Add call stack error chain

This allows us to write out errors that follow the call stack and give
locations to where errors originated.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/hello.sy

@@ -1,4 +1,4 @@
-[ "Hell world" . ]
+[ "Hell world" println! ]
     :say
 
 say!

src/compile.rs

@@ -82,12 +82,6 @@ impl<'s> Compile<'s> {
                 Inst::Store(word).into()
             }
             Atom::Word(text) => {
-                // XXX : probably something better than this
-                // Check builtins
-                if text == "." {
-                    return Inst::Print.into();
-                }
-
                 // Look for locally defined symbols first. One can't be found,
                 // then create a local variable. The local variable *should* be
                 // defined already, but sometimes things happen.

src/main.rs

@@ -8,7 +8,7 @@ use std::io::Read;
 use std::path::PathBuf;
 use structopt::StructOpt;
 use syn::parser::Parser;
-use vm::machine::MachineBuilder;
+use vm::{error::RuntimeSpanError, machine::MachineBuilder};
 
 #[derive(Debug, StructOpt)]
 struct Opt {
@@ -46,7 +46,21 @@ fn main() -> Result {
         .max_arena_objects(opt.max_arena_objects)
         .finish();
 
-    machine.eval(stmts)?;
+    if let Err(e) = machine.eval(stmts) {
-
+        // print error message
+        print_error(&e);
+        Ok(())
+    } else {
         Ok(())
     }
+}
+
+fn print_error(error: &RuntimeSpanError) {
+    match error {
+        RuntimeSpanError::Span(span, error) => {
+            println!("at {}", span);
+            print_error(error)
+        }
+        RuntimeSpanError::Error(error) => println!("{}", error),
+    }
+}

src/syn/span.rs

@@ -4,7 +4,7 @@
 #![allow(dead_code)]
 
 use std::cmp::{Ord, Ordering, PartialOrd};
-use std::fmt::{self, Debug};
+use std::fmt::{self, Debug, Display};
 use std::rc::Rc;
 
 #[cfg_attr(not(test), derive(PartialEq))]
@@ -96,6 +96,20 @@ impl Span {
     }
 }
 
+impl Display for Span {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        if self.start.line == self.end.line {
+            write!(fmt, "line {} in {}", self.start.line, self.source)
+        } else {
+            write!(
+                fmt,
+                "lines {}-{} in {}",
+                self.start.line, self.end.line, self.source
+            )
+        }
+    }
+}
+
 #[derive(Clone, PartialEq, Eq)]
 pub struct Spanned<T> {
     span: Span,

src/vm/error.rs

@@ -1,3 +1,4 @@
+use crate::syn::span::Span;
 use thiserror::Error;
 
 #[derive(Error, Debug, Clone)]
@@ -15,7 +16,44 @@ pub enum RuntimeError {
     CannotCall(String),
 }
 
-pub type Result<T, E = RuntimeError> = std::result::Result<T, E>;
+pub type Result<T, E = RuntimeSpanError> = std::result::Result<T, E>;
+
+pub enum RuntimeSpanError {
+    Span(Span, Box<RuntimeSpanError>),
+    Error(RuntimeError),
+}
+
+/// Add a location to the implemented value.
+pub trait WithLocation {
+    type Out;
+    fn with_location(self, span: Span) -> Self::Out;
+}
+
+impl<T> WithLocation for Result<T, RuntimeSpanError> {
+    type Out = Result<T, RuntimeSpanError>;
+
+    fn with_location(self, span: Span) -> Self::Out {
+        if let Err(e) = self {
+            Err(RuntimeSpanError::Span(span, Box::new(e)))
+        } else {
+            self
+        }
+    }
+}
+
+impl WithLocation for RuntimeError {
+    type Out = RuntimeSpanError;
+
+    fn with_location(self, span: Span) -> Self::Out {
+        RuntimeSpanError::Span(span, Box::new(self.into()))
+    }
+}
+
+impl From<RuntimeError> for RuntimeSpanError {
+    fn from(other: RuntimeError) -> Self {
+        RuntimeSpanError::Error(other)
+    }
+}
 
 // TODO
 // Error building idea:

src/vm/inst.rs

@@ -16,9 +16,6 @@ pub enum Inst {
     //Dup,
     /// Applies the top stack value, which should be a macro.
     Call,
-
-    /// Pops and prints the top stack value.
-    Print,
 }
 
 // TODO - do we want separate function definition syntax? We would be able to

src/vm/machine.rs

@@ -93,7 +93,7 @@ impl Machine {
     pub fn stack_push(&mut self, value: Value) -> Result<()> {
         if let Some(max) = self.max_stack_size() {
             if self.stack().len() >= max {
-                return Err(RuntimeError::StackOverflow);
+                return Err(RuntimeError::StackOverflow.into());
             }
         }
         self.stack_mut().push(value);
@@ -103,7 +103,7 @@ impl Machine {
     pub fn stack_pop(&mut self) -> Result<Value> {
         self.stack_mut()
             .pop()
-            .ok_or_else(|| RuntimeError::StackUnderflow)
+            .ok_or_else(|| RuntimeError::StackUnderflow.into())
     }
 
     pub fn lookup_word(&self, word: &Word) -> Option<&Value> {
@@ -267,12 +267,8 @@ impl Machine {
                 Value::BuiltinFn(builtin) => {
                     self.call_native(builtin);
                 }
-                value => return Err(RuntimeError::CannotCall(value.name().to_string())),
+                value => return Err(RuntimeError::CannotCall(value.name().to_string()).into()),
             },
-            Inst::Print => {
-                let value = self.stack_pop()?;
-                println!("{}", value);
-            }
         };
 
         // Update this frame's PC
@@ -333,9 +329,22 @@ impl MachineBuilder {
             } else {
                 return Err(RuntimeError::CannotCall(
                     "if statement requires quote value".to_string(),
-                ));
+                )
+                .into());
             }
         });
+
+        self.register_builtin_fun("print", |machine, _| {
+            let value = machine.stack_pop()?;
+            print!("{}", value);
+            Ok(BuiltinExit::Return)
+        });
+
+        self.register_builtin_fun("println", |machine, _| {
+            let value = machine.stack_pop()?;
+            println!("{}", value);
+            Ok(BuiltinExit::Return)
+        });
     }
 
     fn register_builtin_fun(