diff --git a/src/builtin_cells.rs b/src/builtin_cells.rs
new file mode 100644
index 0000000..037cdc7
--- /dev/null
+++ b/src/builtin_cells.rs
@@ -0,0 +1,54 @@
+use crate::frontend::{Callable, CallArgument, Type};
+use crate::rtlil;
+
+fn builtin_unop_cell(celltype: &str, id: &str, a: &str, y: &str) -> rtlil::Cell {
+    let mut cell = rtlil::Cell::new(id, celltype);
+    cell.add_param("\\A_SIGNED", "0");
+    cell.add_param("\\A_WIDTH", "1");
+    cell.add_param("\\Y_WIDTH", "1");
+    cell.add_connection("\\A", a);
+    cell.add_connection("\\Y", y);
+    cell
+}
+
+fn instantiate_binop(celltype: &str, id: &str, args: &[String], ret: &str) -> rtlil::Cell {
+    let a = args.get(0).expect("wrong argcount slipped through type check");
+    let b = args.get(1).expect("wrong argcount slipped through type check");
+
+    let mut cell = rtlil::Cell::new(id, celltype);
+    cell.add_param("\\A_SIGNED", "0");
+    cell.add_param("\\A_WIDTH", "1");
+    cell.add_param("\\B_SIGNED", "0");
+    cell.add_param("\\B_WIDTH", "1");
+    cell.add_param("\\Y_WIDTH", "1");
+    cell.add_connection("\\A", a);
+    cell.add_connection("\\B", b);
+    cell.add_connection("\\Y", ret);
+    cell
+}
+
+fn make_binop_callable(name: &str, celltype: &'static str) -> Callable {
+    let args = vec![
+        CallArgument {
+            name: "A".to_owned(),
+            atype: Type::wire(),
+        },
+        CallArgument {
+            name: "B".to_owned(),
+            atype: Type::wire(),
+        },
+    ];
+    Callable {
+        name: name.to_owned(),
+        args,
+        ret: Type::wire(),
+        instantiate: Box::new(move |id, args, ret| instantiate_binop(celltype, id, args, ret))
+    }
+}
+
+pub fn get_builtins() -> Vec<Callable> {
+    vec![
+        make_binop_callable("and", "$and"),
+        make_binop_callable("xor", "$xor"),
+    ]
+}
diff --git a/src/frontend.rs b/src/frontend.rs
index 8aeb19b..46fc152 100644
--- a/src/frontend.rs
+++ b/src/frontend.rs
@@ -1,30 +1,8 @@
+use std::collections::BTreeMap;
+
 use crate::parser;
 use crate::rtlil;
-
-fn builtin_binop_cell(celltype: &str, id: &str, a: &str, b: &str, y: &str) -> rtlil::Cell {
-    let mut cell = rtlil::Cell::new(id, celltype);
-    cell.add_param("\\A_SIGNED", "0");
-    cell.add_param("\\A_WIDTH", "1");
-    cell.add_param("\\B_SIGNED", "0");
-    cell.add_param("\\B_WIDTH", "1");
-    cell.add_param("\\Y_WIDTH", "1");
-    cell.add_connection("\\A", a);
-    cell.add_connection("\\B", b);
-    cell.add_connection("\\Y", y);
-    cell
-}
-
-fn builtin_unop_cell(celltype: &str, id: &str, a: &str, y: &str) -> rtlil::Cell {
-    let mut cell = rtlil::Cell::new(id, celltype);
-    cell.add_param("\\A_SIGNED", "0");
-    cell.add_param("\\A_WIDTH", "1");
-    cell.add_param("\\Y_WIDTH", "1");
-    cell.add_connection("\\A", a);
-    cell.add_connection("\\Y", y);
-    cell
-}
-
-// the hacky way
+use crate::builtin_cells::get_builtins;
 
 fn make_pubid(id: &str) -> String {
     "\\".to_owned() + id
@@ -58,45 +36,46 @@ pub enum Type {
     Wire(GenericParam<u32>)
 }
 
-// module that can be instantiated like a function
-pub struct Callable {
+impl Type {
+    pub fn wire() -> Self {
+        Self::Wire(GenericParam::Unsolved)
+    }
 }
 
-fn lower_expression(module: &mut rtlil::Module, expr: &parser::Expression) -> Result<String, CompileError> {
+pub struct CallArgument {
+    pub name: String,
+    pub atype: Type,
+}
+
+// module that can be instantiated like a function
+pub struct Callable {
+    pub name: String,
+    pub args: Vec<CallArgument>,
+    pub ret: Type,
+    pub instantiate: Box<dyn Fn(&str, &[String], &str) -> rtlil::Cell>
+}
+
+struct Context {
+    callables: BTreeMap<String, Callable>
+}
+
+fn lower_expression(ctx: &Context, module: &mut rtlil::Module, expr: &parser::Expression) -> Result<String, CompileError> {
     match expr {
         parser::Expression::Ident(ident) => Ok(make_pubid(&ident)),
         parser::Expression::Call(call) => {
             let output_gen_id = module.make_genid("cell");
             module.add_wire(rtlil::Wire::new(&output_gen_id, 1, None));
 
-            let mut args_resolved = call.args.iter().map(|expr| lower_expression(module, expr));
+            let args_resolved = call.args.iter()
+                .map(|expr| lower_expression(ctx, module, expr)).collect::<Result<Vec<_>, _>>()?;
 
-            // TODO: make this sensible
-            let cell = match *call.name.fragment() {
-                "and" => {
-                    let arg_a = args_resolved.next().unwrap()?;
-                    let arg_b = args_resolved.next().unwrap()?;
-                    let cell_id = module.make_genid("and");
-                    builtin_binop_cell("$and", &cell_id, &arg_a, &arg_b, &output_gen_id)
-                }
-                "xor" => {
-                    let arg_a = args_resolved.next().unwrap()?;
-                    let arg_b = args_resolved.next().unwrap()?;
-                    let cell_id = module.make_genid("xor");
-                    builtin_binop_cell("$xor", &cell_id, &arg_a, &arg_b, &output_gen_id)
-                }
-                "not" => {
-                    let arg_a = args_resolved.next().unwrap()?;
-                    let cell_id = module.make_genid("not");
-                    builtin_unop_cell("$not", &cell_id, &arg_a, &output_gen_id)
-                }
-                "reduce_or" => {
-                    let arg_a = args_resolved.next().unwrap()?;
-                    let cell_id = module.make_genid("reduce_or");
-                    builtin_unop_cell("$reduce_or", &cell_id, &arg_a, &output_gen_id)
-                }
-                name => return Err(CompileError::new(CompileErrorKind::UndefinedReference(name.to_owned()))),
-            };
+            let callable = ctx.callables.get(call.name.fragment() as &str).ok_or_else(|| {
+                CompileError::new(CompileErrorKind::UndefinedReference(call.name.fragment().to_string()))
+            })?;
+
+            let cell_id = module.make_genid(&callable.name);
+
+            let cell = (*callable.instantiate)(&cell_id, args_resolved.as_slice(), &output_gen_id);
             module.add_cell(cell);
             Ok(output_gen_id)
         }
@@ -104,9 +83,9 @@ fn lower_expression(module: &mut rtlil::Module, expr: &parser::Expression) -> Re
     }
 }
 
-fn lower_assignment(module: &mut rtlil::Module, assignment: parser::Assign) -> Result<(), CompileError> {
+fn lower_assignment(ctx: &Context, module: &mut rtlil::Module, assignment: parser::Assign) -> Result<(), CompileError> {
     let target_id = make_pubid(&assignment.lhs);
-    let return_wire = lower_expression(module, &assignment.expr)?;
+    let return_wire = lower_expression(ctx, module, &assignment.expr)?;
     module.add_connection(target_id, return_wire);
     Ok(())
 }
@@ -114,6 +93,7 @@ fn lower_assignment(module: &mut rtlil::Module, assignment: parser::Assign) -> R
 pub fn lower_module(pa_module: parser::Module) -> Result<String, CompileError> {
     let mut writer = rtlil::ILWriter::new();
     let mut ir_module = rtlil::Module::new(make_pubid(&pa_module.name));
+    let mut context = Context { callables: get_builtins().into_iter().map(|clb| (clb.name.to_owned(), clb)).collect() };
     writer.write_line("autoidx 1");
     for (idx, port) in pa_module.ports.iter().enumerate() {
         let dir_option = match port.direction {
@@ -129,7 +109,7 @@ pub fn lower_module(pa_module: parser::Module) -> Result<String, CompileError> {
     }
     for stmt in pa_module.statements {
         match stmt {
-            parser::Statement::Assign(assignment) => lower_assignment(&mut ir_module, assignment)?,
+            parser::Statement::Assign(assignment) => lower_assignment(&context, &mut ir_module, assignment)?,
         }
     }
     ir_module.write_rtlil(&mut writer);