diff --git a/data/test.lang b/data/test.lang
index fd635ae..f780cae 100644
--- a/data/test.lang
+++ b/data/test.lang
@@ -30,6 +30,7 @@ fn start() {
       x = add(x, 1);
    };
    println("after");
+   let infer_me: slice<_> = "hello";
    print(tester());
    let test: Test = Test {
       a: 10,
diff --git a/src/ir/upper/ty.rs b/src/ir/upper/ty.rs
index 9be1473..9168d85 100644
--- a/src/ir/upper/ty.rs
+++ b/src/ir/upper/ty.rs
@@ -127,12 +127,23 @@ pub fn match_types(dest: &Type, src: &Type) -> Option<Type> {
             if dest_id != src_id {
                 return None;
             }
-            None
-            // TODO
-            // let mut args = Vec::new();
-            // for (darg, sarg) in dest_args.iter().zip(src_args) {
-            // }
-            // Some(Type::Struct { id: *dest_id, args })
+            let mut args = Vec::new();
+            let mut changed = false;
+            for (darg, sarg) in dest_args.iter().zip(src_args) {
+                if let Some(ty) = match_types(darg, sarg) {
+                    args.push(ty);
+                    changed = true;
+                } else if darg != sarg {
+                    return None;
+                } else {
+                    args.push(darg.clone());
+                }
+            }
+            if changed {
+                Some(Type::Struct { id: *dest_id, args })
+            } else {
+                None
+            }
         }
         (
             Type::Fn {
@@ -147,13 +158,13 @@ pub fn match_types(dest: &Type, src: &Type) -> Option<Type> {
             // TODO
             None
         }
-        (Type::Ref(dest), Type::Ref(src)) => Some(match_types(dest, src)?),
-        (Type::Slice(dest), Type::Slice(src)) => Some(match_types(dest, src)?),
+        (Type::Ref(dest), Type::Ref(src)) => Some(match_types(dest, src)?.rf()),
+        (Type::Slice(dest), Type::Slice(src)) => Some(match_types(dest, src)?.slice()),
         (Type::Array(dest, dlen), Type::Array(src, slen)) => {
             if dlen != slen {
                 return None;
             }
-            match_types(dest, src)
+            Some(match_types(dest, src)?.arr(*dlen))
         }
         _ => None,
     }
diff --git a/src/ir/upper/validate.rs b/src/ir/upper/validate.rs
index 35aa280..fea2172 100644
--- a/src/ir/upper/validate.rs
+++ b/src/ir/upper/validate.rs
@@ -15,6 +15,12 @@ impl UProgram {
                     spans: vec![var.origin],
                 });
             }
+            if var.ty == Type::Infer {
+                output.err(CompilerMsg {
+                    msg: format!("Var {:?} cannot be inferred", id),
+                    spans: vec![var.origin],
+                });
+            }
         }
         output
     }
diff --git a/src/parser/v3/lower/def.rs b/src/parser/v3/lower/def.rs
index 45cff8a..ef3b004 100644
--- a/src/parser/v3/lower/def.rs
+++ b/src/parser/v3/lower/def.rs
@@ -67,6 +67,7 @@ impl PType {
                             let inner = self.args[0].lower(namespace, output);
                             Type::Slice(Box::new(inner))
                         }
+                        "_" => Type::Infer,
                         _ => {
                             output.err(CompilerMsg::from_span(span, "Type not found".to_string()));
                             Type::Error