diff --git a/TODO.md b/TODO.md
index 73e0583..ae9969a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -20,6 +20,11 @@ Inspect is probably the correct location for this.
 * Flags should allow different seperators for prefixed flags.
 * HCL export
 
+# Documentation TODO
+
+* Recursive Modules
+* Schema Checking With Modules
+
 # Release Checklist
 
 * Cargo test
diff --git a/src/build/mod.rs b/src/build/mod.rs
index 7182ddd..3eeec91 100644
--- a/src/build/mod.rs
+++ b/src/build/mod.rs
@@ -1711,11 +1711,12 @@ impl<'a> FileBuilder<'a> {
         def: &BinaryOpDef,
         scope: &Scope,
     ) -> Result<Rc<Val>, Box<dyn Error>> {
-        let typ = match def.right.as_ref() {
-            Expression::Simple(Value::Str(ref s)) => s.val.clone(),
+        let tval = self.eval_expr(def.right.as_ref(), scope)?;
+        let typ = match tval.as_ref() {
+            Val::Str(ref s) => s.clone(),
             _ => {
                 return Err(Box::new(error::BuildError::new(
-                    format!("Expected string expression but got {}", def.right),
+                    format!("Expected string expression but got {}", tval),
                     error::ErrorType::TypeFail,
                     def.right.pos().clone(),
                 )));
diff --git a/std/tests/tuples_test.ucg b/std/tests/tuples_test.ucg
index b94cb52..53192bc 100644
--- a/std/tests/tuples_test.ucg
+++ b/std/tests/tuples_test.ucg
@@ -19,4 +19,23 @@ assert t.equal{
 assert t.equal{
     left = tpl.strip_nulls{tpl={foo="bar", bar=NULL}}.result,
     right = {foo="bar"},
+};
+
+assert t.ok{
+    test = tpl.has_fields{tpl={foo=1, bar=2}, fields=["foo", "bar"]}.result,
+    desc = "tuple has fields has foo and bar fields",
+};
+
+assert t.not_ok{
+    test = tpl.has_fields{tpl={blah=1, bar=2}, fields=["foo", "bar"]}.result,
+    desc = "tuple does not have fields foo and bar",
+};
+
+assert t.ok{
+    test = tpl.field_type{
+        tpl={foo=1},
+        field="foo",
+        type="int",
+    }.result,
+    desc = "tuple has field of type int",
 };
\ No newline at end of file
diff --git a/std/tuples.ucg b/std/tuples.ucg
index 056bc1d..7e40fc5 100644
--- a/std/tuples.ucg
+++ b/std/tuples.ucg
@@ -1,8 +1,15 @@
+let assert_tuple = macro(tpl) => select tpl is "tuple", NULL, {
+    false = fail "@ is not a tuple" % (tpl),
+};
 
 // return a list of the fields in a tuple.
 let fields = module{
     tpl = NULL,
 } => {
+    let assert_tuple = import "std/tuples.ucg".assert_tuple;
+    // First we check that mod.tpl is a tuple.
+    assert_tuple(mod.tpl);
+    
     let reducer = macro(acc, field, value) =>  acc + [field];
 
     let result = reduce reducer [], (mod.tpl);
@@ -12,6 +19,10 @@ let fields = module{
 let values = module{
     tpl = NULL,
 } => {
+    let assert_tuple = import "std/tuples.ucg".assert_tuple;
+    // First we check that mod.tpl is a tuple.
+    assert_tuple(mod.tpl);
+    
     let reducer = macro(acc, field, value) =>  acc + [value];
 
     let result = reduce reducer [], (mod.tpl);
@@ -20,6 +31,10 @@ let values = module{
 let iter = module{
     tpl = NULL,
 } => {
+    let assert_tuple = import "std/tuples.ucg".assert_tuple;
+    // First we check that mod.tpl is a tuple.
+    assert_tuple(mod.tpl);
+    
     let reducer = macro(acc, field, value) =>  acc + [[field, value]];
     
     let result = reduce reducer [], (mod.tpl);
@@ -28,7 +43,59 @@ let iter = module{
 let strip_nulls = module{
     tpl = NULL,
 } => {
+    let assert_tuple = import "std/tuples.ucg".assert_tuple;
+    // First we check that mod.tpl is a tuple.
+    assert_tuple(mod.tpl);
+    
     let filterer = macro(name, value) =>  value != NULL;
 
     let result = filter filterer (mod.tpl);
+};
+
+let has_fields = module{
+    tpl = NULL,
+    fields = [],
+} => {
+    let lib = import "std/tuples.ucg";
+    // First we check that mod.tpl is a tuple.
+    lib.assert_tuple(mod.tpl);
+
+    let fs = lib.fields{tpl=mod.tpl}.result;
+
+    let reducer = macro(acc, f) => acc && (f in fs);
+
+    let result = reduce reducer true, (mod.fields);
+};
+
+let field_type = module{
+    tpl = NULL,
+    field = NULL,
+    type = NULL,
+} => {
+    let lib = import "std/tuples.ucg";
+    // First we check that mod.tpl is a tuple.
+    lib.assert_tuple(mod.tpl);
+
+    // Next we assert that mod.field is a string.
+    select mod.field is "str", NULL, {
+        false = fail "@ is not a string" % (mod.field),
+    };
+
+    // and finally that mod.type is a string
+    select mod.type is "str", NULL, {
+        false = fail "@ is not a string" % (mod.type),
+    };
+
+    let has_field = lib.has_fields{tpl=mod.tpl, fields=[mod.field]}.result; 
+    
+    let it = lib.iter{tpl=mod.tpl}.result;
+    
+    let reducer = macro(acc, l) => acc && (select l.0 == mod.field, NULL, {
+        true = l.1 is mod.type, // if this is the field then we propagate if it's the right type.
+        false = true, // if this isn't the field then we propagate true
+    });
+
+    let is_type = reduce reducer true, it;
+
+    let result = has_field && is_type;
 };
\ No newline at end of file