Merge pull request #24 from AlexBethel/master

src/error.rs

@@ -14,4 +14,5 @@ pub enum ErrorKind {
     UnknownVariable(String),
     MeloVariable(String),
     TypeError(String),
+    TopLevelBreak,
 }

src/interpret.rs

@@ -1,10 +1,10 @@
 //! Expression evaluator and statement interpreter.
 //!
 //! To interpret a piece of AbleScript code, you first need to
-//! construct a [Scope], which is responsible for storing the list of
-//! variable and function definitions accessible from an AbleScript
-//! snippet. You can then call [Scope::eval_items] to evaluate or
-//! execute any number of expressions or statements.
+//! construct an [ExecEnv], which is responsible for storing the stack
+//! of local variable and function definitions accessible from an
+//! AbleScript snippet. You can then call [ExecEnv::eval_items] to
+//! evaluate or execute any number of expressions or statements.
 
 #[deny(missing_docs)]
 use std::collections::HashMap;
@@ -16,41 +16,91 @@ use crate::{
     variables::{Value, Variable},
 };
 
-/// A set of visible variable and function definitions, which serves
-/// as a context in which expressions can be evaluated.
-pub struct Scope {
-    /// The mapping from variable names to values.
-    variables: HashMap<String, Variable>,
-
-    // In the future, this will store functio definitions, a link to a
-    // parent scope (so we can have nested scopes), and possibly other
-    // information.
+/// An environment for executing AbleScript code.
+pub struct ExecEnv {
+    /// The stack, ordered such that `stack[stack.len() - 1]` is the
+    /// top-most (newest) stack frame, and `stack[0]` is the
+    /// bottom-most (oldest) stack frame.
+    stack: Vec<Scope>,
 }
 
-impl Scope {
+/// A set of visible variable and function definitions in a single
+/// stack frame.
+#[derive(Default)]
+struct Scope {
+    /// The mapping from variable names to values.
+    variables: HashMap<String, Variable>,
+    // In the future, this will store functio definitions and possibly
+    // other information.
+}
+
+/// The reason a successful series of statements halted.
+enum HaltStatus {
+    /// The last statement in the list evaluated to this value.
+    Value(Value),
+
+    /// A `break` statement occurred and was not caught by a `loop`
+    /// statement.
+    Break,
+
+    /// A `hopback` statement occurred and was not caught by a `loop`
+    /// statement.
+    Hopback,
+}
+
+impl ExecEnv {
     /// Create a new Scope with no predefined variable definitions or
     /// other information.
     pub fn new() -> Self {
         Self {
-            variables: HashMap::new(),
+            stack: Default::default(),
         }
     }
 
-    /// Evaluate a set of Items. Returns the value of the last Item
-    /// evaluated, or an error if one or more of the Items failed to
-    /// evaluate.
+    /// Evaluate a set of Items in their own stack frame. Return the
+    /// value of the last Item evaluated, or an error if one or more
+    /// of the Items failed to evaluate or if a `break` or `hopback`
+    /// statement occurred at the top level.
     pub fn eval_items(&mut self, items: &[Item]) -> Result<Value, Error> {
-        items
-            .iter()
-            .map(|item| self.eval_item(item))
-            .try_fold(Value::Nul, |_, result| result)
+        match self.eval_items_hs(items)? {
+            HaltStatus::Value(v) => Ok(v),
+            HaltStatus::Break | HaltStatus::Hopback => Err(Error {
+                // It's an error to issue a `break` outside of a
+                // `loop` statement.
+                kind: ErrorKind::TopLevelBreak,
+                position: 0..0,
+            }),
+        }
+    }
+
+    /// The same as `eval_items`, but report "break" and "hopback"
+    /// exit codes as normal conditions in a HaltStatus enum.
+    ///
+    /// `interpret`-internal code should typically prefer this
+    /// function over `eval_items`.
+    fn eval_items_hs(&mut self, items: &[Item]) -> Result<HaltStatus, Error> {
+        let init_depth = self.stack.len();
+
+        self.stack.push(Default::default());
+        let mut final_result = Ok(HaltStatus::Value(Value::Nul));
+        for item in items {
+            final_result = self.eval_item(item);
+            if !matches!(final_result, Ok(HaltStatus::Value(_))) {
+                break;
+            }
+        }
+        self.stack.pop();
+
+        // Invariant: stack size must have net 0 change.
+        debug_assert_eq!(self.stack.len(), init_depth);
+        final_result
     }
 
     /// Evaluate a single Item, returning its value or an error.
-    fn eval_item(&mut self, item: &Item) -> Result<Value, Error> {
+    fn eval_item(&mut self, item: &Item) -> Result<HaltStatus, Error> {
         match item {
-            Item::Expr(expr) => self.eval_expr(expr),
-            Item::Stmt(stmt) => self.eval_stmt(stmt).map(|_| Value::Nul),
+            Item::Expr(expr) => self.eval_expr(expr).map(|v| HaltStatus::Value(v)),
+            Item::Stmt(stmt) => self.eval_stmt(stmt),
         }
     }
 
@@ -82,46 +132,131 @@ impl Scope {
             }
             Eq { left, right } => Bool(self.eval_expr(left)? == self.eval_expr(right)?),
             Neq { left, right } => Bool(self.eval_expr(left)? != self.eval_expr(right)?),
-            And { left, right } => Bool(
-                bool::try_from(self.eval_expr(left)?)? && bool::try_from(self.eval_expr(right)?)?,
-            ),
-            Or { left, right } => Bool(
-                bool::try_from(self.eval_expr(left)?)? || bool::try_from(self.eval_expr(right)?)?,
-            ),
-            Not(expr) => Bool(!bool::try_from(self.eval_expr(expr)?)?),
+            And { left, right } => {
+                Bool(bool::from(self.eval_expr(left)?) && bool::from(self.eval_expr(right)?))
+            }
+            Or { left, right } => {
+                Bool(bool::from(self.eval_expr(left)?) || bool::from(self.eval_expr(right)?))
+            }
+            Not(expr) => Bool(!bool::from(self.eval_expr(expr)?)),
             Literal(value) => value.clone(),
-            Identifier(Iden(name)) => self
-                .variables
-                .get(name)
-                .ok_or_else(|| Error {
-                    kind: ErrorKind::UnknownVariable(name.to_owned()),
-                    // TODO: figure out some way to avoid this 0..0
-                    // dumbness
+            Identifier(Iden(name)) => self.get_var(name)?.value.clone(),
+        })
+    }
+
+    /// Perform the action indicated by a statement.
+    fn eval_stmt(&mut self, stmt: &Stmt) -> Result<HaltStatus, Error> {
+        match stmt {
+            Stmt::Print(expr) => {
+                println!("{}", self.eval_expr(expr)?);
+            }
+            Stmt::VariableDeclaration { iden, init } => {
+                let init = match init {
+                    Some(e) => self.eval_expr(e)?,
+                    None => Value::Nul,
+                };
+
+                // There's always at least one stack frame on the
+                // stack if we're evaluating something, so we can
+                // `unwrap` here.
+                self.stack.iter_mut().last().unwrap().variables.insert(
+                    iden.0.clone(),
+                    Variable {
+                        melo: false,
+                        value: init,
+                    },
+                );
+            }
+            Stmt::FunctionDeclaration {
+                iden: _,
+                args: _,
+                body: _,
+            } => todo!(),
+            Stmt::BfFDeclaration { iden: _, body: _ } => todo!(),
+            Stmt::If { cond, body } => {
+                if self.eval_expr(cond)?.into() {
+                    return self.eval_items_hs(body);
+                }
+            }
+            Stmt::FunctionCall { iden: _, args: _ } => todo!(),
+            Stmt::Loop { body } => loop {
+                let res = self.eval_items_hs(body)?;
+                match res {
+                    HaltStatus::Value(_) => {}
+                    HaltStatus::Break => break,
+                    HaltStatus::Hopback => continue,
+                }
+            },
+            Stmt::VarAssignment { iden, value } => {
+                self.get_var_mut(&iden.0)?.value = self.eval_expr(value)?;
+            }
+            Stmt::Break => {
+                return Ok(HaltStatus::Break);
+            }
+            Stmt::HopBack => {
+                return Ok(HaltStatus::Hopback);
+            }
+            Stmt::Melo(iden) => {
+                self.get_var_mut(&iden.0)?.melo = true;
+            }
+        }
+
+        Ok(HaltStatus::Value(Value::Nul))
+    }
+
+    /// Get a shared reference to the value of a variable. Throw an
+    /// error if the variable is inaccessible or banned.
+    fn get_var(&self, name: &str) -> Result<&Variable, Error> {
+        match self
+            .stack
+            .iter()
+            .rev()
+            .find_map(|scope| scope.variables.get(name))
+        {
+            Some(var) => {
+                if !var.melo {
+                    Ok(var)
+                } else {
+                    Err(Error {
+                        kind: ErrorKind::MeloVariable(name.to_owned()),
+                        // TODO: figure out some way to avoid this
+                        // 0..0 dumbness
                         position: 0..0,
                     })
-                .and_then(|var| {
+                }
+            }
+            None => Err(Error {
+                kind: ErrorKind::UnknownVariable(name.to_owned()),
+                position: 0..0,
+            }),
+        }
+    }
+
+    /// Get a mutable reference to a variable. Throw an error if the
+    /// variable is inaccessible or banned.
+    fn get_var_mut(&mut self, name: &str) -> Result<&mut Variable, Error> {
+        // This function is almost exactly 22 lines of duplicated code
+        // from get_var, which I feel like is a bad sign...
+        match self
+            .stack
+            .iter_mut()
+            .rev()
+            .find_map(|scope| scope.variables.get_mut(name))
+        {
+            Some(var) => {
                 if !var.melo {
-                        Ok(var.value.clone())
+                    Ok(var)
                 } else {
                     Err(Error {
                         kind: ErrorKind::MeloVariable(name.to_owned()),
                         position: 0..0,
                     })
                 }
-                })?,
-        })
-    }
-
-    /// Perform the action indicated by a statement.
-    fn eval_stmt(&mut self, stmt: &Stmt) -> Result<(), Error> {
-        match stmt {
-            Stmt::Print(expr) => {
-                println!("{}", self.eval_expr(expr)?);
-                Ok(())
-            }
-            _ => {
-                todo!()
             }
+            None => Err(Error {
+                kind: ErrorKind::UnknownVariable(name.to_owned()),
+                position: 0..0,
+            }),
         }
     }
 }

src/main.rs

@@ -10,7 +10,7 @@ mod repl;
 mod variables;
 
 use clap::{App, Arg};
-use interpret::Scope;
+use interpret::ExecEnv;
 use logos::Source;
 use parser::Parser;
 
@@ -41,8 +41,8 @@ fn main() {
             match ast {
                 Ok(ast) => {
                     println!("{:#?}", ast);
-                    let mut ctx = Scope::new();
-                    println!("{:?}", ctx.eval_items(&ast));
+                    let mut env = ExecEnv::new();
+                    println!("{:?}", env.eval_items(&ast));
                 }
                 Err(e) => {
                     println!(

src/repl.rs

@@ -1,11 +1,11 @@
 use logos::Source;
 use rustyline::Editor;
 
-use crate::{interpret::Scope, parser::Parser};
+use crate::{interpret::ExecEnv, parser::Parser};
 
 pub fn repl() {
     let mut rl = Editor::<()>::new();
-    let mut ctx = Scope::new();
+    let mut env = ExecEnv::new();
     loop {
         let readline = rl.readline(":: ");
         match readline {
@@ -19,7 +19,7 @@ pub fn repl() {
                 match ast {
                     Ok(ast) => {
                         println!("{:?}", ast);
-                        println!("{:?}", ctx.eval_items(&ast));
+                        println!("{:?}", env.eval_items(&ast));
                     },
                     Err(e) => {
                         println!(

src/variables.rs

@@ -71,17 +71,23 @@ impl TryFrom<Value> for i32 {
     }
 }
 
-impl TryFrom<Value> for bool {
-    type Error = Error;
-
-    fn try_from(value: Value) -> Result<Self, Self::Error> {
+// Coercions from a value to a boolean always succeed, so every value
+// can be used in an `if` statement. C does things that way, so how
+// could it possibly be a bad idea?
+impl From<Value> for bool {
+    fn from(value: Value) -> Self {
         match value {
-            Value::Bool(b) => Ok(b),
-            Value::Abool(b) => Ok(b.into()),
-            _ => Err(Error {
-                kind: ErrorKind::TypeError(format!("Expected bool, got {}", value)),
-                position: 0..0,
-            }),
+            // Booleans and abooleans have a trivial conversion.
+            Value::Bool(b) => b,
+            Value::Abool(b) => b.into(),
+            // The empty string is falsey, other strings are truthy.
+            Value::Str(s) => s.len() != 0,
+            // 0 is falsey, nonzero is truthy.
+            Value::Int(x) => x != 0,
+            // And nul is truthy as a symbol of the fact that the
+            // deep, fundamental truth of this world is nothing but
+            // the eternal void.
+            Value::Nul => true,
         }
     }
 }