diff --git a/nac3core/src/inference_core.rs b/nac3core/src/inference_core.rs
new file mode 100644
index 000000000..1442a966b
--- /dev/null
+++ b/nac3core/src/inference_core.rs
@@ -0,0 +1,200 @@
+use crate::context::InferenceContext;
+use crate::typedef::{TypeEnum::*, *};
+use std::collections::HashMap;
+
+fn find_subst(
+    ctx: &InferenceContext,
+    valuation: &Option<(VariableId, Type)>,
+    sub: &mut HashMap<VariableId, Type>,
+    mut a: Type,
+    mut b: Type,
+) -> Result<(), String> {
+    // TODO: fix error messages later
+    if let TypeVariable(id) = a.as_ref() {
+        if let Some((assumption_id, t)) = valuation {
+            if assumption_id == id {
+                a = t.clone();
+            }
+        }
+    }
+
+    let mut substituted = false;
+    if let TypeVariable(id) = b.as_ref() {
+        if let Some(c) = sub.get(&id) {
+            b = c.clone();
+            substituted = true;
+        }
+    }
+
+    match (a.as_ref(), b.as_ref()) {
+        (BotType, _) => Ok(()),
+        (TypeVariable(id_a), TypeVariable(id_b)) => {
+            if substituted {
+                return if id_a == id_b {
+                    Ok(())
+                } else {
+                    Err("different variables".to_string())
+                };
+            }
+            let v_a = ctx.get_variable_def(*id_a);
+            let v_b = ctx.get_variable_def(*id_b);
+            if !v_b.bound.is_empty() {
+                if v_a.bound.is_empty() {
+                    return Err("unbounded a".to_string());
+                } else {
+                    let diff: Vec<_> = v_a
+                        .bound
+                        .iter()
+                        .filter(|x| !v_b.bound.contains(x))
+                        .collect();
+                    if !diff.is_empty() {
+                        return Err("different domain".to_string());
+                    }
+                }
+            }
+            sub.insert(*id_b, a.clone());
+            Ok(())
+        }
+        (TypeVariable(id_a), _) => {
+            let v_a = ctx.get_variable_def(*id_a);
+            if v_a.bound.len() == 1 && v_a.bound[0].as_ref() == b.as_ref() {
+                Ok(())
+            } else {
+                Err("different domain".to_string())
+            }
+        }
+        (_, TypeVariable(id_b)) => {
+            let v_b = ctx.get_variable_def(*id_b);
+            if v_b.bound.is_empty() || v_b.bound.contains(&a) {
+                sub.insert(*id_b, a.clone());
+                Ok(())
+            } else {
+                Err("different domain".to_string())
+            }
+        }
+        (_, VirtualClassType(id_b)) => {
+            let mut parents;
+            match a.as_ref() {
+                ClassType(id_a) => {
+                    parents = [*id_a].to_vec();
+                }
+                VirtualClassType(id_a) => {
+                    parents = [*id_a].to_vec();
+                }
+                _ => {
+                    return Err("cannot substitute non-class type into virtual class".to_string());
+                }
+            };
+            while !parents.is_empty() {
+                if *id_b == parents[0] {
+                    return Ok(());
+                }
+                let c = ctx.get_class_def(parents.remove(0));
+                parents.extend_from_slice(&c.parents);
+            }
+            Err("not subtype".to_string())
+        }
+        (ParametricType(id_a, param_a), ParametricType(id_b, param_b)) => {
+            if id_a != id_b || param_a.len() != param_b.len() {
+                Err("different parametric types".to_string())
+            } else {
+                for (x, y) in param_a.iter().zip(param_b.iter()) {
+                    find_subst(ctx, valuation, sub, x.clone(), y.clone())?;
+                }
+                Ok(())
+            }
+        }
+        (_, _) => {
+            if a == b {
+                Ok(())
+            } else {
+                Err("not equal".to_string())
+            }
+        }
+    }
+}
+
+fn resolve_call_rec(
+    ctx: &InferenceContext,
+    valuation: &Option<(VariableId, Type)>,
+    obj: Option<Type>,
+    func: &str,
+    args: &[Type],
+) -> Result<Option<Type>, String> {
+    let mut subst = obj
+        .as_ref()
+        .map(|v| v.get_subst(ctx))
+        .unwrap_or_else(HashMap::new);
+
+    let fun = match &obj {
+        Some(obj) => {
+            let base = match obj.as_ref() {
+                TypeVariable(id) => {
+                    let v = ctx.get_variable_def(*id);
+                    if v.bound.is_empty() {
+                        return Err("unbounded type var".to_string());
+                    }
+                    let results: Result<Vec<_>, String> = v
+                        .bound
+                        .iter()
+                        .map(|ins| {
+                            resolve_call_rec(
+                                ctx,
+                                &Some((*id, ins.clone())),
+                                Some(ins.clone()),
+                                func,
+                                args.clone(),
+                            )
+                        })
+                        .collect();
+                    let results = results?;
+                    if results.iter().all(|v| v == &results[0]) {
+                        return Ok(results[0].clone());
+                    }
+                    let mut results = results.iter().zip(v.bound.iter()).map(|(r, ins)| {
+                        r.as_ref()
+                            .map(|v| v.inv_subst(&[(ins.clone(), obj.clone())]))
+                    });
+                    let first = results.next().unwrap();
+                    if results.all(|v| v == first) {
+                        return Ok(first);
+                    } else {
+                        return Err("divergent type after substitution".to_string());
+                    }
+                }
+                PrimitiveType(id) => &ctx.get_primitive_def(*id),
+                ClassType(id) | VirtualClassType(id) => &ctx.get_class_def(*id).base,
+                ParametricType(id, _) => &ctx.get_parametric_def(*id).base,
+                _ => return Err("not supported".to_string()),
+            };
+            base.methods.get(func)
+        }
+        None => ctx.get_fn_def(func),
+    }
+    .ok_or_else(|| "no such function".to_string())?;
+
+    if args.len() != fun.args.len() {
+        return Err("incorrect parameter number".to_string());
+    }
+    for (a, b) in args.iter().zip(fun.args.iter()) {
+        find_subst(ctx, valuation, &mut subst, a.clone(), b.clone())?;
+    }
+    let result = fun.result.as_ref().map(|v| v.subst(&subst));
+    Ok(result.map(|result| {
+        if let SelfType = result {
+            obj.unwrap()
+        } else {
+            result.into()
+        }
+    }))
+}
+
+pub fn resolve_call(
+    ctx: &InferenceContext,
+    obj: Option<Type>,
+    func: &str,
+    args: &[Type],
+) -> Result<Option<Type>, String> {
+    resolve_call_rec(ctx, &None, obj, func, args)
+}
+
diff --git a/nac3core/src/lib.rs b/nac3core/src/lib.rs
index 73495624c..b3fcf1a53 100644
--- a/nac3core/src/lib.rs
+++ b/nac3core/src/lib.rs
@@ -5,6 +5,7 @@ extern crate num_bigint;
 extern crate inkwell;
 extern crate rustpython_parser;
 
+pub mod inference_core;
 pub mod primitives;
 pub mod typedef;
 pub mod context;