diff --git a/crates/mist-api/src/transpiler.mist b/crates/mist-api/src/transpiler.mist
index f2975fe..6ce29aa 100644
--- a/crates/mist-api/src/transpiler.mist
+++ b/crates/mist-api/src/transpiler.mist
@@ -206,6 +206,10 @@ pub fn transpile_code<'a>(path *PathBuf, source *'a str, mut output_file PathBuf
 
     let parsed = mist_parser::parse(source)?;
 
+    for i in &parsed {
+        let _ = mist_parser::semantics::check_class_semantics(i);
+    }
+
     let output = gc.generate(parsed);
 
     gc.mapping.shift_rust(mod_decl.lines().count() as isize, 0);
diff --git a/crates/parser/src/lib.rs b/crates/parser/src/lib.rs
index ea39710..cf905f0 100644
--- a/crates/parser/src/lib.rs
+++ b/crates/parser/src/lib.rs
@@ -5,6 +5,7 @@ pub mod ast;
 pub mod error;
 pub mod parser;
 pub mod rev_mapper;
+pub mod semantics;
 
 use ast::*;
 
diff --git a/crates/parser/src/semantics.rs b/crates/parser/src/semantics.rs
new file mode 100644
index 0000000..6a2c664
--- /dev/null
+++ b/crates/parser/src/semantics.rs
@@ -0,0 +1,157 @@
+use crate::{
+    ast::{
+        Block, ClassItem, Expression, FunctionDecl, Identifier, Postfix, Spanned, TopLevel,
+        TopLevelKind,
+    },
+    error::AstError,
+};
+
+pub trait GetMutability {
+    fn get_mutability(&self) -> Vec<Identifier>;
+}
+
+/// Class semantics - Ensuring fields are initialized
+
+pub fn find_function_call<'a>(
+    items: &'a Vec<ClassItem>,
+    name: &Identifier,
+) -> Option<&'a Spanned<FunctionDecl>> {
+    items.iter().find_map(|v| {
+        if let ClassItem::Method(m) = v {
+            if m.item.is_using_self() && &m.item.name == name {
+                Some(m)
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    })
+}
+
+pub fn check_class_semantics<'a>(top_level: &TopLevel) -> Result<(), AstError<'a, ()>> {
+    if let TopLevelKind::ClassDecl {
+        fields,
+        constructor,
+        items,
+        ..
+    } = &top_level.0.item
+    {
+        if let Some(constructor) = constructor {
+            let fields = fields
+                .iter()
+                .map(|field| Spanned {
+                    item: field.item.decl.name.clone(),
+                    line: field.line,
+                    column: field.column,
+                })
+                .collect::<Vec<_>>();
+
+            let mut mutability = constructor.item.body.get_mutability();
+
+            for name in mutability.clone() {
+                if let Some(method) = find_function_call(items, &name) {
+                    if let Some(body) = &method.item.body {
+                        mutability.append(&mut body.get_mutability());
+                    }
+                }
+            }
+
+            let mut mutability_iter = mutability.into_iter();
+
+            for field in &fields {
+                if mutability_iter.find(|v| v == &field.item).is_none() {
+                    panic!("Unable to find set for field {}", field.item.0);
+                }
+            }
+        }
+
+        Ok(())
+    } else {
+        Ok(())
+    }
+}
+
+impl<T: GetMutability> GetMutability for Vec<T> {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        self.iter().flat_map(T::get_mutability).collect()
+    }
+}
+
+impl GetMutability for Block {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        let mut items: Vec<Identifier> = self
+            .statements
+            .iter()
+            .flat_map(|v| v.item.get_mutability())
+            .collect();
+
+        if let Some(v) = &self.soft_return {
+            items.append(&mut v.item.get_mutability());
+        }
+
+        items
+    }
+}
+
+impl Expression {
+    fn get_self_ref(&self) -> Option<Identifier> {
+        match self {
+            Self::Fix {
+                initial, postfixes, ..
+            } => match &**initial {
+                Self::Path(v) => {
+                    if v.0[0].ident.0 == "self" {
+                        if let Some(field) = postfixes.get(0) {
+                            match field {
+                                Postfix::FieldAccess(field, _) => return Some(field.clone()),
+                                _ => {}
+                            }
+                        }
+                    }
+                }
+                _ => {}
+            },
+            _ => {}
+        }
+
+        None
+    }
+}
+
+impl GetMutability for Expression {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        match self {
+            Self::Array(v) => v.get_mutability(),
+            Self::ArrayRepeat(v, i) => {
+                let mut a = v.get_mutability();
+                a.append(&mut i.get_mutability());
+                a
+            }
+            Self::Binary { lhs, op, rhs } => {
+                if mutates_lhs(op) {
+                    let mut a = if let Some(self_ref) = lhs.get_self_ref() {
+                        vec![self_ref]
+                    } else {
+                        lhs.get_mutability()
+                    };
+
+                    a.append(&mut rhs.get_mutability());
+                    a
+                } else {
+                    let mut a = lhs.get_mutability();
+                    a.append(&mut rhs.get_mutability());
+                    a
+                }
+            }
+            _ => Vec::new(),
+        }
+    }
+}
+
+pub fn mutates_lhs(op: &str) -> bool {
+    matches!(
+        op,
+        "=" | "+=" | "-=" | "*=" | "/=" | "%=" | "&=" | "|=" | "^=" | "<<=" | ">>="
+    )
+}