang form for custom language preludes. Here, we put this machinery to use with a language that has an adhoc surface syntax.

Let's call our language scuby, because it takes the ruby approach in delineating basic blocks using end tokens. The language itself is a toy, but it's complex enough to benefit from a proper parser and a lexer.

Gerbil provides extensive facilities for parsing and lexing in the standard library, as part of the :std/parser package. The package provides facilities for lexing and parsing while accurately tracking source location, macros for parser and lexer generation, and a custom language prelude for writing language grammars in declarative fashion (i was quite fond of silex and lalr).

Some Scuby Code

So let's look at our yet-to-be designed language, and write some code in it. We want to keep things simple and minimal, so we have one def form for function definitions and variables, while we have exactly one special form with if. Our datums can be booleans, null, integers, and strings.

There is some scuby code in the example module.

For instance, here is a function that computes the nth Fibonacci number:

def (fibo n)
  def (fibo-r x y k)
    if (< k n)
      (fibo-r (+ x y) x (1+ k))
    else x
  (fibo-r 1 0 1)

And here are closures with the classic stateful counter:

def (make-counter x)
  def (counter)
    def next = x
    (set! x (1+ x))

The Grammar

So let's take a look at the grammar.

First, let's take a look at the parser specification. The syntax is declarative, with mutually recursive rules and productions. Each rule can have multiple productions, which take the form of a pattern and an optional action.

The Parser Specification Grammar

So the general form of a parser specification can be given in a declarative form of EBNF notation:

Parser        <- Rule*
Rule          <- Production AltProduction*
Production    <- Pattern Action
              |  Pattern
AltProduction <- '|' Production

Action     <- '=>' ActionExpr
ActionExpr <- '!'                     ; cut (rule scoped)

Pattern <- SimplePattern '+'          ; one or more
        |  SimplePattern '*'          ; zero or more
        |  SimplePattern '?'          ; zero or one
        |  SimplePattern
SimplePattern <- '(' Ident Ident ')'  ; rule match with variable binding
              |  Ident                ; rule match
              |  Quote                ; token value eq? 'symbolic-value'
              |  String               ; token value equal? "string-value"
              |  EOF

EOF    <- '$$'                        ; the end of input token
Ident  <- %                           ; identifier (lexer token)
Quote  <- %                           ; symbolic equality (token)
String <- %                           ; string equality (token)

Incidentally, this is a valid but incomplete (it is has a reference to SExpression) parser specification for the parser specification language. This parser would recognize the language but doesn't produce an AST.

The Scuby Parser

Here is the high level logic for our scuby parser:

Program <- (Form $1)* EOF
           => ['begin $1 ...]

Form <- Defn
     |  Expr

Defn <- DEF (VarIdent $1) '=' (Expr $2)
        => ['def $1 $2]
     |  DEF '(' (VarIdent $1) (VarIdent $2)* ')' (Defn $3)* (Expr $4)+ END
        => ['def [$1 $2 ...] $3 ... $4 ...]
     |  DEF
        ;; hard cut to provide better error locality
        => (raise-parse-error 'parse-scuby "malformed definition" @@)

VarIdent <- Reserved
            => (raise-parse-error 'parse-scuby "illegal variable identifier: reserved keyword" @@)
         |  (Ident $1)
            => $1  ; wrap identifier to track source location

Expr <- BeginExpr
     |  IfExpr
     |  AppExpr
     |  RefExpr
     |  DatumExpr

BeginExpr <- BEGIN (Expr $1)+ END
             => ['begin $1 ...]

IfExpr <- IF (Expr $1) (Expr $2) ELSE (Expr $3) END
          => ['if $1 $2 $3]
       |  IF (Expr $1) (Expr $2) END
          => ['if $1 $2]

AppExpr <- '(' (Expr $1) (Expr $2)* ')'
           => [$1 $2 ...]

RefExpr <- Reserved
           => !
        |  (Ident $1)
           => $1  ; wrap identifier to track source location

Reserved <- DEF
         |  BEGIN
         |  IF
         |  ELSE
         |  END

DatumExpr <- Int
          |  String
          |  TRUE
          |  FALSE
          |  NULL

The Scuby Lexer

The lexer specification language is almost identical to silex, which should be obvious by example. The basic form is

Lexer <- Rule*
Rule <- Regex '->' Production
Production <- '$'                 ; ignored token
           |  '$$'                ; eof token
           |  '{' Identifier '}'  ; macro definition
           |  (Ident Sexpression) ; token production

And here is our lexer specification:

[\ \t\n]+                       -> $                             ; whitespace
\;[^\n]*\n                      -> $                             ; comments
true                            -> (TRUE #t)                     ; #t
false                           -> (FALSE #f)                    ; #f
null                            -> (NULL '())                    ; '()
[(]                             -> (LPAREN '|(|)                 ; LPAREN token
[)]                             -> (RPAREN '|)|)                 ; RPAREN token
[-]?[0-9]+                      -> (Int (string->number @@))     ; integers
[-+*/=?<>~!@$%^&:_a-zA-Z0-9]+   -> (Ident (string->symbol @@))   ; identifiers
"([^"]|\\")*"                   -> (String (string-e @@ @loc))   ; strings

The Scuby Prelude

The scuby prelude defines a custom module reader which parses scuby syntax and produces an AST. The reader is straightforward:

(import :gerbil/core
        (phi: +1

  ;; the module reader: uses the parser defined in scuby-grammar to parse the module
  (def (read-module-body port)
     (let (progn (parse-scuby port))
       (case (token-t progn)
         ((Program)                     ; we parsed a program
          [(token-e progn)])
         (($$)                          ; the empty program
         (else                          ; that really shouldn't happen with our grammar
          (raise-syntax-error #f "Bad syntax; unexpected token" (token->syntax progn)))))
     (catch (parse-error? exn)
       (apply raise-syntax-error #f
              (string-append "Bad syntax; " (error-message exn))
              (map token->syntax (error-irritants exn)))))))


And putting it all together, we can import our small corpus of scuby code:

$ gerbil env gxi
> (import "example/my-scuby")
> (import :std/iter)
> (for (x (in-iota 10 1)) (displayln (fibo x)))
> (def my-counter (make-counter 1))
> (for (x (in-iota 10)) (displayln (my-counter)))