GDLisp

A Lisp-style language and REPL written using GDScript and Godot 3.3.

Quickstart

Run the main scene. Start writing Lisp-style stuff.

Integration into Godot games/apps

Take the gdlisp.gd file and drop it into your project. Create a new gdlisp object.

A dictionary of initialization variables can be passed as an argument (e.g. var gdlisp = GDLISP.new({"some-identifier": myobject})). If you want to be able to modify objects from GDLisp, these initialization variables must be some sort of reference (i.e. not a primitive).

Call gdlisp.parse_string('(print "hello world")) to start execution. The parse_string method will return an array of return values, depending on how many root S-expressions were passed.

Motivation

It seemed like a cool idea at the time. I also wanted to write a language that had goto statements in it. This website was a big help for the initial interpreter.. I tried to avoid referencing any explicit Make a Lisp (MAL) implementations.

Implementation notes

• Arrays and Dictionaries are wrapped in the GDLArray and GDLDictionary classes respectively. This was done so that set/get can be called on Arrays and Dictionaries in GDLisp.

TODO

• finish implementing macros
• finish implementing a way to access lists and tables
• allow for compilation to GDScript
• self host the interpreter

Syntax

Define a variable (def <identifier> <value>)

Creates a variable in the current scope and sets it to the specified value. Can shadow outer variables if used in an inner scope.

The below example will set variable x to value 0 in the current scope.

(def x 0)

Update a variable (= <identifier> <value>)

Finds a variable in the current or any outer scope (in that order) and sets it to the specified value.

The below example will set variable x to value 10.

(= x 10)

Create a list [ <value> ... ] or (list <value> ... )

Creates and returns a list with the specified values. An empty list can be created if no values are specified.

The below example creates a list with the values 1, 2, and 3 and stores it in variable x.

(def x [1 2 3])

Create a table { <key> <value> ... } or (table <key> <value> ...)

Creates and returns a table (aka a dictionary) with the specified values. An empty table can be created if no values are specified.

The below example creates a table with the key/value pair "hello": "world" and stores it in variable x.

(def x { "hello" "world" })

Create a lambda expression (lam [<parameters>] <expression> ...)

Creates and returns a lambda expression that takes the specified parameters and runs the specified expressions. A lambda expression does not need any parameters but must have at least 1 expression.

The below example creates a lambda expression and assigns it to the variable double. The lambda expression is then used to double the value 2. The returned value will equal 4.

(def double (lam [ x ] (+ x x)))

(double 2)

Create a macro (macro [<parameters>] <expression> ...)

Not yet implemented.

Call a method from a raw value (raw <object> <method> <optional param> ...)

Gets an object and calls the callv function on it.

If statements (if <condition> <expression if true> <expression if false>)

Evaluates the condition and selects an expression to execute.

The below example checks to see if 1 is less than 2. If this is true, the double lambda expression will be executed, otherwise the built-in print function will be called.

(if (< 1 2) (double 1) (print "this is not possible"))

While statements (while <condition> <expression> ...)

Executes the given expressions while the condition is true. There must be at least 1 expression to execute.

The below example creates and increments a variable x from 0 to 5.

(def x 0)
(while (<= x 5) (= x (+ x 1)))

Do block (do <expression> ...)

Executes the given expressions. There must be at least 1 expression to execute. Do blocks create an implicit inner scope.

The below example creates a variable x and sets it equal to 0. Then the do block will shadow the variable x and set it equal to 10. Outside of the do block, the built-in function print will print the value of x, which will be 0.

(def x 0)
(do (def x 10))
(print x)

Label (label <identifier>)

Not yet implemented. Marks the current expression as a goto location.

Goto (goto <label>)

Not yet implemented. Moves execution to the specified label. The label must be in the same scope or an outer scope. If the label has not be interpreted yet, then the interpreter will conduct a breadth-first search for the label.