first draft of partial application, is working in easy cases

src/main.rs

@@ -76,7 +76,9 @@ pub fn run(src: &'static str) {
 pub fn main() {
     env::set_var("RUST_BACKTRACE", "1");
     let src = "
+ fn add2 (x, y) -> add (x, y)   
 
+ add2 (_, 2) (2)
 
     ";
     run(src);

src/parser.rs

@@ -203,7 +203,7 @@ impl fmt::Display for Ast {
                     .collect::<Vec<_>>()
                     .join("\n")
             ),
-            Placeholder => todo!(),
+            Placeholder => write!(f, "Placeholder"),
             LBox(_name, _rhs) => todo!(),
             Match(value, clauses) => {
                 write!(

src/value.rs

@@ -52,6 +52,7 @@ impl LFn {
     }
 
     pub fn chunk(&self, arity: u8) -> &Chunk {
+        // println!("Getting chunk of {arity} from {:?}", self);
         match self {
             LFn::Declared { .. } => unreachable!(),
             LFn::Defined { chunks, .. } => &chunks.iter().find(|(a, _)| *a == arity).unwrap().1,
@@ -68,9 +69,9 @@ impl LFn {
 
 #[derive(Clone, Debug, PartialEq)]
 pub struct Partial {
-    args: Vec<Value>,
+    pub args: Vec<Value>,
-    name: &'static str,
+    pub name: &'static str,
-    function: Value,
+    pub function: Value,
 }
 
 #[derive(Clone, Debug)]

src/vm.rs

@@ -2,7 +2,7 @@ use crate::base::BaseFn;
 use crate::compiler::{Chunk, Op};
 use crate::parser::Ast;
 use crate::spans::Spanned;
-use crate::value::{LFn, Value};
+use crate::value::{LFn, Partial, Value};
 use chumsky::prelude::SimpleSpan;
 use imbl::{HashMap, Vector};
 use num_traits::FromPrimitive;
@@ -705,7 +705,19 @@ impl Vm {
                     self.ip += 1;
                 }
                 Partial => {
-                    todo!();
+                    let arity = self.chunk().bytecode[self.ip + 1];
+                    self.ip += 2;
+                    let the_fn = self.pop();
+                    let Value::Fn(ref inner) = the_fn else {
+                        return self.panic("only functions may be partially applied");
+                    };
+                    let args = self.stack.split_off(self.stack.len() - arity as usize);
+                    let partial = crate::value::Partial {
+                        args,
+                        name: inner.name(),
+                        function: the_fn,
+                    };
+                    self.push(Value::Partial(Rc::new(partial)));
                 }
                 Call => {
                     let arity = self.chunk().bytecode[self.ip + 1];
@@ -742,6 +754,34 @@ impl Vm {
                             };
                             self.push(value);
                         }
+                        Value::Partial(partial) => {
+                            let last_arg = self.pop();
+                            let args = &partial.args;
+                            for arg in args {
+                                if *arg == Value::Nothing {
+                                    self.push(last_arg.clone());
+                                } else {
+                                    self.push(arg.clone());
+                                }
+                            }
+                            let the_fn = partial.function.clone();
+                            let mut frame = CallFrame {
+                                function: the_fn,
+                                arity: args.len() as u8,
+                                stack_base: self.stack.len() - arity as usize - 1,
+                                ip: 0,
+                            };
+
+                            swap(&mut self.frame, &mut frame);
+                            frame.ip = self.ip;
+
+                            self.call_stack.push(frame);
+                            self.ip = 0;
+
+                            if crate::DEBUG_RUN {
+                                println!("== calling into {} ==", self.frame.function.show());
+                            }
+                        }
                         _ => return self.panic_with(format!("{} is not a function", val.show())),
                     }
                 }