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Indentation-based Racket Syntax
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Indentation-based Racket Syntax with Macros and Infix Operators

Not sweet-exps (see Asumu's racket implementation). Not srfi-49. More inspired by Python and Haskell.

(The name "something" is temporary, and likely to be repurposed.)


Check out the repository. Then, in the directory containing Makefile,

make link


raco pkg install --link -n something `pwd`/src

The main idea

S-expressions, but with usually-implicit parentheses. Indentation for grouping is explicitly represented in the S-expression returned from the reader.

This program:

#lang something
for { x: 1 .. 10 }
  def y: x + 1
  printf "x ~a y ~a\n" x y

... reads as this S-expression:

(module something-module something/base
   (for (block (x (block (1 .. 10))))
        (block (def y (block (x + 1)))
               (printf "x ~a y ~a\n" x y)))))

The #%rewrite-body macro, together with its companion #%rewrite-infix, consults an operator table, extendable via the def-operator macro, to rewrite infix syntax into standard prefix S-expressions.

The block syntax has many different interpretations. It has a macro binding that turns it into a Racket match-lambda*, and it is used as literal syntax as input to other macro definitions.

For example, here's one possible implementation of that for syntax:

#lang something


  for-syntax something/lang/implicit
  prefix-in base_ racket/base

def-syntax for stx
  syntax-case stx (block)
    _ (block (v (block exp)) ...) (block body ...)
      (syntax (base_for ((v exp) ...) body ...))

def-operator .. 10 nonassoc in-range

Notice how the block S-expressions are rewritten into a normal S-expression compatible with the underlying for from racket/base.

Generally, all of these forms are equivalent

x y z          x y z:          x y z { a; b }
  a              a
  b              b

and they are read as

(x y z (block a b))

and are then made available to the normal macro-expansion process (which involves a new infix-rewriting semi-phase).

Colons are optional to indicate a following suite at the end of an indentation-sensitive line. Indentation-sensitivity is disabled inside parentheses. If inside a parenthesised expression, indentation-sensitivity can be reenabled with a colon at the end of a line:

a b (c d:

= (a b (c d (block e f)))

a b (c d

= (a b (c d e f))

Conversely, long lines may be split up and logically continued over subsequent physical lines with a trailing \:

a b c \
  d \

= (a b c d e)

Semicolons may also appear in vertically-laid-out suites; these two are equivalent:

x y z
  b; c

x y z { a; b; c; d }

Suites may begin on the same line as their colon. Any indented subsequent lines become children of the portion after the colon, rather than the portion before.

This example:

x y z: a b
  c d

reads as

(x y z (block (a b (block (c d) e))))

Square brackets are syntactic sugar for a #%seq macro:

[a; b; c; d e f]    →        (#%seq a b c (d e f))

[                   →        (#%seq a (b (block c)) (d e f))
  d e f

Forms starting with block in expression context expand into match-lambda* like this:

  pat1a pat1b

→ (match-lambda*
    [(list pat1a pat1b) exp1a exp1b]
    [(list pat2a) exp2])

The map* function exported from something/lang/implicit differs from map in racket/base in that it takes its arguments in the opposite order, permitting maps to be written

map* [1; 2; 3; 4]
    item + 1

map* [1; 2; 3; 4]
  item: item + 1

map* [1; 2; 3; 4]: item: item + 1

map* [1; 2; 3; 4] { item: item + 1 }

A nice consequence of all of the above is that curried functions have an interesting appearance:

def curried x:: y:: z:
  [x; y; z]

require rackunit
check-equal? (((curried 1) 2) 3) [1; 2; 3]

A larger example

More examples can be found in the examples and src/something/test directories.

#lang something

  for-syntax something/lang/implicit
  except-in xml document

def-syntax single-xexpr stx
  syntax-case stx (= block)
    _ str
      string? (syntax-e . (syntax str))
      syntax str
    _ (= expr)
      syntax expr
    _ (tag (attr attr-expr) ... (block xexpr ...))
      syntax (list (quote tag) (list (list (quote attr) attr-expr) ...) (single-xexpr xexpr) ...)
    _ (tag (attr attr-expr) ...)
      syntax (list (quote tag) (list (list (quote attr) attr-expr) ...))

def-syntax xexpr stx
  syntax-case stx (block)
    _ (block xexpr)
      syntax (single-xexpr xexpr)

def-operator ++ 50 left string-append
def-operator = 10 prefix =

def document
        meta (http-equiv "Content-Type") (content "text/html; charset=utf-8")
        title: "Test page"
        h1: "Hello"
        p: "Hello, world"
        h2: "Testing"
          = "Hello, " ++ number->string (3 + 4)
          "! This rules."
          "Another way of putting it would be to say that 3 + 4 = "
          = (number->string (3 + 4))

pretty-print document
printf "\n~a\n" (xexpr->string document)

A note on lexical syntax

The lexical syntax of this reader is not exactly that of Racket. For example, comments start with // rather than ;, and the set of allowable non-escaped identifiers is different (smaller).

I will likely revise this decision to bring it to be much closer to Racket's lexical syntax.

Emacs mode

See sth8.el.


Copyright (C) 2016–2019 Tony Garnock-Jones

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program (see the files "lgpl.txt" and "gpl.txt"). If not, see

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