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Schemy

Schemy is a lightweight Scheme-like scripting language interpreter for embedded use in .NET applications. It's built from scratch without any external dependency. Its primary goal is to serve as a highly flexible configuration language. Example scenarios are to describe computational graph, workflow, or to represent some complex configuration.

Its design goals are:

easy to embed and extend in .NET
extensible in Scheme via macro expansion
safe without the need of complicated AppDomain sandboxing. It's safe because IO functions are not implemented.
runs reasonably fast and low memory footprint

Non-goals:

be highly optimized - it's designed to load configurations and not part of any heavy computation, so being optimized is not the goal - e.g., there's no JIT compiling, etc.

Schemy's implementation is inspired by Peter Norvig's article on Lisp interpreter, but is heavily adapted to .NET and engineered to be easily extensible and embeddable in .NET applications.

Scheme Features

It has most features that a language would support:

number, boolean, string, list types
varaible, function definition
tail call optimization
macro definition
lexical scoping

Many Scheme features are not (yet) supported. Among those are:

continuation (call/cc)
use square brackets [...] in place of parenthesis (...)

Embedding and Extending Schemy

Schemy is primarily designed to be embedded into a .NET application for configuration or as a shell-like interactive environment (REPL). To use Schemy, you can either:

Reference schemy.dll, or
Copy src/schemy/*.cs source code to include in your application. Since Schemy code base is small. This approach is very feasible (don't forget to also include the resource file init.ss).

The below sections describes how to embed and extend Schemy in .NET applications and in Scheme scripts. For a comprehensive example, please refer to src/examples/command_server.

Extending Schemy in .NET

Schemy can be extended by feeding the interpreter symbols with predefined .NET objects. Variables could be any .NET type. Procedures must implement ICallable.

An example procedure implementation:

new NativeProcedure(args => args, "list");

This implements the Scheme procedure list, which converts its arguments into a list:

schemy> (list 1 2 3 4)
(1 2 3 4)

To "register" extensions, one can pass them to the Interpreter's constructor:

Interpreter.CreateSymbolTableDelegate extension = itpr => new Dictionary<Symbol, object>
{
    { Symbol.FromString("list"), new NativeProcedure(args => args, "list") },
};

var interpreter = new Interpreter(new[] { extension });

Extending Schemy in Scheme

When launched, the interpreter tries to locate and load Scheme file .init.ss in the same directory as the executing assembly. You can extend Schemy by putting function, variable, macro definition inside this file.

Extending with functions

For example, this function implements the standard Scheme list reversion function reverse (with proper tail call optimization):

(define (reverse ls)
  (define loop
    (lambda (ls acc)
      (if (null? ls) acc
        (loop (cdr ls) (cons (car ls) acc)))))
  (loop ls '()))

Use it like so:

Schemy> (reverse '(1 2 "foo" "bar"))
("bar" "foo" 2 1)

Syntax augmentation in Scheme

For example, we want to augment Schemy with a new syntax for local variable definition, let. Here's what we want to achieve:

Schemy> (let ((x 1)     ; let x = 1
              (y 2))    ; let y = 2
          (+ x y))      ; evaluate x + y
3

The following macro implements the let form by using lambda invocation:

(define-macro let
  (lambda args
    (define specs (car args))  ; ((var1 val1), ...)
    (define bodies (cdr args)) ; (expr1 ...)
    (if (null? specs)
      `((lambda () ,@bodies))
      (begin
        (define spec1 (car specs)) ; (var1 val1)
        (define spec_rest (cdr specs)) ; ((var2 val2) ...)
        (define inner `((lambda ,(list (car spec1)) ,@bodies) ,(car (cdr spec1))))
        `(let ,spec_rest ,inner)))))

Use Interactively (REPL)

The interpreter can be run interactively, when given a TextReader for input and a TextWriter for output.

/// <summary>Starts the Read-Eval-Print loop</summary>
/// <param name="input">the input source</param>
/// <param name="output">the output target</param>
/// <param name="prompt">a string prompt to be printed before each evaluation</param>
/// <param name="headers">a head text to be printed at the beginning of the REPL</param>
public void REPL(TextReader input, TextWriter output, string prompt = null, string[] headers = null)

This can be useful for expose a remote "shell" for the application, or as debugging purposes (see how src/examples/command_server/ uses the --repl command line argument).

There is an example REPL application in src/examples/repl/ that can be started as a REPL interpreter:

$ schemy.repl.exe
-----------------------------------------------
| Schemy - Scheme as a Configuration Language |
| Press Ctrl-C to exit                        |
-----------------------------------------------

Schemy> (define (sum-to n acc)
           (if (= n 0) 
              acc 
              (sum-to (- n 1) (+ acc n))))

Schemy> (sum-to 100 0)
5050

Schemy> (sum-to 10000 0)  ; proper tail call optimization prevents stack overflow
50005000

Run a script:

$ schemy.repl.exe <some_file>

Contributing

This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit https://cla.microsoft.com.

When you submit a pull request, a CLA-bot will automatically determine whether you need to provide a CLA and decorate the PR appropriately (e.g., label, comment). Simply follow the instructions provided by the bot. You will only need to do this once across all repos using our CLA.

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

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