Skeem

Linux	Windows

Skeem will be an interpreter of a subset of the Scheme programming language.

Installation

Add this line to your application's Gemfile:

gem 'skeem'

And then execute:

$ bundle

Or install it yourself as:

$ gem install skeem

The Skeem project has started recently and at this stage, the gem supports a small Scheme subset.

About Scheme

The Scheme programming language is a Lisp dialect that supports multiple paradigms, including functional programming and imperative programming.

Resources on Scheme

Here are a few pointers for the Scheme programming language:

Wikipedia article on Scheme
Latest official Scheme standard: R7RS #### Online tutorials and books:
The Scheme Programming Language, 4th Edition by Kent Dybvig. A complete, introductory textbook on Scheme based on the older R5RS standard.
Teach Yourself Scheme in Fixnum Days by Dorai Sitaram
Yet Another Scheme Tutorial by Shido Takafumi
An Introduction to Scheme and its Implementation by Paul R. Wilson

Usage

Example 1 (Variable definition)

  require 'skeem'

  schemer = Skeem::Interpreter.new

  scheme_code ="    ; This heredoc consists of Scheme code...\n    ; Let's define a Scheme variable\n    (define foobar (* 2 3 7))\n\n    ; Now test its value against a lower value\n    (if (> foobar 40) #true #false)\n  SKEEM\n\n  # Ask Ruby to execute Scheme code\n  result = schemer.run(scheme_code)\n  puts result.value # => true\n\n  # The interpreter object keeps the bindings of variable\n  # Let's test that...\n  scheme_code = '(* foobar foobar)'\n  result = schemer.run(scheme_code)\n  puts result.value # => 1764\n"

Example 2 (Defining a function)

  require 'skeem'

  schemer = Skeem::Interpreter.new

  scheme_code ="    ; Let's implement the 'min' function\n    (define min (lambda (x y) (if (< x y) x y)))\n\n    ; What is the minimum of 2 and 3?\n    (min 2 3)\n  SKEEM\n\n  # Ask Ruby to execute Scheme code\n  result = schemer.run(scheme_code)\n  puts result.value # => 2\n\n  # Let's retry with other values\n  scheme_code = '(min 42 3)'\n  result = schemer.run(scheme_code)\n  puts result.value # => 3\n"

Example 3 (Defining a recursive function)

  require 'skeem'

  schemer = Skeem::Interpreter.new
  scheme_code = "    ; Compute the factorial of 100\n    (define fact (lambda (n)\n      (if (<= n 1) 1 (* n (fact (- n 1))))))\n    (fact 100)\n  SKEEM\n\n  result = schemer.run(scheme_code)\n  puts result.value # => 9332621544394415268169923885626670049071596826438162146859296389521759999322991560894146397615651828625369792082722375825118521091686400000000000000000000000\n"

Example 4 (Defining a procedure that holds its own environment)

  require 'skeem'

  schemer = Skeem::Interpreter.new
  scheme_code = "  (define make-counter\n    ;; create a procedure that will bind count and\n    ;; return a new procedure that will iself increment the\n    ;; variable and return its newest value\n    (lambda ()\n       (let ((count 0))\n          (lambda ()\n             (set! count (+ count 1))\n             count))))\n\n  (define c1 (make-counter))\n  (define c2 (make-counter))\n  (define c3 (make-counter))\n\n  ;; Notice how each procedure keeps track of its \"own\" counter.\n  (c1) ; => 1\n  (c2) ; => 1\n  (c1) ; => 2\n  (c3) ; => 1\n  (c1) ; => 3\n  SKEEM\n\n  result = schemer.run(scheme_code)\n  puts result.last.value # => 3\n"

Example 5 (Conditional branching)

  require 'skeem'

  schemer = Skeem::Interpreter.new

  scheme_code ="    ; Let's implement the 'signum' function\n    (define signum (lambda (x)\n       (cond\n         ((positive? x) 1)\n         ((zero? x) 0)\n         (else -1))))\n\n    (signum -3)\n  SKEEM\n\n  result = schemer.run(scheme_code)\n  puts result.value # => -1\n"

Currently implemented R7RS features

Data type literals

Booleans: #t, #true, #f, #false
Of the number hierarchy:
real (e.g. 2.718, 6.671e-11),
integer (42, -3)
Lists (quoted) : '(1 two "three")
Strings: "Hello, world."
Identifiers (symbols): really-cool-procedure
Vectors: #(1 2 "three")

Scheme Expressions

Constant literals
Quotations
Quasiquotation (without unquote-splicing)
Variable references
Procedure calls
Lambda expressions
Conditionals (if, cond)
Definitions
Assignments
Control procedures

Standard syntactic forms

define

Purpose: Create a new variable and bind an expression/value to it.
Syntax:

(define )
(define ( ) )

if

Purpose: Conditional evaluation based on a test expression.
Syntax:

(if )
(if )

lambda

Purpose: Definition of a procedure.
Syntax:

(lambda )

quote

Purpose: Quoting an expression (leave it unevaluated).
Syntax:

(quote )
'

set!

Purpose: Assign to an existing variable an expression/value to it.
Syntax:

(set! )

Derived expressions

cond

Purpose: Define one or more branchings.
Syntax:

(cond ( )+)
(cond ()* (else ))

let

Purpose: Define one or more variable local to the block.
Syntax:

(let () )

let*

Purpose: Define one or more variable local to the block.
Syntax:

(let* () )

Standard library

This section lists the implemented standard procedures

Equivalence predicates

eqv?, equal?

Boolean procedures

boolean?, and, or, not

Numerical operations

Number-level: number?, real?, integer?, zero?, +, -, *, /, =, square, number->string
Real-level: positive?, negative?, <, >, <=, >=, abs, floor-remainder
Integer-level: even?, odd?

List procedures

list?, null?, pair?, append, car, cdr, caar, cadr, cdar, cddr, cons, length, list, list-copy, list->vector, set-car!, set-cdr! , assq, assv

String procedures

string?, string=?, string-append, string-length, string->symbol

Symbol procedures

symbol?, symbol=?, symbol->string

Vector procedures

vector?, make-vector, vector, vector-length, vector-set!, vector->list

Control procedures

procedure?, apply, map

Input/output procedures

newline

Special procedures

assert

Roadmap:

Extend language support
Implement REPL
Implement an equivalent of lis.py
Implement an equivalent of lispy
Make it pass the test suite
Extend the language in order to support Minikanren
Make it pass all examples from the Reasoned Schemer book.

Other Scheme implementations in Ruby

Skeem isn't the sole implementation of the Scheme language in Ruby.
Here are a few other ones:

Heist gem -- Probably one of best Scheme implementation in Ruby. Really worth a try. Alas, the project seems to be dormant for several years.
Schemerald gem. The last commit for the project is October 2017.
rubic gem. The last commit for the project is June 2015.
RLisp ...Simple scheme interpreter written in ruby

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/famished-tiger/Skeem. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

Copyright

Code of Conduct

Everyone interacting in the Skeem project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.