diff --git a/src/repl/mod.rs b/src/repl/mod.rs
index ec44d89..747f189 100755
--- a/src/repl/mod.rs
+++ b/src/repl/mod.rs
@@ -1,7 +1,9 @@
-use std::io::{self, Write};
+use ::std::io::{self, Write};
 
 use crate::codegen::Transpilable;
 use crate::error_handling::PrintableError;
+use crate::semantic::std;
+use crate::semantic::symbol_table::SymbolTable;
 
 use super::lexic;
 use super::syntax;
@@ -12,6 +14,8 @@ use crate::php_ast::transformers::PHPTransformable;
 pub fn run() -> io::Result<()> {
     let stdin = io::stdin();
     let mut buffer = String::new();
+    let mut repl_symbol_table = SymbolTable::new();
+    std::populate(&mut repl_symbol_table);
 
     println!("REPL: Enter expressions to evaluate. Type Ctrl-D to exit.");
     loop {
@@ -26,7 +30,7 @@ pub fn run() -> io::Result<()> {
                 break Ok(());
             }
             Ok(_) => {
-                compile(&buffer);
+                compile(&buffer, &mut repl_symbol_table);
             }
             Err(error) => {
                 eprintln!("Error reading stdin.");
@@ -37,7 +41,7 @@ pub fn run() -> io::Result<()> {
 }
 
 /// Full pipeline from THP source code to PHP output
-fn compile(input: &String) {
+fn compile(input: &String, symbol_table: &mut SymbolTable) {
     //
     // Lexical analysis
     //
@@ -63,7 +67,7 @@ fn compile(input: &String) {
     //
     // Semantic analysis
     //
-    let res1 = crate::semantic::check_semantics(&ast);
+    let res1 = crate::semantic::check_semantics_with(&ast, symbol_table);
     match res1 {
         Ok(_) => {}
         Err(error) => {
diff --git a/src/semantic/mod.rs b/src/semantic/mod.rs
index 6331d61..683e099 100644
--- a/src/semantic/mod.rs
+++ b/src/semantic/mod.rs
@@ -2,11 +2,12 @@ use crate::{error_handling::MistiError, syntax::ast::ModuleAST};
 
 mod checks;
 mod impls;
-mod std;
-mod symbol_table;
+pub mod std;
+pub mod symbol_table;
 mod types;
 
 use impls::SemanticCheck;
+use symbol_table::SymbolTable;
 
 // What to do?
 // 1. Create a mutable symbol table
@@ -25,6 +26,15 @@ pub fn check_semantics(ast: &ModuleAST) -> Result<(), MistiError> {
     ast.check_semantics(&global_scope)
 }
 
+/// Checks that the AST is semantically correct.
+/// Accepts a handle to a symbol table to operate with.
+pub fn check_semantics_with(
+    ast: &ModuleAST,
+    symbol_table: &mut SymbolTable,
+) -> Result<(), MistiError> {
+    ast.check_semantics(&symbol_table)
+}
+
 #[cfg(test)]
 mod tests {
     use crate::semantic::types::Type;
diff --git a/src/semantic/std.rs b/src/semantic/std.rs
index a499851..e89e0d4 100644
--- a/src/semantic/std.rs
+++ b/src/semantic/std.rs
@@ -19,4 +19,8 @@ pub fn populate(table: &mut SymbolTable) {
     // + operator (Int, Int) -> Int
     let plus_op = Type::Function(vec![INT.into(), INT.into()], INT.into());
     table.insert("+".into(), plus_op);
+
+    // - operator (Int, Int) -> Int
+    let plus_op = Type::Function(vec![INT.into(), INT.into()], INT.into());
+    table.insert("-".into(), plus_op);
 }