Start re-writing reduced ast and evaluator

schala-lang/language/src/ast/mod.rs

@@ -1,13 +1,15 @@
 #![allow(clippy::upper_case_acronyms)]
 #![allow(clippy::enum_variant_names)]
 
+use std::rc::Rc;
+use std::fmt;
+
 mod visitor;
 mod operators;
 
 pub use operators::{PrefixOp, BinOp};
 pub use visitor::{walk_ast, walk_block, ASTVisitor};
 
-use std::rc::Rc;
 use crate::derivative::Derivative;
 use crate::tokenizing::Location;
 
@@ -15,7 +17,7 @@ use crate::tokenizing::Location;
 /// the u32 index limits the size of an AST to 2^32 nodes.
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Default)]
 pub struct ItemId {
-  idx: u32,
+  pub idx: u32,
 }
 
 impl ItemId {
@@ -24,6 +26,12 @@ impl ItemId {
   }
 }
 
+impl fmt::Display for ItemId {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    write!(f, "ItemId:{}", self.idx)
+  }
+}
+
 pub struct ItemIdStore {
   last_idx: u32
 }

schala-lang/language/src/lib.rs

@@ -25,6 +25,8 @@ mod builtin;
 mod error;
 mod eval;
 mod reduced_ast;
+mod reduced_ir;
+mod tree_walk_eval;
 
 mod schala;
 

schala-lang/language/src/reduced_ir/mod.rs

@@ -0,0 +1,274 @@
+use crate::ast;
+use crate::symbol_table::{DefId, Symbol, SymbolSpec, SymbolTable};
+use crate::builtin::Builtin;
+
+use std::str::FromStr;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+mod test;
+
+pub fn reduce(ast: &ast::AST, symbol_table: &SymbolTable) -> ReducedIR {
+    let reducer = Reducer::new(symbol_table);
+    reducer.reduce(ast)
+}
+
+struct Reducer<'a> {
+    symbol_table: &'a SymbolTable,
+    functions: HashMap<DefId, Callable>,
+}
+
+impl<'a> Reducer<'a> {
+    fn new(symbol_table: &'a SymbolTable) -> Self {
+        Self {
+            symbol_table,
+            functions: HashMap::new(),
+        }
+    }
+
+    fn reduce(mut self, ast: &ast::AST) -> ReducedIR {
+        // First reduce all functions
+        // TODO once this works, maybe rewrite it using the Visitor
+        for statement in ast.statements.iter() {
+            self.top_level_statement(&statement);
+        }
+
+        // Then compute the entrypoint statements (which may reference previously-computed
+        // functions by ID)
+        let mut entrypoint = vec![];
+        for statement in ast.statements.iter() {
+            let ast::Statement { id: item_id, kind, .. } = statement;
+            match &kind {
+                ast::StatementKind::Expression(expr) => {
+                    entrypoint.push(Statement::Expression(self.expression(&expr)));
+                },
+                ast::StatementKind::Declaration(ast::Declaration::Binding { name, constant, expr, ..}) => {
+                    let symbol = self.symbol_table.lookup_symbol(item_id).unwrap();
+                    entrypoint.push(Statement::Binding { id: symbol.def_id.clone(), constant: *constant, expr: self.expression(&expr) });
+                },
+                _ => ()
+            }
+        }
+
+        ReducedIR {
+            functions: self.functions,
+            entrypoint,
+        }
+    }
+
+    fn top_level_statement(&mut self, statement: &ast::Statement) {
+        let ast::Statement { id: item_id, kind, .. } = statement;
+        match kind {
+            ast::StatementKind::Expression(_expr) => {
+                //TODO expressions can in principle contain definitions, but I won't worry
+                //about it now
+                ()
+            },
+            ast::StatementKind::Declaration(decl) => match decl {
+                ast::Declaration::FuncDecl(_, statements) => {
+                    self.insert_function_definition(item_id, statements);
+                },
+                _ => ()
+            },
+            ast::StatementKind::Import(..) => (),
+            ast::StatementKind::Module(modspec) => {
+                //TODO handle modules
+                ()
+            }
+        }
+    }
+
+    fn function_internal_statement(&mut self, statement: &ast::Statement) -> Option<Statement> {
+        let ast::Statement { id: item_id, kind, .. } = statement;
+        match kind {
+            ast::StatementKind::Expression(expr) => {
+                Some(Statement::Expression(self.expression(expr)))
+            },
+            ast::StatementKind::Declaration(decl) => match decl {
+                ast::Declaration::FuncDecl(_, statements) => {
+                    self.insert_function_definition(item_id, statements);
+                    None
+                },
+                _ => None
+            },
+            _ => None
+        }
+    }
+
+    fn insert_function_definition(&mut self, item_id: &ast::ItemId, statements: &ast::Block) {
+        let symbol = self.symbol_table.lookup_symbol(item_id).unwrap();
+        let def_id = symbol.def_id.clone();
+        let callable = Callable::UserDefined {
+            body: self.function(statements)
+        };
+        self.functions.insert(def_id, callable);
+    }
+
+    fn expression(&mut self, expr: &ast::Expression) -> Expression {
+        use crate::ast::ExpressionKind::*;
+        use Expression::{Unimplemented};
+
+        match &expr.kind {
+            NatLiteral(n) => Expression::Literal(Literal::Nat(*n)),
+            FloatLiteral(f) => Expression::Literal(Literal::Float(*f)),
+            StringLiteral(s) => Expression::Literal(Literal::StringLit(s.clone())),
+            BoolLiteral(b) => Expression::Literal(Literal::Bool(*b)),
+            BinExp(binop, lhs, rhs) =>  self.binop(binop, lhs, rhs),
+            PrefixExp(op, arg) => self.prefix(op, arg),
+            Value(qualified_name) => self.value(qualified_name),
+            Call { f, arguments } => Unimplemented, // self.reduce_call_expression(f, arguments),
+            TupleLiteral(exprs) => Expression::Tuple(exprs.iter().map(|e| self.expression(e)).collect()),
+            IfExpression {
+                discriminator,
+                body,
+            } => Unimplemented, //self.reduce_if_expression(deref_optional_box(discriminator), body),
+            Lambda { params, body, .. } => Unimplemented, //self.reduce_lambda(params, body),
+            NamedStruct { name, fields } => Unimplemented, //self.reduce_named_struct(name, fields),
+            Index { .. } => Unimplemented,
+            WhileExpression { .. } => Unimplemented,
+            ForExpression { .. } => Unimplemented,
+            ListLiteral { .. } => Unimplemented,
+        }
+    }
+
+    fn function(&mut self, statements: &ast::Block) -> Vec<Statement> {
+        statements.iter().filter_map(|stmt| self.function_internal_statement(stmt)).collect()
+    }
+
+    fn prefix(&mut self, prefix: &ast::PrefixOp, arg: &ast::Expression) -> Expression {
+        let builtin: Option<Builtin> = TryFrom::try_from(prefix).ok();
+        match builtin {
+            Some(op) => {
+                Expression::Call {
+                    f: Function::Builtin(op),
+                    args: vec![self.expression(arg)],
+                }
+            }
+            None => {
+                //TODO need this for custom prefix ops
+                Expression::Unimplemented
+            }
+        }
+    }
+
+    fn binop(&mut self, binop: &ast::BinOp, lhs: &ast::Expression, rhs: &ast::Expression) -> Expression {
+        use Expression::*;
+        let operation = Builtin::from_str(binop.sigil()).ok();
+        match operation {
+            Some(Builtin::Assignment) => {
+                let lval = match &lhs.kind {
+                    ast::ExpressionKind::Value(qualified_name) => {
+                        if let Some(Symbol { def_id, .. }) = self.symbol_table.lookup_symbol(&qualified_name.id) {
+                            def_id.clone()
+                        } else {
+                            return ReductionError(format!("Couldn't look up name: {:?}", qualified_name));
+                        }
+                    },
+                    _ => return ReductionError("Trying to assign to a non-name".to_string()),
+                };
+
+                Assign {
+                    lval,
+                    rval: Box::new(self.expression(rhs)),
+                }
+            },
+            Some(op) => {
+                Expression::Call {
+                    f: Function::Builtin(op),
+                    args: vec![self.expression(lhs), self.expression(rhs)],
+                }
+            }
+            None => {
+                //TODO handle a user-defined operation
+                Unimplemented
+            }
+        }
+    }
+
+    fn value(&mut self, qualified_name: &ast::QualifiedName) -> Expression {
+        let ast::QualifiedName { id, components, .. } = qualified_name;
+        println!("Qualified name: {:?}", qualified_name);
+        let maybe_symbol = self.symbol_table.lookup_symbol(&qualified_name.id);
+        println!("Symbol? {:?}", maybe_symbol);
+
+        Expression::Unimplemented
+    }
+}
+
+/// The reduced intermediate representation consists of a list of function definitions, and a block
+/// of entrypoint statements.  In a repl or script context this can be an arbitrary list of
+/// statements, in an executable context will likely just be a pointer to the main() function.
+#[derive(Debug)]
+pub struct ReducedIR {
+    functions: HashMap<DefId, Callable>,
+    pub entrypoint: Vec<Statement>,
+}
+
+impl ReducedIR {
+    fn debug(&self, symbol_table: &SymbolTable) {
+        println!("Reduced IR:");
+        println!("Functions:");
+        println!("-----------");
+        for (id, callable) in self.functions.iter() {
+            let name = &symbol_table.lookup_symbol_by_def(id).unwrap().local_name;
+            println!("{}({}) -> {:?}", id, name, callable);
+        }
+        println!("");
+        println!("Entrypoint:");
+        println!("-----------");
+        for stmt in self.entrypoint.iter() {
+            println!("{:?}", stmt);
+        }
+        println!("-----------");
+    }
+}
+
+#[derive(Debug)]
+pub enum Callable {
+    Builtin(Builtin),
+    UserDefined {
+        body: Vec<Statement>
+    }
+}
+
+#[derive(Debug)]
+pub enum Statement {
+    Expression(Expression),
+    Binding {
+        id: DefId,
+        constant: bool,
+        expr: Expression
+    },
+}
+
+#[derive(Debug)]
+pub enum Expression {
+    Literal(Literal),
+    Tuple(Vec<Expression>),
+    Assign {
+        lval: DefId,
+        rval: Box<Expression>,
+    },
+    Call {
+        f: Function,
+        args: Vec<Expression>
+    },
+    Unimplemented,
+    ReductionError(String),
+}
+
+#[derive(Debug)]
+pub enum Function {
+    Builtin(Builtin),
+    Defined(DefId),
+}
+
+
+#[derive(Debug)]
+pub enum Literal {
+    Nat(u64),
+    Int(i64),
+    Float(f64),
+    Bool(bool),
+    StringLit(Rc<String>),
+}

schala-lang/language/src/reduced_ir/test.rs

@@ -0,0 +1,42 @@
+#![cfg(test)]
+
+use crate::symbol_table::SymbolTable;
+use super::*;
+
+fn build_ir(input: &str) -> ReducedIR {
+  let ast = crate::util::quick_ast(input);
+
+  let mut symbol_table = SymbolTable::new();
+  symbol_table.process_ast(&ast).unwrap();
+
+  let reduced = reduce(&ast, &symbol_table);
+  reduced.debug(&symbol_table);
+  reduced
+}
+
+
+#[test]
+fn test_ir() {
+    let src = r#"
+
+    let global_one = 10 + 20
+    let global_two = "the string hello"
+
+    fn a_function(i, j, k) {
+        fn nested(x) {
+            x + 10
+        }
+        i + j * nested(k)
+    }
+
+    fn another_function(e) {
+        let local_var = 420
+        e * local_var
+    }
+
+    another_function()
+    "#;
+
+    let reduced = build_ir(src);
+    assert!(1 == 2);
+}

schala-lang/language/src/schala.rs

@@ -1,14 +1,14 @@
 use stopwatch::Stopwatch;
 
 use crate::error::SchalaError;
-use crate::{eval, parsing, reduced_ast, symbol_table, tokenizing, typechecking};
+use crate::{eval, parsing, reduced_ast, reduced_ir, tree_walk_eval, symbol_table, tokenizing, typechecking};
 use schala_repl::{
     ComputationRequest, ComputationResponse, GlobalOutputStats, LangMetaRequest, LangMetaResponse,
     ProgrammingLanguageInterface,
 };
 
 /// All the state necessary to parse and execute a Schala program are stored in this struct.
-pub struct Schala {
+pub struct Schala<'a> {
     /// Holds a reference to the original source code, parsed into line and character
     source_reference: SourceReference,
     /// Execution state for AST-walking interpreter
@@ -19,8 +19,12 @@ pub struct Schala {
     type_context: typechecking::TypeContext<'static>,
     /// Schala Parser
     active_parser: parsing::Parser,
+
+    eval_state: tree_walk_eval::State<'a>,
+
 }
 
+/*
 impl Schala {
     //TODO implement documentation for language items
     /*
@@ -31,23 +35,25 @@ impl Schala {
     }
     */
 }
+*/
 
-impl Schala {
+impl<'a> Schala<'a> {
     /// Creates a new Schala environment *without* any prelude.
-    fn new_blank_env() -> Schala {
+    fn new_blank_env() -> Schala<'a> {
         Schala {
             source_reference: SourceReference::new(),
             symbol_table: symbol_table::SymbolTable::new(),
             state: eval::State::new(),
             type_context: typechecking::TypeContext::new(),
             active_parser: parsing::Parser::new(),
+            eval_state: tree_walk_eval::State::new(),
         }
     }
 
     /// Creates a new Schala environment with the standard prelude, which is defined as ordinary
     /// Schala code in the file `prelude.schala`
     #[allow(clippy::new_without_default)]
-    pub fn new() -> Schala {
+    pub fn new() -> Schala<'a> {
         let prelude = include_str!("../source-files/prelude.schala");
         let mut env = Schala::new_blank_env();
 
@@ -88,6 +94,22 @@ impl Schala {
             .typecheck(&ast)
             .map_err(SchalaError::from_type_error);
 
+        let reduced_ir = reduced_ir::reduce(&ast, &self.symbol_table);
+        println!("Reduced IR: {:?}", reduced_ir);
+
+        let evaluation_outputs = self.eval_state.evaluate(reduced_ir, true);
+        let text_output: Result<Vec<String>, String> = evaluation_outputs.into_iter().collect();
+
+        let text_output: Result<Vec<String>, SchalaError> =
+            text_output.map_err(|err| SchalaError::from_string(err, Stage::Evaluation));
+
+        let eval_output: String =
+            text_output.map(|v| Iterator::intersperse(v.into_iter(), "\n".to_owned()).collect())?;
+
+        Ok(eval_output)
+
+        /*
+
         // Reduce AST - TODO this doesn't produce an error yet, but probably should
         let reduced_ast = reduced_ast::reduce(&ast, &self.symbol_table);
 
@@ -102,6 +124,7 @@ impl Schala {
             text_output.map(|v| Iterator::intersperse(v.into_iter(), "\n".to_owned()).collect())?;
 
         Ok(eval_output)
+        */
     }
 }
 
@@ -151,7 +174,7 @@ fn stage_names() -> Vec<&'static str> {
     ]
 }
 
-impl ProgrammingLanguageInterface for Schala {
+impl<'a> ProgrammingLanguageInterface for Schala<'a> {
     //TODO flesh out Config
     type Config = ();
     fn language_name() -> String {

schala-lang/language/src/symbol_table/mod.rs

@@ -15,6 +15,35 @@ mod symbol_trie;
 use symbol_trie::SymbolTrie;
 mod test;
 
+
+/// ID used for definitions
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Default)]
+pub struct DefId {
+  idx: u32,
+}
+
+impl fmt::Display for DefId {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    write!(f, "DefId:{}", self.idx)
+  }
+}
+
+pub struct DefIdStore {
+  last_idx: u32
+}
+
+impl DefIdStore {
+  pub fn new() -> Self {
+    Self { last_idx: 0 }
+  }
+
+  pub fn fresh(&mut self) -> DefId {
+    let idx = self.last_idx;
+    self.last_idx += 1;
+    DefId { idx }
+  }
+}
+
 /// Fully-qualified symbol name
 #[derive(Debug, Clone, Eq, PartialEq, Hash, PartialOrd, Ord)]
 pub struct Fqsn {
@@ -113,6 +142,9 @@ impl<K> NameTable<K> {
 
 //cf. p. 150 or so of Language Implementation Patterns
 pub struct SymbolTable {
+
+    def_id_store: DefIdStore,
+
     /// Used for import resolution.
     symbol_trie: SymbolTrie,
 
@@ -131,6 +163,7 @@ pub struct SymbolTable {
 impl SymbolTable {
     pub fn new() -> SymbolTable {
         SymbolTable {
+            def_id_store: DefIdStore::new(),
             symbol_trie: SymbolTrie::new(),
             fq_names: NameTable::new(),
             types: NameTable::new(),
@@ -148,13 +181,18 @@ impl SymbolTable {
         if !errs.is_empty() {
             return Err(errs);
         }
-        self.resolve_symbol_ids(ast);
+        self.resolve_scopes(ast);
         Ok(())
     }
 
     pub fn lookup_symbol(&self, id: &ItemId) -> Option<&Symbol> {
         self.id_to_symbol.get(id).map(|s| s.as_ref())
     }
+
+    pub fn lookup_symbol_by_def(&self, def: &DefId) -> Option<&Symbol> {
+        self.id_to_symbol.iter().find(|(_, sym)| sym.def_id == *def)
+            .map(|(_, sym)| sym.as_ref())
+    }
 }
 
 #[allow(dead_code)]
@@ -163,6 +201,7 @@ pub struct Symbol {
     pub local_name: Rc<String>,
     fully_qualified_name: Fqsn,
     pub spec: SymbolSpec,
+    pub def_id: DefId,
 }
 
 impl fmt::Display for Symbol {
@@ -224,6 +263,7 @@ impl SymbolTable {
             local_name: fqsn.local_name(),
             fully_qualified_name: fqsn.clone(),
             spec,
+            def_id: self.def_id_store.fresh(),
         });
         self.symbol_trie.insert(&fqsn);
         self.fqsn_to_symbol.insert(fqsn, symbol.clone());
@@ -232,8 +272,8 @@ impl SymbolTable {
 
     /// Walks the AST, matching the ID of an identifier used in some expression to
     /// the corresponding Symbol.
-    fn resolve_symbol_ids(&mut self, ast: &ast::AST) {
-        let mut resolver = resolver::Resolver::new(self);
+    fn resolve_scopes(&mut self, ast: &ast::AST) {
+        let mut resolver = resolver::ScopeResolver::new(self);
         resolver.resolve(ast);
     }
 
@@ -367,6 +407,7 @@ impl SymbolTable {
                         kind: NameKind::Binding,
                     },
                 )?;
+                println!("Adding Binding symbol: {:?}", fq_binding);
                 self.add_symbol(
                     id,
                     fq_binding,

schala-lang/language/src/symbol_table/resolver.rs

@@ -6,12 +6,12 @@ use crate::util::ScopeStack;
 
 type FqsnPrefix = Vec<Scope>;
 
-pub struct Resolver<'a> {
+pub struct ScopeResolver<'a> {
     symbol_table: &'a mut super::SymbolTable,
     name_scope_stack: ScopeStack<'a, Rc<String>, FqsnPrefix>,
 }
 
-impl<'a> Resolver<'a> {
+impl<'a> ScopeResolver<'a> {
     pub fn new(symbol_table: &'a mut SymbolTable) -> Self {
         let name_scope_stack: ScopeStack<'a, Rc<String>, FqsnPrefix> = ScopeStack::new(None);
         Self {
@@ -47,7 +47,9 @@ impl<'a> Resolver<'a> {
     }
 
     fn qualified_name(&mut self, name: &QualifiedName) {
+        println!("Handling qualified_name in resolver.rs: {:?}", name);
         let fqsn = self.lookup_name_in_scope(name);
+        println!("Computed FQSN: {:?}", fqsn);
         let symbol = self.symbol_table.fqsn_to_symbol.get(&fqsn);
         if let Some(symbol) = symbol {
             self.symbol_table.id_to_symbol.insert(name.id.clone(), symbol.clone());
@@ -55,7 +57,7 @@ impl<'a> Resolver<'a> {
     }
 }
 
-impl<'a> ASTVisitor for Resolver<'a> {
+impl<'a> ASTVisitor for ScopeResolver<'a> {
     //TODO need to un-insert these - maybe need to rethink visitor
     fn import(&mut self, import_spec: &ImportSpecifier) {
         let ImportSpecifier {

schala-lang/language/src/tree_walk_eval/mod.rs

@@ -0,0 +1,144 @@
+use crate::reduced_ir::{ReducedIR, Expression, Function, Statement, Literal};
+use crate::symbol_table::{DefId};
+use crate::util::ScopeStack;
+
+use std::fmt::Write;
+use std::convert::From;
+
+type EvalResult<T> = Result<T, String>;
+
+#[derive(Debug)]
+pub struct State<'a> {
+  environments: ScopeStack<'a, DefId, RuntimeValue>,
+}
+
+#[derive(Debug)]
+struct RuntimeError {
+    msg: String
+}
+
+impl From<String> for RuntimeError {
+    fn from(msg: String) -> Self {
+        Self {
+            msg
+        }
+    }
+}
+
+impl RuntimeError {
+    fn get_msg(&self) -> String {
+        format!("Runtime error: {}", self.msg)
+    }
+}
+
+fn paren_wrapped(terms: impl Iterator<Item=String>) -> String {
+  let mut buf = String::new();
+  write!(buf, "(").unwrap();
+  for term in terms.map(Some).intersperse(None) {
+    match term {
+      Some(e) => write!(buf, "{}", e).unwrap(),
+      None => write!(buf, ", ").unwrap(),
+    };
+  }
+  write!(buf, ")").unwrap();
+  buf
+}
+
+#[derive(Debug)]
+enum RuntimeValue {
+    Expression(Expression),
+}
+
+impl From<Expression> for RuntimeValue {
+    fn from(ex: Expression) -> Self {
+        Self::Expression(ex)
+    }
+}
+
+fn expr_to_repl(expr: &Expression) -> String {
+    match expr {
+        Expression::Unimplemented => format!("Expression {:?} not implemented", expr),
+        Expression::Literal(lit) => match lit {
+            Literal::Nat(n) => format!("{}", n),
+            Literal::Int(i) => format!("{}", i),
+            Literal::Float(f) => format!("{}", f),
+            Literal::Bool(b) => format!("{}", b),
+            Literal::StringLit(s) => format!("\"{}\"", s),
+        }
+        Expression::Tuple(terms) => paren_wrapped(terms.iter().map(|x| expr_to_repl(x))),
+        Expression::Assign { lval, rval } => {
+            "".to_string()
+        },
+        e => format!("Expression {:?} shouldn't be here", e),
+    }
+}
+
+impl RuntimeValue {
+    fn to_repl(&self) -> String {
+        match self {
+            RuntimeValue::Expression(ref expr) => expr_to_repl(expr)
+        }
+    }
+}
+
+impl<'a> State<'a> {
+    pub fn new() -> Self {
+        Self {
+            environments: ScopeStack::new(Some("global".to_string()))
+        }
+    }
+
+    pub fn evaluate(&mut self, reduced: ReducedIR, repl: bool) -> Vec<Result<String, String>> {
+        let mut acc = vec![];
+
+        for statement in reduced.entrypoint.into_iter() {
+            match self.statement(statement) {
+                Ok(Some(output)) if repl => {
+                    acc.push(Ok(output.to_repl()))
+                },
+                Ok(_) => (),
+                Err(error) => {
+                    acc.push(Err(error.into()));
+                    return acc;
+                }
+            }
+        }
+        acc
+    }
+
+    fn statement(&mut self, stmt: Statement) -> EvalResult<Option<RuntimeValue>> {
+        match stmt {
+            Statement::Binding { id, expr, constant } => {
+                println!("eval() binding id: {}", id);
+                let evaluated = self.expression(expr)?;
+                self.environments.insert(id, evaluated.into());
+                Ok(None)
+            },
+            Statement::Expression(expr) => {
+                let evaluated = self.expression(expr)?;
+                Ok(Some(evaluated.into()))
+            }
+        }
+    }
+
+  fn expression(&mut self, expression: Expression) -> EvalResult<Expression> {
+      use Expression::Unimplemented;
+      Ok(match expression {
+          lit @ Expression::Literal(_) => lit,
+          Expression::Tuple(items) => Expression::Tuple(items.into_iter().map(|expr| self.expression(expr)).collect::<EvalResult<Vec<Expression>>>()?),
+          Expression::Assign { lval, box rval } => {
+              let mut env = self.environments.lookup(&lval);
+              Unimplemented
+          },
+          Expression::Call { f, args } => self.call_expression(f, args)?,
+          Unimplemented => Unimplemented,
+          Expression::ReductionError(e) => return Err(e.into()),
+          _ => Expression::Literal(Literal::Nat(69420)),
+      })
+  }
+
+  fn call_expression(&mut self, f: Function, args: Vec<Expression>) -> EvalResult<Expression> {
+      Err("Call expression not implemented".to_string().into())
+  }
+}
+