helvm / SandScript

A lisp dialect written in PureScript

SandScript

A lisp-like language that runs on node, written in Purescript (which is written in Haskell).

Using

First make sure you have purescript (>= 0.8.5) and pulp (>= 8.1.0) installed, as well as node, npm and bower.

Run bower install then pulp run --interactive to get a REPL.

Alternatively, you can interpret a file with pulp run $(cat <filename>).

Building the interpreter

pulp build --optimise --to ssc.js
sed -i '1s/^/#!\/usr\/bin\/env node\n/' ssc.js
chmod +x ssc.js

Syntax

Everything is an S-expression. Quoted lists are supported.

Values are immutable.

Defining variables is done with def: (def x 3)

Defining functions is similar:

(def (f x y) (+ x y)

(def (compose f g)
  (lambda (x) (f (g x))))

Semantics

Data types

Primitive types come in the following flavors:

1. Atoms => 'atom
2. Strings => "hello world"
3. Bools => True, False
4. Ints => 2, -2

There's one type of collection, a linked list. List literals can be made as follows: '(1 True "string").

Builtin functions:

1. + add 2 or more numbers
2. - subtract 2 or more numbers
3. * multiply 2 or more numbers
4. / divide 2 or more numbers
5. % mod 2 or more numbers
6. = check equality between numbers
7. < check if the first number is less than the second
8. > check if the first number is greater than the second
9. <= check if the first number is greater than the second
10. >= check if the first number is less than the second
11. /= check equality between numbers
12. && truth-functional and
13. || truth-functional or
14. string=? check equality between strings
15. string>? check if the first string is greater than the second
16. string<? check if the first string is less than the second
17. string>=? check if the first string is less than the second
18. string<=? check if the first string is greater than the second
19. head get the first element in a nonempty list
20. tail get the tail of a list
21. cons add an element to the front of the given list
22. eqv check equality of two terms of the same type
23. == same as eqv

Hacking

While based on Write Yourself A Scheme, there are quite a few differences:

1. Instead of using Refs to store the environment, the environment is stored in a StateT transformer monad.
2. Instead of using an association list as the environment, StrMaps are used
3. No support for dotted lists
4. "IO primitives" are handled by the REPL as directives instead of given as language primitives.

In particular, the base monad for the transformer stack is left polymorphic: The REPL uses Aff to deal with user input, and the source-code file interpreter uses Eff. In principle this means one could use, for example, the Trampoline monad at the base to handle stack overflows in the language (which were a serious problem in a previous version), though I haven't tested this yet.