#+Title: Relish: Rusty Expressive LIsp SHell
#+Author: Ava Hahn

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:
#+BEGIN_SRC lisp
  (top-level element1 "element2" 3 (nested 2 5 2) (peer-nested))
#+END_SRC

As in memory
#+BEGIN_SRC
top-level -> element1 -> "element2" -> 3 -> [] -> [] ->
                                            |     ^-> peer-nested ->
                                            \-> nested -> 2 -> 5 -> 2 ->
#+END_SRC

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:
#+BEGIN_SRC lisp
(dothing arg1 arg2 arg3)
#+END_SRC

Function calls are executed as soon as the tree is evaluated. See the following example:
#+BEGIN_SRC lisp
(add 3 (add 5 2))
#+END_SRC

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.

#+BEGIN_SRC lisp
  ;; 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))
#+END_SRC

**** 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.

#+BEGIN_SRC lisp
  (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
#+END_SRC

**** 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:

#+BEGIN_SRC lisp
  ;; 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
#+END_SRC

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:
#+BEGIN_SRC lisp
  (circuit
     (eq? (exit-code my-shell-command) 0)
     (get-state-flag global-state)
     (eq? (some-big-calculation) result))
#+END_SRC

**** 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:
#+BEGIN_SRC lisp
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>
#+END_SRC

*** Quote and Eval
As stated previously: Lisp, and consequently Relish, is homoiconic. This means that code can be passed around (and modified) as data.
This allows us to write self programming programs, or construct entire procedures on the fly. The primary means to do so are with *quote* and *eval*.
The *quote* function allows data (code) to be passed around without evaluating it. It is used to pass unevaluated code around as data that can then be evaluated later.
To be specific, typing ~(a)~ usually results in a symbol lookup for ~a~, and then possibly even a function call. However, if we *quote* ~a~, we can pass around the symbol itself:

#+BEGIN_SRC lisp
  (quote a)         ;; returns the symbol a
  (quote (add 1 2)) ;; returns the following tree: (add 1 2)
  (q a)             ;; returns the symbol a
#+END_SRC
(note that ~quote~ may be shortened to ~q~)

We can use this to build structures that evaluate into new data:
#+BEGIN_SRC lisp
  (let ((mylist (q (add)))                  ;; store a list starting with the add function
        (myiter 0))                         ;; store an iterator starting at 0
     (while (lt? myiter 4)                  ;; loop until the iterator >= 4
       (inc myiter)                         ;; increment the iterator
       (def mylist '' (cons mylist myiter)) ;; add to the list
       (echo mylist))                       ;; print the current state of the list
     (echo (eval mylist)))                  ;; print the eval result
#+END_SRC

Notice the final body in the let form: ~(echo (eval mylist))~
The above procedure outputs the following:
#+BEGIN_EXAMPLE
  (add 1)
  (add 1 2)
  (add 1 2 3)
  (add 1 2 3 4)
  10
#+END_EXAMPLE

*** 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:
#+BEGIN_SRC lisp
  (lambda (x y) (add x y))
  ;;       |     ^ this is the function body
  ;;       +-> this is the argument list
#+END_SRC

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:
#+BEGIN_SRC lisp
  ((lambda (x y) (add x y)) 1 2)
#+END_SRC
This call returns ~3~.

Here is the lambda bound to a variable inside a let statement:
#+BEGIN_SRC lisp
  (let ((adder (lambda (x y) (add x y)))) ;; let form contains one local var
            (adder 1 2))                  ;; local var (lambda) called here
#+END_SRC

*** Defining variables and functions
In Relish, both variables and functions are stored in a table of symbols.
All Symbols defined with ~def~ are *GLOBAL*. The only cases when symbols are local is when they are defined as part of *let* forms or as arguments to functions.
In order to define a symbol, the following arguments are required:
- A name
- A docstring (absolutely required)
- A list of arguments (only needed to define a function)
- A value

Regarding the *value*: A function may be defined with several trees of code to execute.
In this case, the value derived from the final form in the function will be returned.

#+BEGIN_SRC lisp
  (def my-iter 'an iterator to use in my while loop' 0) ;; a variable
  (def plus-one 'adds 1 to a number' (x) (add 1 x))     ;; a function
  (def multi-func 'example of multi form function'
       (x y)              ;; args
       (inc my-iter)      ;; an intermediate calculation
       (add x y my-iter)) ;; the final form of the function. X+Y+MYITER is returned
#+END_SRC

Make sure to read the *Configuration* section for information on how symbols are linked to environment variables.

**** Naming conventions
- Symbol names are case sensitive
- Symbols may contain alphanumeric characters
- Symbols may contain one or more of the following: - _ ?
- The idiomatic way to name symbols is ~all-single-case-and-hyphenated~

**** Undefining variables and functions
Removing a symbol consists of a call to ~def~ with no additional arguments:

#+BEGIN_SRC lisp
(def my-iter 'an iterator' 0)
(inc my-iter) ;; my-iter = 1
(def my-iter) ;; removes my-iter
(inc my-iter) ;; UNDEFINED SYMBOL ERROR
#+END_SRC

** Builtin functions
As opposed to listing every builtin function here, it is suggested to the user to do one of two things:
- Call ~env~ from a fresh shell: ~(env)~
  This will output all variables and functions defined
- Read the std library declaration code:
  file:src/stl.rs

** Documentation
*** Tests
Most of the tests evaluate small scripts (single forms) and check their output.
Perusing them may yield answers on all the cases a given builtin can handle.
file:tests/

*** Help function
Relish is self documenting. The *help* function can be used to inspect any variable or function.
It will show the name, current value, docstring, arguments, and definition of any builtin or user defined function or variable.
#+BEGIN_EXAMPLE
> (help my-adder)
NAME: my-adder

ARGS: 2 args of any type

DOCUMENTATION:

adds two numbers

CURRENT VALUE AND/OR BODY:
args: x y 
form: ((add x y))
#+END_EXAMPLE
#+BEGIN_EXAMPLE
> (help CFG_RELISH_ENV)                                                                                           <
NAME: CFG_RELISH_ENV

ARGS: (its a variable)

DOCUMENTATION:

my env settings

CURRENT VALUE AND/OR BODY:
true
#+END_EXAMPLE

Every single symbol in Relish can be inspected in this way, unless some third party developer purposefully left a docstring blank.

*** Snippets directory
The *snippets directory* may also yield some interesting examples.
Within it are several examples that the authors and maintainers wanted to keep around but didnt know where.
It is sort of like a lint roller.
It also contains considerably subpar implementations of Relish's internals that are kept around for historical reasons.

*** Userlib
The *Userlib* was added as a script containing many valuable functions such as ~set~ and ~prepend~.
You can use it by loading it in your shell config (See file:snippets/basic_minimal_configuration.rls for more info).

** Easy patterns
This section contains examples of common composites of control flow that can be used to build more complex or effective applications 
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
#+BEGIN_SRC lisp
          ;;  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
#+END_SRC

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.

*** 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.
  #+BEGIN_SRC lisp
    ;; 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))
  #+END_SRC

*** if-set?
One common pattern seen in bash scripts and makefiles is the set-variable-if-not-set pattern.
#+BEGIN_SRC shell
  MYVAR ?= MY_SPECIAL_VALUE
#+END_SRC
Translated, can be seen below 
#+BEGIN_SRC lisp
    (if (set? myvar)
        () ;; no need to do anything... or add a call here
        (def myvar "MY_SPECIAL_VALUE"))
#+END_SRC

Alternatively this combination can be used to process flags in a script or application:
#+BEGIN_SRC lisp
  (if (set? myflag)
      (process-flag myflag)
      ())
#+END_SRC


** 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 false): when true, enables POSIX style job control.
- CFG_RELISH_ENV (default true): when true, interpreter's variable table and environment variable table are kept in sync.
- CFG_RELISH_L_PROMPT (default 'λ'): a function that is called with no arguments to output the left hand of the prompt
- CFG_RELISH_R_PROMPT (default ''): a function that is called with no arguments to output the right hand of the prompt
- CFG_RELISH_PROMPT_DELIMITER (default '>'): a function that is called with no arguments to output the delimiter separating prompt from user input

*** Prompt design
For an example of prompt design see file:snippets/mood-prompt.rls

*** 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.

#+BEGIN_SRC lisp
  (load "my-extra-library-functions.rls")
#+END_SRC

** Compilation 
#+BEGIN_SRC sh
  cargo build
#+END_SRC

** Testing
#+BEGIN_SRC sh
  cargo test
#+END_SRC

** Running (the main shell)
#+BEGIN_SRC sh
  cargo run src/bin/main.rs
#+END_SRC

* The codebase
** file:tests directory
Start here if you are new.

*** 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 and work to prove the functionality of builtin functions in the language.

** 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 data structures that the interpreter operates on.
Representation of code trees, traversals, and type annotations all live here.

*** lib: file:src/lib.rs
This defines a library that can be included to provide an interpreter interface within any Rust project.
The components defined here can certainly be used to support language development for other LISP (or non LISP) langauges.
Your project can use or not use any number of these components.

*** sym: file:src/sym.rs
This file contains all code related to symbol expansion and function calling.
The types defined in this file include SymTable, Args, Symbol, and more.

*** 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 stdlib functions).
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 lighter 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 Pre-alpha tasks
- Shell module
  - circuit casts int to bool automatically
  - pipe control flow construct
  - l and load functions for binaries, call for scripts
  - load-to-string function
  - ignore job control signals (if needed)
  - background processes
  - be able to list all background processes with j function
  - be able to fg a bg process
  - be able to bg and fg process (ctrl z?)
  - changedir/cd
  - pwd
  - call-with (let style fd redirection)
  - Documentation!
- logging library
- make const all the error messages
- make presentation on relish
  - lisp basics
    - s expressions
    - data structures
  - control flow
    - let, circuit, etc
  - homoiconicity
    - quote/eval
    - lambda
  - putting it all together
    - CI tests
    - repl configuration
  - self documenting
    - env
    - help
  - beyond lisp
    - POSIX
      - ergonomics of control flow as shell command
      - circuit -> pipe
      - let -> call-with
      - putting it all together
        - job control in shell
        - shell scripts
    - To infinity and beyond
      - NGINX modules
      - bootable?
  - give a pretty pastel white, pink and blue theme
  - store in repo after giving presentation
        
** TODO alpha tasks
- Can pass args to relish scripts (via interpreter)
- Can pass args to relish scripts (via command line)
- History length configurable
- Search delim configurable
- Escape sequences in strings
- Rename to Flesh
- master branch -> main branch
- Create a dedicated community channel on matrix.sunnypup.io
- get type function
- Lex function
- Read function (Input + Lex)

** TODO post-alpha tasks
- Post to relevant channels
- Custom ast pretty print
- Implement Compose for lambdas
  Document this in relevant readme sections
- File operations
  - read-to-string
  - write-to-file
- color control library
- emacs syntax highlighting and/or LSP implementation
- GNU Guix package

** TODO Second release tasks
- Network library
  - HTTP Client
  - TCP Stream client
  - UDP Client
  - TCP Listener
  - HTTP Listener
  - UDP Listener