diff --git a/nac3core/src/context/inference_context.rs b/nac3core/src/context/inference_context.rs
new file mode 100644
index 0000000000..d056e3d4e8
--- /dev/null
+++ b/nac3core/src/context/inference_context.rs
@@ -0,0 +1,228 @@
+use super::TopLevelContext;
+use crate::typedef::*;
+use std::boxed::Box;
+use std::collections::HashMap;
+
+struct ContextStack<'a> {
+    /// stack level, starts from 0
+    level: u32,
+    /// stack of variable definitions containing (id, def, level) where `def` is the original
+    /// definition in `level-1`.
+    var_defs: Vec<(usize, VarDef<'a>, u32)>,
+    /// stack of symbol definitions containing (name, level) where `level` is the smallest level
+    /// where the name is assigned a value
+    sym_def: Vec<(&'a str, u32)>,
+}
+
+pub struct InferenceContext<'a> {
+    /// top level context
+    top_level: TopLevelContext<'a>,
+
+    /// list of primitive instances
+    primitives: Vec<Type>,
+    /// list of variable instances
+    variables: Vec<Type>,
+    /// identifier to (type, readable) mapping.
+    /// an identifier might be defined earlier but has no value (for some code path), thus not
+    /// readable.
+    sym_table: HashMap<&'a str, (Type, bool)>,
+    /// resolution function reference, that may resolve unbounded identifiers to some type
+    resolution_fn: Box<dyn FnMut(&str) -> Result<Type, String>>,
+    /// stack
+    stack: ContextStack<'a>,
+}
+
+// non-trivial implementations here
+impl<'a> InferenceContext<'a> {
+    /// return a new `InferenceContext` from `TopLevelContext` and resolution function.
+    pub fn new(
+        top_level: TopLevelContext,
+        resolution_fn: Box<dyn FnMut(&str) -> Result<Type, String>>,
+    ) -> InferenceContext {
+        let primitives = (0..top_level.primitive_defs.len())
+            .map(|v| TypeEnum::PrimitiveType(PrimitiveId(v)).into())
+            .collect();
+        let variables = (0..top_level.var_defs.len())
+            .map(|v| TypeEnum::TypeVariable(VariableId(v)).into())
+            .collect();
+        InferenceContext {
+            top_level,
+            primitives,
+            variables,
+            sym_table: HashMap::new(),
+            resolution_fn,
+            stack: ContextStack {
+                level: 0,
+                var_defs: Vec::new(),
+                sym_def: Vec::new(),
+            },
+        }
+    }
+
+    /// execute the function with new scope.
+    /// variable assignment would be limited within the scope (not readable outside), and type
+    /// variable type guard would be limited within the scope.
+    /// returns the list of variables assigned within the scope, and the result of the function
+    pub fn with_scope<F, R>(&mut self, f: F) -> (Vec<&'a str>, R)
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        self.stack.level += 1;
+        let result = f(self);
+        self.stack.level -= 1;
+        while !self.stack.var_defs.is_empty() {
+            let (_, _, level) = self.stack.var_defs.last().unwrap();
+            if *level > self.stack.level {
+                let (id, def, _) = self.stack.var_defs.pop().unwrap();
+                self.top_level.var_defs[id] = def;
+            } else {
+                break;
+            }
+        }
+        let mut poped_names = Vec::new();
+        while !self.stack.sym_def.is_empty() {
+            let (_, level) = self.stack.sym_def.last().unwrap();
+            if *level > self.stack.level {
+                let (name, _) = self.stack.sym_def.pop().unwrap();
+                self.sym_table.remove(name).unwrap();
+                poped_names.push(name);
+            } else {
+                break;
+            }
+        }
+        (poped_names, result)
+    }
+
+    /// assign a type to an identifier.
+    /// may return error if the identifier was defined but with different type
+    pub fn assign(&mut self, name: &'a str, ty: Type) -> Result<Type, String> {
+        if let Some((t, x)) = self.sym_table.get_mut(name) {
+            if t == &ty {
+                if !*x {
+                    self.stack.sym_def.push((name, self.stack.level));
+                }
+                *x = true;
+                Ok(ty)
+            } else {
+                Err("different types".into())
+            }
+        } else {
+            self.stack.sym_def.push((name, self.stack.level));
+            self.sym_table.insert(name, (ty.clone(), true));
+            Ok(ty)
+        }
+    }
+
+    /// check if an identifier is already defined
+    pub fn defined(&self, name: &str) -> bool {
+        self.sym_table.get(name).is_some()
+    }
+
+    /// get the type of an identifier
+    /// may return error if the identifier is not defined, and cannot be resolved with the
+    /// resolution function.
+    pub fn resolve(&mut self, name: &str) -> Result<Type, String> {
+        if let Some((t, x)) = self.sym_table.get(name) {
+            if *x {
+                Ok(t.clone())
+            } else {
+                Err("may not have value".into())
+            }
+        } else {
+            self.resolution_fn.as_mut()(name)
+        }
+    }
+
+    /// restrict the bound of a type variable by replacing its definition.
+    /// used for implementing type guard
+    pub fn restrict(&mut self, id: VariableId, mut def: VarDef<'a>) {
+        std::mem::swap(self.top_level.var_defs.get_mut(id.0).unwrap(), &mut def);
+        self.stack.var_defs.push((id.0, def, self.stack.level));
+    }
+}
+
+// trivial getters:
+impl<'a> InferenceContext<'a> {
+    pub fn get_primitive(&self, id: PrimitiveId) -> Type {
+        self.primitives.get(id.0).unwrap().clone()
+    }
+    pub fn get_variable(&self, id: VariableId) -> Type {
+        self.variables.get(id.0).unwrap().clone()
+    }
+
+    pub fn get_fn_def(&self, name: &str) -> Option<&FnDef> {
+        self.top_level.fn_table.get(name)
+    }
+    pub fn get_primitive_def(&self, id: PrimitiveId) -> &TypeDef {
+        self.top_level.primitive_defs.get(id.0).unwrap()
+    }
+    pub fn get_class_def(&self, id: ClassId) -> &ClassDef {
+        self.top_level.class_defs.get(id.0).unwrap()
+    }
+    pub fn get_parametric_def(&self, id: ParamId) -> &ParametricDef {
+        self.top_level.parametric_defs.get(id.0).unwrap()
+    }
+    pub fn get_variable_def(&self, id: VariableId) -> &VarDef {
+        self.top_level.var_defs.get(id.0).unwrap()
+    }
+    pub fn get_type(&self, name: &str) -> Option<Type> {
+        self.top_level.get_type(name)
+    }
+}
+
+impl TypeEnum {
+    pub fn subst(&self, map: &HashMap<VariableId, Type>) -> TypeEnum {
+        match self {
+            TypeEnum::TypeVariable(id) => map.get(id).map(|v| v.as_ref()).unwrap_or(self).clone(),
+            TypeEnum::ParametricType(id, params) => TypeEnum::ParametricType(
+                *id,
+                params
+                    .iter()
+                    .map(|v| v.as_ref().subst(map).into())
+                    .collect(),
+            ),
+            _ => self.clone(),
+        }
+    }
+
+    pub fn inv_subst(&self, map: &[(Type, Type)]) -> Type {
+        for (from, to) in map.iter() {
+            if self == from.as_ref() {
+                return to.clone();
+            }
+        }
+        match self {
+            TypeEnum::ParametricType(id, params) => TypeEnum::ParametricType(
+                *id,
+                params.iter().map(|v| v.as_ref().inv_subst(map)).collect(),
+            ),
+            _ => self.clone(),
+        }
+        .into()
+    }
+
+    pub fn get_subst(&self, ctx: &InferenceContext) -> HashMap<VariableId, Type> {
+        match self {
+            TypeEnum::ParametricType(id, params) => {
+                let vars = &ctx.get_parametric_def(*id).params;
+                vars.iter()
+                    .zip(params)
+                    .map(|(v, p)| (*v, p.as_ref().clone().into()))
+                    .collect()
+            }
+            // if this proves to be slow, we can use option type
+            _ => HashMap::new(),
+        }
+    }
+
+    pub fn get_base<'b: 'a, 'a>(&'a self, ctx: &'b InferenceContext) -> Option<&'b TypeDef> {
+        match self {
+            TypeEnum::PrimitiveType(id) => Some(ctx.get_primitive_def(*id)),
+            TypeEnum::ClassType(id) | TypeEnum::VirtualClassType(id) => {
+                Some(&ctx.get_class_def(*id).base)
+            }
+            TypeEnum::ParametricType(id, _) => Some(&ctx.get_parametric_def(*id).base),
+            _ => None,
+        }
+    }
+}
diff --git a/nac3core/src/context/mod.rs b/nac3core/src/context/mod.rs
index 79f26bf5be..f59140d94a 100644
--- a/nac3core/src/context/mod.rs
+++ b/nac3core/src/context/mod.rs
@@ -1,2 +1,4 @@
+mod inference_context;
 mod top_level_context;
+pub use inference_context::InferenceContext;
 pub use top_level_context::TopLevelContext;