tree-traversing LISP interpreter geared toward embeddability and system scripting

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Readme.org

Relish: Rusty Expressive LIsp SHell

Purpose statement
Goals
- Stretch Goals
Contact
How to use
Guide to codebase
- file:tests directory
- file:src directory
Current Status / TODO list

Note: this document is best read using a dedicated ORG mode editor

Purpose statement

The purpose of Relish is to create a highly portable, easy to integrate language that can be used in many environments.

Goals

Iterate on the ideas and designs that were tested with SHS https://gitlab.com/whom/shs
Create a usable POSIX shell
Create usable applications/scripts
To have quality code coverage
No unsafe code

Stretch Goals

Create an interpreter that can be booted on one or more SOCs
Create an interpreter that can be embedded in another application
Create UI bindings

Contact

Matrix chat: #vomitorium:matrix.sunnypup.io https://matrix.to/#/#vomitorium:matrix.sunnypup.io

How to use

Syntax

S-Expressions

Relish fits within the LISP family of languages alongside venerable languages like Scheme or Common Lisp. Lisps are HOMOICONIC which means that the code is data, and that there is a direct correlation between the code as written and the program as stored in memory. This is achieved through S-EXPRESSIONS. An S-Expression (or symbolic expression) is a tree of nested lists. Programs in Relish (and most other lisps) are written with S-Expressions, and are then represented in memory as trees of nested linked lists.

An example:

  (top-level element1 "element2" 3 (nested 2 5 2) (peer-nested))

As in memory

top-level -> element1 -> "element2" -> 3 -> [] -> [] ->
                                            |     ^-> peer-nested ->
                                            \-> nested -> 2 -> 5 -> 2 ->

Each node in memory has type information and potentially a cooresponding entry in a global symbol table.

Data types

Relish leverages the following data types:

Strings: delimited by ', ", or `
Integers: up to 128 bit signed integers
Floats: all floats are stored as 64 bit floats
Booleans: true or false
Symbols: an un-delimited chunk of text containing alphanumerics, -, _, or ?

Symbols and Functions can contain data of any type. there is no restriction on what can be set/passed to what….. However, internally Relish is typed, and many builtin functions will get very picky about what types are passed to them.

Calling a function

S-Expressions can represent function calls in addition to trees of data. A function call is a list of data starting with a symbol that is defined to be a function:

(dothing arg1 arg2 arg3)

Function calls are executed as soon as the tree is evaluated. See the following example:

(add 3 (add 5 2))

In this example, (add 5 2) is evaluated first, its result is then passed to (add 3 ...). In infix form: 3 + (5 + 2).

Control flow

If

An if form is the most basic form of conditional evaluation offered by Relish. It is a function that takes lazily evaluated arguments: a condition, a then clause, and an else clause. If the condition evaluates to true, the then clause is evaluated and the result returned. Otherwise the else clause is evaluated and the result is returned. If the condition evaluates to neither true nor false (a non-boolean value) a type error is returned.

  ;; simple condition
  (if true
      (echo "its true!")
      (echo "its false!"))

  ;; more advanced condition, with hypothetical data
  (if (get-my-flag global-state)
      (echo "my flag is already on!")
      (turn-on-my-flag global-state))

While

Another popular control flow structure is the while loop. This is implemented as a condition followed by one or more bodies that are lazily evaluated only if the condition is true. Like the if form, if the conditional returns a non-boolean value the while loop will return an error.

  (while (get-my-flag global-state) ;; if false, returns (nothing) immediately
    (dothing) ;; this is evaluated
    "simple token" ;; this is also evaluated
    (toggle-my-flag global-state)) ;; this is also evaluated

Let

Let is one of the most powerful forms Relish offers. The first body in a call to let is a list of lists. Specifically, a list of variable declarations that lookf like this: (name value). Each successive variable definition can build off of the last one, like this: ((step1 "hello") (step2 (concat step1 " ")) (step3 (concat step2 "world"))). In said example, the resulting value of step3 is "hello world". After the variable declaration list, the next for is one or more unevaluated trees of code to be evaluated. Here is an example of a complete let statement using hypothetical data and methods:

  ;; Example let statement accepts one incoming connection on a socket and sends one response
  (let ((conn (accept-conn listen-socket))                      ;; start the var decl list, decl first var
        (hello-pfx "hello from ")                               ;; start the var decl list, declare second var
        (hello-msg (concat hello-pfx (get-server-name)))        ;; declare third var from the second var
        (hello-response (make-http-response 200 hello-msg)))    ;; declare fourth var from the third, end list
    (log (concat "response to " (get-dst conn) ": " hello-msg)) ;; evaluates a function call using data from the first and third vars
    (send-response conn hello-response))                        ;; evaluates a function call using data from the first and fourth vars

Here you can see the usefulness of being able to declare multiple variables in quick succession. Each variable is in scope for the duration of the let statement and then dropped when the statement has concluded. Thus, it is little cost to break complex calculations down into reusable parts.

Circuit

Circuit is useful to run a sequence of commands in order. A call to circuit comprises of one or more forms in a sequence. All forms in the call to circuit are expected to evaluate to a boolean. The first form to evaluate to false halts the sequence, and false is returned. If all forms evaluate to true, true is returned.

Example:

  (circuit
     (eq? (exit-code my-shell-command) 0)
     (get-state-flag global-state)
     (eq? (some-big-calculation) result))

Not quite control flow

Several other functions use lazy evaluation of their arguments. The below list is non-exhaustive:

toggle
inc
dec

These functions are mentioned here for their use with control flow.

inc: increment a symbol by one
dec: decrement a symbol by one
toggle: flip a symbol from true to false, or vice versa

For more information on these functions consult the output of the help function:

λ (help toggle)
NAME: toggle

ARGS: 1 args of any type

DOCUMENTATION:

switches a boolean symbol between true or false.
Takes a single argument (a symbol). Looks it up in the variable table.
Either sets the symbol to true if it is currently false, or vice versa.

CURRENT VALUE AND/OR BODY:
<builtin>

TODO Quote and Eval

Lambda

Another form of homoiconicity is the anonymous function. This is a nameless function being passed around as data. It can be bound to a variable, or called directly. An anonymous function is created with the lambda function.

Here is an example of a lambda function:

  (lambda (x y) (add x y))
  ;;       |     ^ this is the function body
  ;;       +-> this is the argument list

The result of the lambda call is returned as a piece of data. It can later be called inline or bound to a variable.

Here is an example of an inline lambda call:

  ((lambda (x y) (add x y)) 1 2)

This call returns 3.

Here is the lambda bound to a variable inside a let statement:

  (let ((adder (lambda (x y) (add x y)))) ;; let form contains one local var
            (adder 1 2))                  ;; local var (lambda) called here

TODO Defining variables and functions

TODO Anatomy

TODO Naming conventions

TODO Undefining variables and functions

Easy patterns

This section can serve as a sort of cookbook for a user who is new to leveraging LISP languages or unsure of where to start with relish. More ideas may be explored in the file:snippets directory of this project. The author encourages any users to contribute their own personal favorites not already in this section either by adding them to the file:snippets folder, or to extend the documentation here.

while-let combo

          ;;  myiter = (1 (2 3 4 5 6))
          (def myiter 'iterator over a list' (head (1 2 3 4 5 6)))

          ;; iterate over each element in mylist
          (while (gt? (len (cdr myiter)) 0) ;; while there are more elements to consume
            (let ((elem (car myiter)) ;; elem = consumed element from myiter
                  (remaining (cdr myiter))) ;; remaining = rest of elements
              (echo elem) ;; do a thing with the element, could be any operation
              (def myiter (head remaining)))) ;; consume next element, loop

The while-let pattern can be used for many purposes. Above it is used to iterate over elements in a list. It can also be used to receive connections to a socket and write data to them.

TODO main loop application

state switch (while-toggle)
state calculation

TODO callback model via eval and passed-in functions

TODO quote/eval for pseudo-anonymous pseudo-functions

TODO short-circuit guard

circuit example
while-not-circuit-do-more-work

let destructuring

let is very useful for destructuring complex return types. If you have a function that may return a whole list of values you can then call it from let to consume the result data. In this example a let form is used to destructure a call to head. head returns a list consisting of (first-element rest-of-list) (for more information see (help head)). The let form starts with the output of head stored in head-struct (short for head-structured). The next variables defined are first and rest which contain individual elements from the return of the call to head. Finally, the bodies evaluated in the let form are able to operate on the head and the rest.

  ;; individually access the top of a list
  (let ((head-struct (head (1 2 3))
      (first (car head-struct))
      (rest (cdr head-struct)))
     (echo "this is 1: " first)
     (echo "this is 2, 3: " rest))

if-set?

One common pattern seen in bash scripts and makefiles is the set-variable-if-not-set pattern.

  MYVAR ?= MY_SPECIAL_VALUE

Translated, can be seen below

    (if (set? myvar)
        () ;; no need to do anything... or add a call here
        (def myvar "MY_SPECIAL_VALUE"))

Alternatively this combination can be used to process flags in a script or application:

  (if (set? myflag)
      (process-flag myflag)
      ())

TODO Builtin functions

TODO Env function

TODO Documentation

TODO Tests

TODO Help function

TODO Snippets directory

Configuration

By default Relish will read from ~/.relishrc for configuration, but the default shell will also accept a filename from the RELISH_CFG_FILE environment variable. See file:snippets/basic_minimal_configuration.rls for an example of a basic minimal configuration file.

The configuration file

The configuration file is a script containing arbitrary Relish code. On start, any shell which leverages the configuration code in the config module (file:src/config.rs) will create a clean seperate context, including default configuration values, within which the standard library will be initialized. The configuration file is evaluated and run as a standalone script and may include arbitrary executable code. Afterwards, configuration values found in the variable map will be used to configure the standard library function mappings that the shell will use. Errors during configuration are non-terminal. In such a case any defaults which have not been overwritten will remain present.

Important points to note

When the configuration file is run, it will be run with default configuration values.
The user/script interpreter will be run with the standard library configured to use the previously defined configuration variables.
The standard library will then be re-processed and re-added to the symbol table with new configuration.
Variables and functions defined during configuration will carry over to the user/script interpreter, allowing the user to load any number of custom functions and variables.

Configuration Values

CFG_RELISH_POSIX (default 0): when on, enables POSIX style job control.
CFG_RELISH_ENV (default 1): when on, interpreter's variable table and environment variable table are kept in sync.
CFG_RELISH_PROMPT (default (echo "λ ")): A function definition which is called in order to output the prompt for each loop of the REPL. This function will be reloaded each REPL loop and will be called by the interpreter with no arguments.

Further configuration

Further configuration can be done by loading scripts that contain more functions and data to evaluate. Variables and functions defined in an external script loaded by your interpreter will persist in the symbol table.

  (load "my-extra-library-functions.rls")

Compilation

  cargo build

Testing

  cargo test

Running (the main shell)

  cargo run src/bin/main.rs

Guide to codebase

file:tests directory

Start here if you are new. Most of these files have unimplemented tests commented out in them. Contributions that help fill out all of these tests

Eval tests: file:tests/test_eval.rs

These are particularly easy to read and write tests.

Func tests: file:tests/test_func.rs

You can consider these to extend the eval tests to cover the co-recursive nature between eval and func calls.

Lex tests: file:tests/test_lex.rs

These tests verify the handling of syntax.

Lib tests: (tests/test_lib*)

These tests are unique per stdlib module.

file:src directory

This directory contains all of the user facing code in relish. Just a few entries of note:

segment: file:src/segment.rs

This file lays out the ~~spiritual~~ ~~theological~~ ~~ideological~~ ~~theoretical~~ mechanical underpinnings of the entire interpreter. The entire LISP machine centers around a singlet or pairing of datum. The Ctr datatype provides an abstraction for which any type of data, including a Seg can be a datum. The Seg datatype provides a mechanism to hold a single datum or a pair of datum. It is implemented as two Ctr~s: ~car and cdr. A primitive type system is provided through the Rust Enum datatype. A number of utility functions follow.

lib: file:src/lib.rs

This defines a library that can be included to provide an interpreter interface within any Rust project. It includes the core interpreter mechanisms, full stdlib, and the configuration system. Your project can use or not use any number of these components. They can certainly be used to support language development for other LISP machines, or even other languages.

config: file:src/config.rs

This file contains default configuration values as well as functions which load and run the configuration file script. For more information see the configuraiton section above in this Readme.

stl: file:src/stl.rs

This defines the static_stdlib function and the dynamic_stdlib function. The static_stdlib function loads all symbols in the standard library which do not need further configuration into the symbol table. The dyanmic_stdlib function loads all symbols in the standard library which do need configuration into the symbol table. The dynamic_stdlib function uses variables saved in the symbol table to configure the functions and variables it loads. For more information see file:src/config. Any new addition to the stdlib must make its way here to be included in the main shell (and any other shell using the included get_stdlib function). You may choose to override these functions if you would like to include your own special functions in your own special interpreter, or if you would like to pare down the stdlib to a small minimal subet of what it is.

You can view the code for standard library functions in file:src/stl/.

bin: file:src/bin/

This contains any executable target of this project. Notably the main shell file:src/bin/main.rs.

Current Status / TODO list

Note: this section will not show the status of each item unless you are viewing it with a proper orgmode viewer. Note: this section only tracks the state of incomplete TODO items. Having everything on here would be cluttered.

TODO list contains via circuit

TODO Input function

TODO Lex function

TODO Read function (Input + Lex)

TODO Caps function (list functions in table)

TODO Default prompt says "<minimum shell> LAMBDA:"

TODO Load (load a script) function

Pull/Refactor the logic out of the configure functions. Optionally return a list of new variables and/or functions? Will need a concatenate function for tables