LispSyntax.jl: A clojure-like lisp syntax for julia

This package provides a lisp-to-julia syntax translator with convenience macros that let you do this:

lisp"(defn fib [a] (if (< a 2) a (+ (fib (- a 1)) (fib (- a 2)))))"
@test lisp"(fib 30)" == 832040
@test fib(30)        == 832040

LispSyntax.jl is implemented as an expression translator between lisp/clojure-like syntax and julia's AST. Julia's compiler, JIT and multiple-dispatch infrastructure is used for code generation and execution. Because of this, LispSyntax.jl is not really clojure or lisp in most meaningful ways. The semantics are entirely julia-based (which are very similar to scheme/lisp in many ways). The net result is that LispSyntax.jl is really an alternative S-expression-like syntax for julia, not an implemention of clojure or lisp.

Special Forms

• (def symbol init)
• (quote form)
• (defn symbol [param*] expr*)
• (defmacro symbol [param*] expr*)
• (lambda [param*] expr*)
• (fn [param*] expr*)
• (let [binding*] expr*)
• (global symbol*)
• (while test expr*)
• (for [binding*] expr*)
• (import package*)

Notable Differences

• Symbol names cannot have -, *, /, ? ... - Julia symbol naming is used for everything, as a result, Julia syntax restrictions are maintained in LispSyntax.jl.
• Reference to global variables in function scopes - Julia requires declaration of global symbols that are referenced in function scope. Because of this functions need to declare which symbols are global. This is done via the special form (global symbol*).
• Binding forms not implemented - Clojure has very awesome destructuring binds that can used in most special forms requiring bindings (e.g. let, fn parameter lists, etc.). This is not currently implemented.
• Lack of loop/recur - Currently, this is not implemented. As with Clojure, julia does not currently support TCO, so something like this may be needed (but a macro-implementation of tail call rewriting may be more appropriate for julia).
• Optional typing - Currently not implemented.
• Named functions are julia methods - For efficiency, functions defined with defn are translated to normal julia function expressions. This means the act as named lambdas in local scope.
• Method definition - Also not currently implemented. If implemented it will probably not be a full implementation of Clojure's sophisticated dispatch system.
• Macros differences - Macros defined in LispSyntax.jl look like standard Lisp macros but because expressions are special objects in julia, S-expressions returned from macros require a special translation step to generate julia expression trees. The result is that LispSyntax.jl macros are directly translated into Julia macros and must be called via special syntax (e.g. (@macro expr)). Macro hygiene follows the Julia approach of hygenic-by-default with explicit escaping using esc. This is the opposite of Clojure's macros which use explicit hygiene with specially named variables.
• Julia's string macro dispatch not supported (yet) - for macros like @r_str which in Julia can be called via r"", it is currently necessary to call these via standard macro syntax: (@r_str "string")

REPL Mode

LispSyntax.jl provides a convenience REPL, alleviating one from having to type lisp"( ... )" for each top level expression. In order to use REPL mode, simply initialize it:

julia> using LispSyntax
julia> LispSyntax.init_repl()
REPL mode Lisp Mode initialized. Press ) to enter and backspace to exit.

At this point, type ), and you're ready to Lisp:

jλ> (* 2 (reduce + (: 1 6)))
42
jλ> (defn fib [a] 
      (if (< a 2) 
        a 
        (+ (fib (- a 1)) (fib (- a 2)))))
fib (generic function with 1 method)
jλ> (fib 10)
55

To return to the Julia prompt, simply type the backspace type or Ctrl-C. Once there, you'll still have access to the fuctions you defined:

julia> fib
fib (generic function with 1 method)
julia> fib(10)
55

You may also create a customized REPL.

TODO

• Support for exceptions: this is straight forward but not currently implemented.
• Optional typing to support method definition
• Structs and aggregate types
• Special dispatch for string macro forms
• Modules
• import vs. using vs. include -- only using is currently implemented and confusingly, it matches Clojure's import form.
• varargs and named arguments