fomoscript

Toy scripting language, built with Rust

• 0 dependencies*
• 1 file
• no_std with alloc

Only a few days old. Not production ready. The goal is to use it in Fomos as a shell.

* except log, doesn't count ;)

Examples

Simple script

java { let x = 0 while x<5 { x = x+1 } x }

returns 5

Support of higher order functions

java { let x = 0 let f = (e) => {e+1} let g = (f,e) => f(e) g(f,x) }

returns 1

Use in Rust

Parse and evaluate a script:

rust let result = fomoscript::parse_eval("{ let x = 0 while x<5 { x = x+1 } x }");

The result will have this type.

Go see the tests for more examples.

parse_eval is a high level function hiding the lower level Ctx.

You can explicitly instantiate an interpreter called Ctx to implement a REPL or explore/modify the state during execution.

By default, there is no side effect possible from the script during eval (except inside ctx)

You can insert native rust closure with (or without) side effects into the Ctx, and use it from there. Example with the print function:

```rust use fomoscript::*; let code = r#" { my_print(1+1) } "#;

let mut ctx = Ctx::new(); ctx.insert_code(code);

let printclosure = Rc::new(|a: N, _, _, _| { println!("{}", a.tostr()); N::Unit }); ctx.setvarabsolute("myprint", N::FuncNativeDef(Native(printclosure)));

let expr = ctx.parsenextexpr().unwrap(); let _ = eval(&expr, &mut ctx); ```

REPL

For simplicity, std is used here, but you can replace it with any input and output impl.

rust use fomoscript::*; let mut ctx = Ctx::new(); let mut buffer = String::new(); loop { buffer.clear(); std::io::stdin().read_line(&mut buffer).unwrap(); ctx.insert_code(&buffer); while let Ok(parent) = ctx.parse_next_expr() { let res = eval(&parent, &mut ctx); println!("> {:?}", res); } }

Cruelly missing

• Arrays
• Months of work
• Pattern matching
• Javascript-like objects (we just have Number and String 😱)
• Error handling (now it just UB if something goes wrong)
• Escape characters in quoted strings

Also the inner workings are not very rust-like, no unsafe though ;) Should be panic free during eval. Don't trust the parser just yet.

Features

• [x] String type
• [x] Number type (f64)
• [x] Scoped variable assignment
• [x] Binary operators +,-,/,*,>,<,==,!=,&,|
• [x] Operator precedence
• [x] Higher order function
• [x] Control flow if/else/while
• [x] Custom native function
• [x] Anonymous function call$
• [x] REPL example

Performance

Parsing is instantaneous (50+GB/sec).

Evaluation is slow, but reasonable for scripting:

• Worst case 1:1000 compared to native
• Common case 1:20 when using native functions reasonably.

See for yourself with cargo bench

Unstructured number crunching will stay slow. Typed arrays (like in js) could be added in the future for fast structured operation.

Fun facts

Everything is an expression. For instance if/else acts as a ternary operator.

let x= if 1 99 else 45

now x is 99

When there is a doubt, the interpreter defaults to N::Unit. For instance let's not put an else branch:

let x= if 0 1

x is now N::Unit

No parenthesis needed for the if condition or body, the previous expression is equivalent to:

rust let x = if 0 { 1 } else { N::Unit }

Same goes for while, it returns N::Unit if it never runs the body, or the last body expression if it runs at least once.

Same goes for brackets :

let x = {1 2 3} is equivalent to let x = {3} or let x = 3 or

let x = { 1 2 3 }

There is no parenthesis, use brackets to force factorization and precedence.

\n is just a whitespace like space. It doesn't separate statements more than space, unlike most languages.

Boolean operation automatically cast operand to bool (lookup to_bool to see how)

the (and,or) operators are (&,|)

1 & 0 evaluate to 0

1 | 0 evaluate to 1

No bitwise operation yet.