plinth is a simple purely-functional Lisp-like language with an interpreter written in Python.

plinth features lexical closure, first-class functions, quoting, run-time macros, as well as all the standard math operations one could want.

To run plinth, you'll need a modern version of Python installed (developed on 2.7.x). Simply run python plinth.py from the root directory, and you'll be in business. You can load the in-progress standard library by passing its filename (stdlib.plinth) into the interpreter as an argument.

plinth supports lots of built-in functionality.

• define gives symbols values in the current scope. It takes as its arguments a symbol name and a value, and assigns the symbol to the given value.

• cons is the basic data-concatenation function in plinth. cons takes as its arguments two symbols, and returns them as a pair. The empty cons, (), is known as 'nil' and is used to mark the end of lists.

• quote (equivalently ') returns its only argument as a literal value.

• quasiquote (equivalently `) functions the same as quote while allowing selective evaluation of its quoted contents via unquote (equivalently ~), and list-replacement via unquote-splicing (equivalently ^).

• cond takes as its arguments a list of lists containing expressions. For every list, if the first element evaluates to #t, then the second element is returned. If the first element evaluates to #f, then the next list of expressions is tried in sequence until one that returns #t is found. If no expressions evaluate to #t, an error is raised and the result is undefined.

• and, or, and not evaluate their arguments using standard logical rules and return at the soonest possible moment. This means that, for example, if and is called without #f as its first argument, it short-circuits and returns its second argument without evaluating it. Hence, (and #t 3) returns 3. The same logic applies to all logical operators.

• lambda returns a function. It takes as its parameters a list of symbols to use as arguments, and an expression to execute over the given argument symbols. Functions are closed over the current scope, and carry their parent scope with them for their lifetime. Functions can receive variadic arguments by using the special ... postfix symbol after symbol names. For example, if a function's arguments are things ..., the function will have access to the symbol things as a list of all the symbols from that position onwards. If arguments are given as a list of symbol and expression, they are considered default arguments, and if not specified when the function is called, will receive the value of the specified expression as evaluated when the function was created.

• macro returns a macro. It takes as its parameters a list of symbols and a body to evaluate with the symbols replaced with the unevaluated parameters to the macro call. Used to extend the language by constructing syntax that is subsequently evaluated. Macros are always expanded using the values of the environment they're expanded in, and have the same creation argument semantics as functions.

• expand takes a macro and some arguments to be passed to it, and returns the expanded form of the macro using the given arguments.

• read takes a string and returns a list of expressions parsed from it.

• eval takes a list of expressions and evaluates them in the current scope.

• boolean?, list?, cons?, symbol?, string?, number?, integer?, float?, complex?, and function? all take a single argument, and return #t if the argument is of the requisite type, and #f otherwise.