Basic Evaluation Logic for the Builder.

src/build.rs

@@ -1,23 +1,30 @@
-use parse::{parse, Statement, Expression, Value, FieldList};
+use parse::{parse, Statement, Expression, Value, FieldList, SelectorList};
 
 use std::error::Error;
-use std::collections::HashMap;
-use std::convert::From;
+use std::collections::{HashMap, VecDeque};
+use std::collections::hash_map::Entry;
+use std::rc::Rc;
+use std::fmt;
+use std::fmt::{Display,Formatter};
 
 quick_error! {
     #[derive(Debug,PartialEq)]
     pub enum BuildError {
         TypeFail(msg: String) {
-            description("Eval Error")
-            display("Eval Error {}", msg)
+            description("Type Error")
+            display("Type Error {}", msg)
+        }
+        Unsupported(msg: String) {
+            description("Unsupported Operation")
+            display("Unsupported Operation {}", msg)
         }
         NoSuchSymbol(msg: String) {
             description("Eval Error")
-            display("Eval Error {}", msg)
+            display("No Such Variable {}", msg)
         }
         TODO(msg: String) {
-            description("Eval Error")
-            display("Eval Error {}", msg)
+            description("TODO Error")
+            display("TODO Error {}", msg)
         }
     }
 }
@@ -26,27 +33,76 @@ quick_error! {
 type BuildResult = Result<(), Box<Error>>;
 
 /// Val is the type of a value for a field in a Tuple.
-#[derive(PartialEq,Debug)]
+#[derive(PartialEq,Debug,Clone)]
 pub enum Val<'a> {
     Int(i64),
     Float(f64),
-    String(&'a str),
-    Symbol(&'a str),
-    Tuple(FieldList<'a>),
+    String(String),
+    Tuple(Vec<(&'a str, Rc<Val<'a>>)>),
 }
 
-impl<'a> From<Value<'a>> for Val<'a> {
-    fn from(v: Value<'a>) -> Self {
-        match v {
-            Value::Int(i) => Val::Int(i),
-            Value::Float(f) => Val::Float(f),
-            Value::String(s) => Val::String(s),
-            Value::Symbol(s) => Val::Symbol(s),
-            Value::Tuple(ref fields) => {
-                Val::Tuple(Vec::new())
-            }
+impl<'a> Val<'a> {
+
+    pub fn type_name(&self) -> String {
+        match self {
+            &Val::Int(_) => "Integer".to_string(),
+            &Val::Float(_) => "Float".to_string(),
+            &Val::String(_) => "String".to_string(),
+            &Val::Tuple(_) => "Tuple".to_string(),
         }
     }
+
+    pub fn type_equal(&self, target: &Self) -> bool {
+        match self {
+            &Val::Int(_) => if let &Val::Int(_) = target { true } else { false },
+            &Val::Float(_) => if let &Val::Float(_) = target { true } else { false },
+            &Val::String(_) => if let &Val::String(_) = target { true } else { false },
+            &Val::Tuple(_) => if let &Val::Tuple(_) = target { true } else { false },
+        }
+    }
+
+    pub fn get_fields(&self) -> Option<&Vec<(&'a str, Rc<Val<'a>>)>> {
+        if let &Val::Tuple(ref fs) = self {
+            Some(fs)
+        } else {
+            None
+        }
+    }
+
+    pub fn is_int(&self) -> bool {
+        if let &Val::Int(_) = self {
+            return true;
+        }
+        return false;
+    }
+
+    pub fn is_float(&self) -> bool {
+        if let &Val::Float(_) = self {
+            return true;
+        }
+        return false;
+    }
+
+    pub fn is_string(&self) -> bool {
+        if let &Val::String(_) = self {
+            return true;
+        }
+        return false;
+    }
+
+    pub fn is_tuple(&self) -> bool {
+        if let &Val::Tuple(_) = self {
+            return true;
+        }
+        return false;
+    }
+}
+
+impl<'a> Display for Val<'a> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        // TODO(jwall): These should render better than this.
+        write!(f, "{}", self.type_name())
+    }
 }
 
 /// Tuple defines a set of fields.
@@ -55,7 +111,7 @@ pub struct Tuple<'a>(HashMap<&'a str, Val<'a>>);
 
 /// Builder parses one or more statements into a out Tuple.
 pub struct Builder<'a> {
-    /// env is the set of key value pairs provided at build time.
+    /// env is the immutable set of key value pairs provided at build time.
     env: HashMap<&'a str, &'a str>,
     /// assets are other parsed files from import statements. They
     /// are keyed by the normalized import path. This acts as a cache
@@ -63,28 +119,66 @@ pub struct Builder<'a> {
     /// multiple times.
     assets: HashMap<&'a str, Tuple<'a>>,
     /// out is our built output.
-    out: Tuple<'a>,
+    out: HashMap<&'a str, Rc<Val<'a>>>,
+}
+
+macro_rules! eval_binary_expr {
+    ($case:pat, $rside:ident, $result:expr, $msg:expr) => {
+        match $rside.as_ref() {
+            $case => {
+                return Ok(Rc::new($result))
+            },
+            val => {
+                return Err(Box::new(
+                    BuildError::TypeFail(
+                        format!("Expected {} but got {}", $msg, val))))
+            }
+        }
+    }
 }
 
 impl<'a> Builder<'a> {
     /// new_builder constructs Builder with initialized fields ready to parse.
-    pub fn new_builder() -> Self {
-        Builder {
-            env: HashMap::new(),
-            assets: HashMap::new(),
-            out: Tuple(HashMap::new()),
+    fn from(&self, v: Value<'a>) -> Result<Rc<Val>, Box<Error>> {
+        match v {
+            Value::Int(i) => Ok(Rc::new(Val::Int(i))),
+            Value::Float(f) => Ok(Rc::new(Val::Float(f))),
+            Value::String(s) => Ok(Rc::new(Val::String(s.to_string()))),
+            Value::Symbol(s) => {
+                self.lookup_sym(s).ok_or(Box::new(
+                    BuildError::NoSuchSymbol(s.to_string())))
+            },
+            Value::Tuple(fields) => {
+                // TODO(jwall): We need to resolve the expressions here.
+                // TODO(jwall): Don't forget we want a stable order for the fields
+                //   in a tuple to make Vec comparisons easier later on.
+                Err(Box::new(
+                    BuildError::TODO(
+                        "Value::Tuple to Val::Tuple is not yet impolemented.".to_string())))
+            },
+            Value::Selector(selector_list) => {
+                self.lookup_selector(selector_list)
+            },
         }
     }
 
-    pub fn build_dir(&self, name: &str) -> BuildResult {
+    pub fn new() -> Self {
+        Builder {
+            env: HashMap::new(),
+            assets: HashMap::new(),
+            out: HashMap::new(),
+        }
+    }
+
+    pub fn build_dir(&mut self, name: &str) -> BuildResult {
         Ok(())
     }
 
-    pub fn build_file(&self, name: &str) -> BuildResult {
+    pub fn build_file(&mut self, name: &str) -> BuildResult {
         Ok(())
     }
 
-    pub fn build(&self, ast: Vec<Statement>) -> BuildResult {
+    pub fn build(&mut self, ast: Vec<Statement>) -> BuildResult {
         // TODO(jwall):
         for stmt in ast.iter() {
             self.build_stmt(stmt);
@@ -92,45 +186,223 @@ impl<'a> Builder<'a> {
         Ok(())
     }
 
-    fn eval_expr(&'a self, expr: Expression<'a>) -> Result<Val, Box<Error>> {
+    fn lookup_sym(&'a self, sym: &str) -> Option<Rc<Val>> {
+        if self.out.contains_key(sym) {
+            Some(self.out[sym].clone())
+        } else {
+            None
+        }
+    }
+
+    fn find_in_fieldlist(target: &str, fs: &Vec<(&str, Rc<Val<'a>>)>) -> Option<Rc<Val<'a>>> {
+        for (key, val) in fs.iter().cloned() {
+            if target == key {
+                return Some(val.clone())
+            }
+        }
+        return None
+    }
+
+    fn lookup_selector(&'a self, sl: SelectorList) -> Result<Rc<Val>, Box<Error>> {
+        let len = sl.len();
+        if len > 0 {
+            println!("Looking up symbol {}", sl[0]);
+            if let Some(v) = self.lookup_sym(sl[0]) {
+                let mut it = sl.iter().skip(1).peekable();
+                if it.peek().is_none() {
+                    return Ok(v.clone());
+                }
+                if let &Val::Tuple(ref fs) = v.as_ref() {
+                    let mut stack = VecDeque::new();
+                    stack.push_back(v.clone());
+                    loop {
+                        let vref = stack.pop_front().unwrap();
+                        if it.peek().is_none() {
+                            return Ok(vref.clone());
+                        }
+                        // This unwrap is safe because we already checked for
+                        // None above.
+                        let k = it.next().unwrap();
+                        if !vref.is_tuple() {
+                            return Err(Box::new(BuildError::NoSuchSymbol(
+                                format!("Attempted to dereference non-tuple {:?} at field {}.", sl, k))));
+                        }
+                        // This unwrap is safe because we already checked for
+                        // Tuple above.
+                        let fs = vref.get_fields().unwrap();
+                        if let Some(vv) = Self::find_in_fieldlist(k, fs) {
+                            if vv.is_tuple() {
+                                stack.push_back(vv.clone());
+                                continue;
+                            }
+                            if it.peek().is_some() {
+                                return Err(Box::new(BuildError::NoSuchSymbol(
+                                    format!("Unable to match selector path {:?}", sl))));
+                            } else {
+                                return Ok(vv.clone());
+                            }
+                        } else {
+                            // TODO(jwall): A better error for this would be nice.
+                            return Err(Box::new(BuildError::NoSuchSymbol(
+                                format!("Unable to match selector path {:?}", sl))));
+                        }
+                    };
+                }
+                return Err(Box::new(BuildError::TypeFail(
+                    format!("{} is not a Tuple", sl[0]))));
+            }
+            return Err(Box::new(BuildError::NoSuchSymbol(
+                format!("Unable to find Symbol {}", sl[0]))));
+        }
+        return Err(Box::new(BuildError::NoSuchSymbol(
+            "Attempted to lookup an empty selector".to_string())));
+    }
+
+    // eval_expr evals a single Expression in the context of a running Builder.
+    // It does not mutate the builders collected state at all.
+    pub fn eval_expr(&'a self, expr: Expression<'a>) -> Result<Rc<Val>, Box<Error>> {
         match expr {
             Expression::Simple(val) => {
-                Ok(Val::from(val))
+                self.from(val)
             },
             Expression::Add(v, expr) => {
-                Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
+                let expr_result = try!(self.eval_expr(*expr));
+                let v = try!(self.from(*v));
+                match *v {
+                    Val::Int(i) => {
+                        eval_binary_expr!(&Val::Int(ii), expr_result,
+                                          Val::Int(i + ii), "Integer")
+                    },
+                    Val::Float(f) => {
+                        eval_binary_expr!(&Val::Float(ff), expr_result,
+                                          Val::Float(f + ff), "Float")
+                    },
+                    Val::String(ref s) => {
+                        match expr_result.as_ref() {
+                            &Val::String(ref ss) => {
+                                return Ok(Rc::new(
+                                    Val::String([s.to_string(), ss.clone()].concat())))
+                            },
+                            val => {
+                                return Err(Box::new(
+                                    BuildError::TypeFail(
+                                        format!("Expected String but got {:?}", val))))
+                            }
+                        }
+                    },
+                    ref expr => {
+                        return Err(Box::new(
+                            BuildError::Unsupported(
+                                format!("{} does not support the '+' operation", expr.type_name()))))
+                    }
+                }
             },
             Expression::Sub(v, expr) => {
-                Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
+                let expr_result = try!(self.eval_expr(*expr));
+                let v = try!(self.from(*v));
+                match *v {
+                    Val::Int(i) => {
+                        eval_binary_expr!(&Val::Int(ii), expr_result,
+                                          Val::Int(i - ii), "Integer")
+                    },
+                    Val::Float(f) => {
+                        eval_binary_expr!(&Val::Float(ff), expr_result,
+                                          Val::Float(f - ff), "Float")
+                    },
+                    ref expr => {
+                        return Err(Box::new(
+                            BuildError::Unsupported(
+                                format!("{} does not support the '-' operation", expr.type_name()))))
+                    }
+                }
             },
             Expression::Mul(v, expr) => {
-                Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
+                let expr_result = try!(self.eval_expr(*expr));
+                let v = try!(self.from(*v));
+                match *v {
+                    Val::Int(i) => {
+                        eval_binary_expr!(&Val::Int(ii), expr_result,
+                                          Val::Int(i * ii), "Integer")
+                    },
+                    Val::Float(f) => {
+                        eval_binary_expr!(&Val::Float(ff), expr_result,
+                                          Val::Float(f * ff), "Float")
+                    },
+                    ref expr => {
+                        return Err(Box::new(
+                            BuildError::Unsupported(
+                                format!("{} does not support the '*' operation", expr.type_name()))))
+                    }
+                }
             },
             Expression::Div(v, expr) => {
-                Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
+                let expr_result = try!(self.eval_expr(*expr));
+                let v = try!(self.from(*v));
+                match *v {
+                    Val::Int(i) => {
+                        eval_binary_expr!(&Val::Int(ii), expr_result,
+                                          Val::Int(i / ii), "Integer")
+                    },
+                    Val::Float(f) => {
+                        eval_binary_expr!(&Val::Float(ff), expr_result,
+                                          Val::Float(f / ff), "Float")
+                    },
+                    ref expr => {
+                        return Err(Box::new(
+                            BuildError::Unsupported(
+                                format!("{} does not support the '*' operation", expr.type_name()))))
+                    }
+                }
             },
-            Expression::Copy(sel, fields) => {
+            Expression::Copy(sel, mut fields) => {
+                let v = try!(self.lookup_selector(sel));
+                if let Val::Tuple(ref src_fields) = *v {
+                    let mut m = HashMap::<&str, Rc<Val>>::new();
+                    // loop through fields and build  up a hasmap
+                    for &(ref key, ref val) in src_fields.iter() {
+                        if let Entry::Vacant(v) = m.entry(*key) {
+                            v.insert(val.clone());
+                        } else {
+                            // TODO(jwall): Is this an error?
+                            return Err(Box::new(
+                                BuildError::TypeFail(
+                                    format!("Duplicate field: {} in tuple", *key))));
+                        }
+                    }
+                    for (key, val) in fields.drain(0..) {
+                        let expr_result = try!(self.eval_expr(val));
+                        match m.entry(key) {
+                            Entry::Vacant(mut v) => {
+                                v.insert(expr_result);
+                            },
+                            Entry::Occupied(mut v) => {
+                                // Ensure that the new type matches the old type.
+                                // TODO(jwall): This copy is ugly but I don't think it's possible
+                                // to both compare and replace this at the same time.
+                                let src_val = v.get().clone();
+                                if src_val.type_equal(&expr_result) {
+                                    v.insert(expr_result);
+                                } else {
+                                    return Err(Box::new(
+                                        BuildError::TypeFail(
+                                            format!("Expected type {} for field {} but got {}",
+                                                    src_val.type_name(), key, expr_result.type_name()))));
+                                }
+                            },
+                        };
+                    }
+                    let mut new_fields: Vec<(&str, Rc<Val>)> = m.drain().collect();
+                    // We want a stable order for the fields to make comparing tuples
+                    // easier in later code. So we sort by the field name before constructing a new tuple.
+                    new_fields.sort_by(|a, b| a.0.cmp(b.0));
+                    return Ok(Rc::new(Val::Tuple(new_fields)));
+                }
                 Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
-            },
-            Expression::Selector(sel) => {
-                Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
+                    BuildError::TypeFail(
+                        format!("Expected Tuple got {}", v))))
             },
             Expression::Grouped(expr) => {
-                Err(Box::new(
-                    BuildError::TODO(
-                        "TODO(jwall): Unimplemented Expression".to_string())))
+                return self.eval_expr(*expr);
             },
             Expression::Call{lambda: sel, arglist: args} => {
                 Err(Box::new(
@@ -165,3 +437,246 @@ impl<'a> Builder<'a> {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::{Builder,Val};
+    use parse::{Expression, Value};
+    use std::rc::Rc;
+
+    fn test_expr_to_val(mut cases: Vec<(Expression,Val)>, b: Builder) {
+        for tpl in cases.drain(0..) {
+            assert_eq!(b.eval_expr(tpl.0).unwrap(), Rc::new(tpl.1));
+        }
+    }
+
+    #[test]
+    fn test_eval_div_expr() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Div(Box::new(Value::Int(2)), Box::new(Expression::Simple(Value::Int(2)))),
+             Val::Int(1)),
+            (Expression::Div(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Float(2.0)))),
+             Val::Float(1.0)),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Expected Float")]
+    fn test_eval_div_expr_fail() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Div(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Int(2)))),
+             Val::Float(1.0)),
+        ], b);
+    }
+
+    #[test]
+    fn test_eval_mul_expr() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Mul(Box::new(Value::Int(2)), Box::new(Expression::Simple(Value::Int(2)))),
+             Val::Int(4)),
+            (Expression::Mul(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Float(2.0)))),
+             Val::Float(4.0)),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Expected Float")]
+    fn test_eval_mul_expr_fail() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Mul(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Int(2)))),
+             Val::Float(1.0)),
+        ], b);
+    }
+
+    #[test]
+    fn test_eval_subtract_expr() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Sub(Box::new(Value::Int(2)), Box::new(Expression::Simple(Value::Int(1)))),
+             Val::Int(1)),
+            (Expression::Sub(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Float(1.0)))),
+             Val::Float(1.0)),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Expected Float")]
+    fn test_eval_subtract_expr_fail() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Sub(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Int(2)))),
+             Val::Float(1.0)),
+        ], b);
+    }
+
+    #[test]
+    fn test_eval_add_expr() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Add(Box::new(Value::Int(1)), Box::new(Expression::Simple(Value::Int(1)))),
+             Val::Int(2)),
+            (Expression::Add(Box::new(Value::Float(1.0)), Box::new(Expression::Simple(Value::Float(1.0)))),
+             Val::Float(2.0)),
+            (Expression::Add(Box::new(Value::String("foo")), Box::new(Expression::Simple(Value::String("bar")))),
+             Val::String("foobar".to_string())),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Expected Float")]
+    fn test_eval_add_expr_fail() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Add(Box::new(Value::Float(2.0)), Box::new(Expression::Simple(Value::Int(2)))),
+             Val::Float(1.0)),
+        ], b);
+    }
+
+    #[test]
+    fn test_eval_simple_expr() {
+        test_expr_to_val(vec![
+            (Expression::Simple(Value::Int(1)), Val::Int(1)),
+            (Expression::Simple(Value::Float(2.0)), Val::Float(2.0)),
+            (Expression::Simple(Value::String("foo")), Val::String("foo".to_string())),
+        ], Builder::new());
+    }
+
+    #[test]
+    fn test_eval_simple_lookup_expr() {
+        let mut b = Builder::new();
+        b.out.entry("var1").or_insert(Rc::new(Val::Int(1)));
+        test_expr_to_val(vec![
+            (Expression::Simple(Value::Symbol("var1")), Val::Int(1)),
+        ], b);
+    }
+
+    #[test]
+    fn test_eval_simple_lookup_error() {
+        let mut b = Builder::new();
+        b.out.entry("var1").or_insert(Rc::new(Val::Int(1)));
+        assert!(b.eval_expr(Expression::Simple(Value::Symbol("var"))).is_err());
+    }
+
+    #[test]
+    fn test_eval_selector_expr() {
+        // TODO(jwall): Tests for this expression.
+        let mut b = Builder::new();
+        b.out.entry("var1").or_insert(Rc::new(Val::Tuple(vec![
+            ("lvl1", Rc::new(Val::Tuple(
+                vec![
+                    ("lvl2", Rc::new(Val::Int(3))),
+                ]
+            ))),
+        ])));
+        b.out.entry("var2").or_insert(Rc::new(Val::Int(2)));
+        b.out.entry("var3").or_insert(Rc::new(Val::Tuple(vec![
+            ("lvl1", Rc::new(Val::Int(4)))
+        ])));
+        test_expr_to_val(vec![
+            (Expression::Simple(Value::Selector(vec!["var1"])), Val::Tuple(
+                vec![
+                    ("lvl1", Rc::new(Val::Tuple(
+                        vec![
+                            ("lvl2", Rc::new(Val::Int(3))),
+                        ]
+                    ))),
+                ]
+            )),
+            (Expression::Simple(Value::Selector(vec!["var1","lvl1"])), Val::Tuple(
+                vec![
+                    ("lvl2", Rc::new(Val::Int(3))),
+                ]
+            )),
+            (Expression::Simple(Value::Selector(vec!["var1","lvl1", "lvl2"])), Val::Int(3)),
+            (Expression::Simple(Value::Selector(vec!["var2"])), Val::Int(2)),
+            (Expression::Simple(Value::Selector(vec!["var3", "lvl1"])), Val::Int(4)),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Unable to find Symbol tpl1")]
+    fn test_expr_copy_no_such_tuple() {
+        let mut b = Builder::new();
+        test_expr_to_val(vec![
+            (Expression::Copy(vec!["tpl1"], Vec::new()),
+             Val::Tuple(Vec::new())),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Expected Tuple got Integer")]
+    fn test_expr_copy_not_a_tuple() {
+        let mut b = Builder::new();
+        b.out.entry("tpl1").or_insert(Rc::new(Val::Int(1)));
+        test_expr_to_val(vec![
+            (Expression::Copy(vec!["tpl1"], Vec::new()),
+             Val::Tuple(Vec::new())),
+        ], b);
+    }
+
+    #[test]
+    #[should_panic(expected = "Expected type Integer for field fld1 but got String")]
+    fn test_expr_copy_field_type_error() {
+        let mut b = Builder::new();
+        b.out.entry("tpl1").or_insert(Rc::new(Val::Tuple(vec![
+            ("fld1", Rc::new(Val::Int(1))),
+        ])));
+        test_expr_to_val(vec![
+            (Expression::Copy(vec!["tpl1"],
+                              vec![("fld1", Expression::Simple(Value::String("2")))]),
+             Val::Tuple(
+                vec![
+                    ("fld1", Rc::new(Val::String("2".to_string()))),
+                ],
+            )),
+        ], b);
+    }
+
+    // TODO(jwall): What about the duplicate field error?
+
+    #[test]
+    fn test_expr_copy() {
+        // TODO(jwall): Tests for this expression.
+        let mut b = Builder::new();
+        b.out.entry("tpl1").or_insert(Rc::new(Val::Tuple(vec![
+            ("fld1", Rc::new(Val::Int(1))),
+        ])));
+        test_expr_to_val(vec![
+            (Expression::Copy(vec!["tpl1"],
+                              vec![("fld2", Expression::Simple(Value::String("2")))]),
+             // Add a new field to the copy
+             Val::Tuple(
+                 // NOTE(jwall): The order of these is important in order to ensure
+                 // that the compare assertion is correct. The ordering has no
+                 // semantics though so at some point we should probably be less restrictive.
+                vec![
+                    ("fld1", Rc::new(Val::Int(1))),
+                    ("fld2", Rc::new(Val::String("2".to_string()))),
+                ],
+            )),
+             // Overwrite a field in the copy
+            (Expression::Copy(vec!["tpl1"],
+                              vec![
+                                  ("fld1", Expression::Simple(Value::Int(3))),
+                                  ("fld2", Expression::Simple(Value::String("2"))),
+                              ]),
+             Val::Tuple(
+                vec![
+                    ("fld1", Rc::new(Val::Int(3))),
+                    ("fld2", Rc::new(Val::String("2".to_string()))),
+                ],
+             )),
+            // The source tuple is still unmodified.
+            (Expression::Simple(Value::Selector(vec!["tpl1"])),
+             Val::Tuple(
+                vec![
+                    ("fld1", Rc::new(Val::Int(1))),
+                ],
+            )),
+        ], b);
+    }
+}