Mickey Scheme is an interpreter for R7RS Scheme written in C++
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Mickey R7RS Scheme

Mickey Scheme is an incomplete, slow and buggy implementation of R7RS Scheme small.

The current project goals are to

  • Provide a correct and complete implementation of R7RS-small (WG1)
  • Emphasize clarity and simplicity in the implementation
  • Be a powerful platform for experimentation and creation of a more advanced scheme compiler.

Note that Mickey is just a codeword for the early stages of this project. The name will change as the project matures.

Current Features

  • Implements 77% of R7RS-small; see compliance report
  • Quotation and quasiquotation
  • All non-syntactic let-forms
  • Variadic functions
  • Two macro systems: syntax-rules (buggy, incomplete support) and define-macro (unhygienic)
  • Lazy evaluation with memoization in (scheme lazy)
  • Full support for the R7RS library system define-library
  • Tail call eliminiation
  • The define-record-type record system

Some of these are demonstrated in the example code section.


The supported SRFIs are

See the SRFI page at http://srfi.schemers.org and the list of accepted ones at http://srfi.schemers.org/final-srfis.html


Hacking on Mickey Scheme is extremely easy. Because of this I couldn't resist adding:

  • First class environments via the library (mickey environment) (see examples)

It's also easy to write wrapper code for using C libraries:

Current Shortcomings

  • It's slow: The code is completely interpreted, without any optimizations. I have plans to change this, but it's currently not a priority.

  • It's incomplete: Some key Scheme features are still missing, and quite some R7RS library functions (currently it supports 426 of 550 R7RS definitions).

  • It's buggy: There are inherent bugs in the engine as well as erronous implementations of library functions.

  • It does not collect garbage: Currently, there is no garbage collector. Adding a simple mark-and-sweep GC is trivial, though, so I'll get to it once I think it's important enough.

  • It doesn't support continuations (yet): I think I'll have to convert to continuation passing style for a proper implementation of this.

  • Other missing features include dynamic-wind, exception handling and so on. These will be added when call/cc is implemented.


The table below shows how many of the definitions in R7RS-small that have been implemented in Mickey Scheme.

The first number shows the coverage in percent, then number of implemented definitions, definitions required by R7RS-small, missing definitions and name of the library.

 68% 161/236  -75 (scheme base)
100%     1/1    0 (scheme case-lambda)
100%   22/22    0 (scheme char)
  0%     0/6   -6 (scheme complex)
100%   24/24    0 (scheme cxr)
100%     2/2    0 (scheme eval)
 20%    2/10   -8 (scheme file)
100%   12/12    0 (scheme inexact)
 40%     2/5   -3 (scheme lazy)
100%     1/1    0 (scheme load)
100%     5/5    0 (scheme process-context)
100%     1/1    0 (scheme read)
100%     1/1    0 (scheme repl)
100%     3/3    0 (scheme time)
 50%     2/4   -2 (scheme write)
 86% 186/217  -31 (scheme r5rs)
 77% 425/550 -125 <all>

In summary, Mickey implements 425 of 550 definitions in R7RS-small. 125 definitions have not been implemented.

This corresponds to 77% coverage.


In short, you need GNU autoconf, automake and libtool to build Mickey from the GitHub sources. If you want to build to a build directory, do this:

$ ./autogen.sh
$ mkdir build
$ ./configure --prefix=`pwd`/build
$ make -j

You should run the test suite as well:

$ make -j check

Then you can install into the build subdirectory with

$ make -j install

If you want to build with different flags, you can take a look at the build-debug.sh and build-release.sh scripts. As an example, if you want to build an optimized version compiled to your native architecture, you can do

  CXXFLAGS="-O3 -march=native -mtune=native -ffast-math" \
    ./configure && make -j all check

To build a redistributable package, you should be able to do

$ make -j distcheck

If you want to start the executable while developing, you do the usual configure and make -j invocations. If you don't use a separate build directory, you should be able to start mickey from the top directory with

$ src/mickey -Llib

Feature flags

See ./configure --help.


Mickey R7RS Scheme Copyright (C) 2011-2015 Christian Stigen Larsen

Distributed under version 2.1 of the Lesser GNU Public License (LGPL) while also allowing anyone to change the license on a particular copy of the code to the LGPL 3.0, the GPL 2.0 or the GPL 3.0.

See the file COPYING for the full text of the LGPL 2.1.


Christian Stigen Larsen csl@csl.name http://csl.name


Here are a few example code snippets for Mickey Scheme.

Besides demonstrating the basic capabilities of Mickey, it also serves as a kind of soft introduction to Scheme.

First, let's start mickey.

If you're on Linux, you may have to use the -L flag to tell Mickey where it can find Scheme and compiled dynamic libraries. It accepts several -L options.

If you've installed mickey, you should be able to simply type

$ mickey
#|                                                                 _
   Mickey Scheme (C) 2011-2012 Christian Stigen Larsen              \
   4.2.1 Compatible Apple Clang 4.0 ((tags/Apple/clang-421.0.57))   /\
   Readline 4.2                                                    /  \_

   To quit, hit CTRL+D or type (exit).  Use (help) for an introduction.
   Distributed under the LGPL 2.1; see LICENSE

#; mickey> 

This is the REPL, short for read-evaluate-print loop.

First, you should note two things: The #| banner inside a comment |# and the #; symbol. They make it possible to copy the code you see here and paste it directly into your own REPL, without needing to remove mickey> and so on.

The symbol #; means "ignore the next symbol"; so #; mickey> (+ 1 2 3) means that Mickey Scheme will skip mickey> and move on to (+ 1 2 3), which it then will evaluate. This is a known trick in scheme systems.

Now, let's type some code.

#; mickey> (display "Hello, world!\n")
Hello, world!


Let's play with lambdas. First we'll create a lambda to square a number and execute it on the fly.

#; mickey> ((lambda (x) (* x x)) 12)

We can bind it to a variable as well. Let's bind it to the variable square.

#; mickey> (define square (lambda (x) (* x x)))
#; mickey> (square 12)
#; mickey> (square 3.1415)

Let's have some fun with lambdas. Let's create a function that creates other functions.

We'll create a general function make-adder that creates a function that can add a static number to another number.

#; mickey> (define make-adder
             (lambda (frozen-number)
               (lambda (x)
                 (+ x frozen-number))))

As you can see, we give it a frozen-number and it returns a lambda that takes a number x and adds the two together.

So to make a function that adds 5 to its argument, we simply do

#; mickey> (define add5 (make-adder 5))
#; mickey> (add5 10)


#; mickey> (define add17 (make-adder 17))
#; mickey> (add17 10)

Writing (define (lambda (x) ...)) is tedious, so a shorter variant is available:

#; mickey> (define (cube x) (* x x x))
#; mickey> (cube 101)

Mickey uses GNU readline, so it offers both tab completion and history. And it actually works, too. Here we write (ca and hit TAB two times to see available definitions.

#; mickey> (ca
caaar  caadr  caar   cadr   car  

Here are car and cdr. They extract the first and remaining items in a list.

#; mickey> (car '(1 2 3))
#; mickey> (cdr '(1 2 3)))
(2 3)

I like the old school "car" and "could-er" forms because you can compose them, so that you can extract the car of the cdr like so:

#; mickey> (cadr '(1 2 3))

They might not be very interesting for flat lists, but they shine for accessing trees.


Here is some simple arithmetic.

#; mickey> (+ 1 2 3 4 5 6 7 8 9 10)

Summation of sequences can be calculated more quickly with

#; mickey> (define (seq-sum n)
          (* n (/ (+ n 1) 2)))

which gives us

#; mickey> (seq-sum 10)
#; mickey> (seq-sum 100)
#; mickey> (seq-sum 127)


Here is an example of the "let star" form. It creates a local variable scope, and evaluates them in the given order.

#; mickey> (let* ((x 2)
                  (y 3)
                  (z (* x y)))
                 (display z))

This, of course, prints 6.

There is also a letrec form that allows for mutually recursive definitions. Or in plain english, expressions that refer to each other.

The typical example of this is to implement even? and odd? in terms of each other:

  • A number is even if the preceding number is odd, and
  • A number is odd if the preceding number is even

Since our definition is going to be mutually recursive, we need to handle base cases of zero (negative values will make the code loop forever, though).

Let's write that out, along with a check-number function.

  ((even? (lambda (n)
            (if (zero? n) #t
                (odd? (- n 1)))))

   (odd? (lambda (n)
           (if (zero? n) #f
               (even? (- n 1)))))

     (lambda (n)
       (display `(The number ,n is ,(if (even? n) 'even 'odd)))

   (check-number 2)
   (check-number 3)
   (check-number 88)
   (check-number 99))

If you run the above code in mickey, you'll get this output:

(The number 2 is even)
(The number 3 is odd)
(The number 88 is even)
(The number 99 is odd)


Now, Scheme doesn't have a when function. The when function checks whether the first argument is true. If it is, then it will evaluate --- or execute --- the code given in the remaining arguments.

You can't do this with a simple function, because it would always evaluate the code body. As an example, let's say we have a boolean variable green-light. If it's true, we'll format the hard drive:

(when green-light (format-drive))

If when was implemented as a function, it would always format the hard drive, because function parameters must be evaluated before entering the function itself.

It is clear that we need a way to control evaluation. Scheme's macros will let ut do exactly that.

So let's implement when as a hygienic macro.

#; mickey> (define-syntax when
             (syntax-rules ()
               ((when test expr ...)
                 (if test (begin expr ...)))))

That's it. To demonstrate that we control the evaluation, let's create a function with a side effect that prints to the console when it's evaluated.

#; mickey> (define (say-hello) (display "Hello\n"))
#; mickey> (say-hello)


#; mickey> (when #f (say-hello))

does not print anything, which is good. In contrast,

#; mickey> (when #t (say-hello))

does indeed print to the console.


Now let's try some examples with quasi-quotation. Since Scheme is a symbolic language, we can easily create syntax trees for languages like SQL by just using quotation. But sometimes we'll want to embed actual computations into them, so therefore we can use the specual unquote prefix with a comma.

Here is an example of just that.

#; mickey> (define (sql-get-user name)
             `(select * from user where name = ,name))

Note: There is currently a bug in the mickey REPL, so that quasiquotation requires an extra closing parenthesis to parse.
This bug is not present if you run this example from a file, though.

Running the function should be self-explanatory.

#; mickey> (sql-get-user "foo")
(select * from user where name = "foo")

Furthermore, sometimes we want to splice two lists together when we quote. We can do that by using unquote splice, or the ,@ prefix.

#; mickey> (define date '(2012 5 17))
#; mickey> date
(2012 5 17)
#; mickey> `(here is a date: ,@date)
(here is a date: 2012 5 17)

Lazy evaluation

Mickey Scheme also supports delayed -- or lazy -- evaluation. That is, computations that are not executed right away.

In fact, all languages that support evaluation control (for instance, via a macro facility) and first class closures should be able to implement lazy evaluation without any external library.

Let's create a list queue that contains some code we want to execute at a later time.

(define queue
  (list (delay (display "One! "))
        (delay (display "Three! "))
        (delay (display "Two! "))))

Now we want to execute the code in the list, but reordered so that it will print "One! Two! Three!":

(force (list-ref queue 0))
(force (list-ref queue 2))
(force (list-ref queue 1))

This outputs:

One! Two! Three!


Mickey Scheme has an experimental library for first-class environments. It bascially implements the API from MIT Scheme, with a few missing features.

Here is an example on how to use it.

First, let's start up mickey with an empty environment (the -z argument). An empty environment means that the REPL only has one bound definition, that of import.

$ ./mickey -z
#|                                                                 _
   Mickey Scheme (C) 2011-2012 Christian Stigen Larsen              \
   4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)   /\
   Readline 4.2                                                    /  \_

   To quit, hit CTRL+D or type (exit).  Use (help) for an

Now, let's just make sure that we don't have any binding for foo.

#; mickey> foo
Unbound definition: foo

Let's import the (mickey environment) library and define foo in a new environment that is a child of the current one.

#; mickey> (import (mickey environment))
#; mickey> (define sub-environment
                       (define foo 123)))

We still should not see foo in the current environment:

#; mickey> foo
Unbound definition: foo

But it should be available in the environment stored in sub-environment.

#; mickey> (environment-bindings sub-environment)
((foo 123))

Let's import (scheme write) so we can call display and the newline function from (scheme base).

#; mickey> (import (scheme write))
#; mickey> (import (only (scheme base) newline))

Now, let's evaluate an expression in the child environment to print the value of foo:

#; mickey> (environment-eval 
               (display foo)
               (newline)) sub-environment)
#; mickey> ^D

So, what's the point with first class environments? You can use it do to stuff like implementing your own module system. The reason they are not part of standard Scheme is because they make it very difficult to reason about what a program does just by reading the code.

Therefore, implementations are allowed to implement them on their own.


It's possible to write libraries in C and call them from Mickey Scheme.

Since Mickey does not use a build system yet, the process is a little bit cumbersome.

First you write a small library in C. Let's say we want to be able to call the uname(3) function from Mickey. We'll just write some wrapper code in C:

#include <sys/utsname.h>
#include "mickey.h"

extern "C" cons_t* proc_uname(cons_t* args, environment_t* env)
  struct utsname p;

  if ( uname(&p) != 0 )
    return boolean(false);

  // Return utsname as an associative list
    cons(cons(symbol("sysname",  NULL), cons(string(p.sysname))),
    cons(cons(symbol("nodename", NULL), cons(string(p.nodename))),
    cons(cons(symbol("release",  NULL), cons(string(p.release))),
    cons(cons(symbol("version",  NULL), cons(string(p.version))),
    cons(cons(symbol("machine",  NULL), cons(string(p.machine))))))));

Note that if you use a C++ compiler with the above code, you must prefix the function with extern "C" to avoid the infamous C++ name-mangling.

To compile this, do something à la

gcc -shared -fPIC -I<mickey path> mickey-uname.c \
    -L<mickey path> -lmickey -o libmickey-uname.so

This should give you a libmickey-uname.so file. To load this file from Mickey, we have to start mickey and then import the (posix dlopen) library.

csl$ ./mickey
#|                                                                 _
   Mickey Scheme (C) 2011-2012 Christian Stigen Larsen              \
   4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)   /\
   Readline 4.2                                                    /  \_

   To quit, hit CTRL+D or type (exit).  Use (help) for an

#; mickey> (import (posix dlopen))

Let's load the library using the dlopen options RTLD_NOW and RTLD_LOCAL. You can omit the options to use default dlopen mode. I'm just showing you how to specify several options.

#; mickey> (define lib-uname (dlopen "libmickey-uname.so" 'now 'local))

Now lib-uname contains a handle to the library.

#; mickey> lib-uname
#<pointer 'dynamic-shared-library-handle' 0x7ffb63436460>

Let's get a reference to the proc_uname function and bind that to the variable uname:

#; mickey> (define uname (dlsym lib-uname "proc_uname"))

If dlsym fails, it will return #f. Let's see if it worked:

#; mickey> uname
#<closure 0x7ffb6343b550>

It did! Let's try executing it.

#; mickey> (uname)
 ((sysname "Darwin") (nodename "Christians-mac-8.local") (release "12.0.0")
 (version "Darwin Kernel Version 12.0.0: Sun Jun 24 23:00:16 PDT 2012;
 root:xnu-2050.7.9~1/RELEASE_X86_64") (machine "x86_64"))

It returns the struct as an a-list, so we can do

#; mickey> (assv 'version (uname))
(version "Darwin Kernel Version 12.0.0: Sun Jun 24 23:00:16 PDT 2012;


#; mickey> (assv 'machine (uname))
(machine "x86_64")


Custom Scheme-libraries in R7RS

To create a small library, use the define-library form as described in the R7RS draft over at http://scheme-reports.org

As an example, put this in a file foo/bar.scm:

(define-library (foo bar)
  (import (only (scheme base) car cdr define))
  (export first last head tail)
    (define (first x) (car x))
    (define (last x) (cdr x))
    (define (head x) (car x))
    (define (tail x) (cdr x))
    (define (rest x) (cdr x))))

Mickey needs to know where to find this library, so you need to add an entry in lib/index.scm:

; ... ((foo bar) "foo/bar.scm") ; ...

There really isn't more to it. You can take a look in the lib/ directory for some ideas. Note that you have to import primitive functions to be able to use them, even core forms such as define. Also note that even though we define rest above, it is not exported, and thus not available outside of the library.

The library system mostly complete, but lacks some intricate features that can be found in R7RS. One thing is that I don't think Mickey gracefully handles circular dependencies.

(define-library ...)

Note that in R7RS, you cannot simply open up a REPL and start writing your own library using (define-library (foo schmoo) ...). That means, the standard doesn't say much about it, so it is most likely up to implementors.

I think this may turn out to be surprising to many newcomers to R7RS, so I will probably allow it in Mickey.