diff --git a/Cargo.lock b/Cargo.lock
index 880dfbd..86310ef 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -131,6 +131,7 @@ dependencies = [
  "chumsky",
  "hir",
  "lexer",
+ "typecheck",
 ]
 
 [[package]]
@@ -163,6 +164,7 @@ dependencies = [
  "serde",
  "serde_derive",
  "toml",
+ "typecheck",
 ]
 
 [[package]]
@@ -361,6 +363,13 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "typecheck"
+version = "0.1.0"
+dependencies = [
+ "hir",
+]
+
 [[package]]
 name = "unicode-xid"
 version = "0.2.2"
diff --git a/Cargo.toml b/Cargo.toml
index 1a1ffc4..1edecab 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,5 +5,6 @@ members = [
     "crates/parser",
     "crates/diagnostic",
     "crates/hir",
+    "crates/typecheck",
     "crates/codegen",
-]
\ No newline at end of file
+]
diff --git a/crates/codegen/src/ts.rs b/crates/codegen/src/ts.rs
index 0115f5f..1e111b7 100644
--- a/crates/codegen/src/ts.rs
+++ b/crates/codegen/src/ts.rs
@@ -31,7 +31,7 @@ impl Codegen {
         macro_rules! semicolon { () => { if should_gen_semicolon { ";" } else { "" } }; }
 
         match ir {
-            IRKind::Define { public, name, type_hint, value, mutable } => {
+            IRKind::Define { public, name, type_hint, value, mutable, .. } => {
                 format!(
                     "{} {} v_{}: {} = {}{}\n",
                     if *public { "export" } else { "" },
@@ -43,7 +43,7 @@ impl Codegen {
                 )
             },
 
-            IRKind::Call { name, args } => {
+            IRKind::Call { name, args, .. } => {
                 format!(
                     "f_{}({}){}",
                     name,
@@ -57,7 +57,7 @@ impl Codegen {
                 )
             },
 
-            IRKind::Intrinsic { name, args } => {
+            IRKind::Intrinsic { name, args, .. } => {
                 match name.as_str() {
                     "write"      => { format!("write({}){}\n"        , self.gen_ir(&args[0], false), semicolon!()) },
                     "write_file" => { format!("writeFile({}, {}){}\n", self.gen_ir(&args[0], false), self.gen_ir(&args[1], false), semicolon!()) },
@@ -71,7 +71,7 @@ impl Codegen {
                 }
             },
 
-            IRKind::Fun { public, name, return_type_hint, args, body } => {
+            IRKind::Fun { public, name, return_type_hint, args, body, .. } => {
                 let args = args
                     .iter()
                     .map(|arg| format!("v_{}: {}", arg.0, arg.1))
@@ -87,14 +87,14 @@ impl Codegen {
                 )
             },
 
-            IRKind::Return { value } => {
+            IRKind::Return { value, .. } => {
                 format!(
                     "return {};\n",
                     self.gen_ir(value, false)
                 )
             },
 
-            IRKind::Do { body } => {
+            IRKind::Do { body, .. } => {
                 let mut out = "{\n".to_string();
                 for expr in body {
                     out.push_str(&self.gen_ir(&expr, true));
@@ -103,7 +103,7 @@ impl Codegen {
                 out
             },
 
-            IRKind::If { cond, body, else_body } => {
+            IRKind::If { cond, body, else_body, .. } => {
                 format!(
                     "if ({}) {{\n{}}} else {{\n{}}}\n",
                     self.gen_ir(cond, true),
@@ -112,7 +112,7 @@ impl Codegen {
                 )
             },
 
-            IRKind::Case { cond, cases, default } => {
+            IRKind::Case { cond, cases, default, .. } => {
                 format!(
                     "switch ({}) {{\n{}{}\n}}\n",
                     self.gen_ir(cond, true),
@@ -131,11 +131,11 @@ impl Codegen {
                 )
             },
 
-            IRKind::Unary { op, right } => {
+            IRKind::Unary { op, right, .. } => {
                 format!("{}{}", op, self.gen_ir(right, false))
             },
 
-            IRKind::Binary { left, op, right } => {
+            IRKind::Binary { left, op, right, .. } => {
                 format!("{} {} {}", self.gen_ir(left, false), op, self.gen_ir(right, false))
             },
 
diff --git a/crates/diagnostic/Cargo.toml b/crates/diagnostic/Cargo.toml
index 8dfae66..af43f71 100644
--- a/crates/diagnostic/Cargo.toml
+++ b/crates/diagnostic/Cargo.toml
@@ -9,4 +9,5 @@ edition = "2021"
 chumsky =  "0.8.0"
 ariadne = "0.1.5"
 lexer = { path = "../lexer" }
-hir = { path = "../hir" }
\ No newline at end of file
+hir = { path = "../hir" }
+typecheck = { path = "../typecheck" }
\ No newline at end of file
diff --git a/crates/diagnostic/src/lib.rs b/crates/diagnostic/src/lib.rs
index 3811794..eb02dd0 100644
--- a/crates/diagnostic/src/lib.rs
+++ b/crates/diagnostic/src/lib.rs
@@ -12,6 +12,7 @@ pub enum Kind {
     LexError(Simple<char>),
     ParseError(Simple<Token>),
     LoweringError(hir::LoweringError),
+    TypecheckError(typecheck::TypecheckError),
 }
 
 impl Diagnostics {
@@ -37,6 +38,10 @@ impl Diagnostics {
         self.errors.push(Kind::LoweringError(error));
     }
 
+    pub fn add_typecheck_error(&mut self, error: typecheck::TypecheckError) {
+        self.errors.push(Kind::TypecheckError(error));
+    }
+
     pub fn display(&self, src: String) {
         let lex_error = self.errors.iter().filter_map(|kind| match kind {
             Kind::LexError(error) => Some(error.clone()), // Using clone() to remove reference
@@ -120,6 +125,11 @@ impl Diagnostics {
             Kind::LoweringError(error) => Some(error.clone()),
             _ => None,
         });
+        let typecheck_error = self.errors.iter().filter_map(|kind| match kind {
+            Kind::TypecheckError(error) => Some(error.clone()),
+            _ => None,
+        });
+        // TODO: so many .iter(), maybe collapse them into one?
 
         lower_error.into_iter()
         .for_each(|e| {
@@ -138,13 +148,27 @@ impl Diagnostics {
                     .with_color(Color::Red)
                 );
 
-            if let Some(note) = &e.note {
-                report
-                .with_note(note)
-                .finish().print(Source::from(&src)).unwrap();
-            } else {
-                report.finish().print(Source::from(&src)).unwrap();
-            }
+            report.finish().print(Source::from(&src)).unwrap();
         });
+
+        typecheck_error.into_iter()
+        .for_each(|e| {
+            let span = &e.span;
+            let message = &e.kind;
+
+            let report = Report::build(ReportKind::Error, (), span.start)
+                .with_message(
+                    format!("{}", message)
+                )
+                .with_label(
+                    Label::new(span.clone())
+                    .with_message(
+                        format!("{}", message)
+                    )
+                    .with_color(Color::Red)
+                );
+
+            report.finish().print(Source::from(&src)).unwrap();
+        })
     }
 }
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 657fe77..0f25e50 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -29,31 +29,62 @@ impl std::fmt::Display for Value {
 pub enum IRKind {
     Value { value: Value },
     Vector { values: Vec<Self> },
-    Unary { op: String, right: Box<Self> },
-    Binary { op: String, left: Box<Self>, right: Box<Self> },
-    Call { name: String, args: Vec<Self> },
-    Intrinsic { name: String, args: Vec<Self> },
+
+    Unary {
+        op: String,
+        right: Box<Self>,
+        span: Range<usize>
+    },
+    Binary {
+        op: String,
+        left: Box<Self>,
+        right: Box<Self>,
+        span: Range<usize>
+    },
+    Call {
+        name: String,
+        args: Vec<Self>,
+        span: Range<usize>
+    },
+    Intrinsic {
+        name: String,
+        args: Vec<Self>,
+        span: Range<usize>
+    },
 
     Define {
         public: bool,
         name: String,
         type_hint: String,
         value: Box<Self>,
-        mutable: bool
+        mutable: bool,
+        span: Range<usize>,
     },
     Fun {
         public: bool,
         name: String,
         return_type_hint: String,
         args: Vec<(String, String)>,
-        body: Box<Self>
+        body: Box<Self>,
+        span: Range<usize>,
     },
 
-    If { cond: Box<Self>, body: Box<Self>, else_body: Box<Self> },
-    Case { cond: Box<Self>, cases: Vec<(Box<Self>, Box<Self>)>, default: Box<Self> },
-    Do { body: Vec<Self> },
+    If {
+        cond: Box<Self>,
+        body: Box<Self>,
+        else_body: Box<Self>,
+        span: Range<usize>,
+    },
+    Case {
+        cond: Box<Self>,
+        cases: Vec<(Box<Self>, Box<Self>)>,
+        default: Box<Self>,
+        span: Range<usize>,
+    },
+    Do { body: Vec<Self>, span: Range<usize> },
 
-    Return { value: Box<Self> },
+    Return { value: Box<Self>, span: Range<usize> },
+    // Error { message: String, note: Option<String>, span: Range<usize> },
 }
 
 #[derive(Debug)]
@@ -62,70 +93,10 @@ pub struct IR {
     pub span: Range<usize>
 }
 
-// S-Expression displaying for IR
-// Used for debugging (run `compile` with `-a`)
-impl std::fmt::Display for IRKind {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        match self {
-            IRKind::Define { ref public, ref name, ref type_hint, ref value, ref mutable } => {
-                write!(f, "(let {} {} {} {} {})",
-                    if *public { "export" } else { "" },
-                    name,
-                    type_hint,
-                    value,
-                    if *mutable { "mut" } else { "" },
-                )
-            },
-            IRKind::Fun { ref public, ref name, ref return_type_hint, ref args, ref body } => {
-                write!(f, "(fun{} {} :{} [{}] {})",
-                    if *public { " export" } else { "" },
-                    name,
-                    return_type_hint,
-                    args.iter().map(|(name, type_hint)| format!(":{} {}", name, type_hint)).collect::<Vec<_>>().join(" "),
-                    body,
-                )
-            },
-            IRKind::Call { ref name, ref args } => {
-                write!(f, "({} {})", name, args.iter().map(|arg| arg.to_string()).collect::<Vec<_>>().join(" "))
-            },
-            IRKind::Intrinsic { ref name, ref args } => {
-                write!(f, "(@{} {})", name, args.iter().map(|arg| arg.to_string()).collect::<Vec<_>>().join(" "))
-            },
-            IRKind::Do { ref body } => {
-                write!(f, "(do {})", body.iter().map(|expr| expr.to_string()).collect::<Vec<_>>().join(" "))
-            },
-            IRKind::If { ref cond, ref body, ref else_body } => {
-                write!(f, "(if {} {} {})", cond, body, else_body)
-            },
-            IRKind::Case { ref cond, ref cases, ref default } => {
-                write!(f, "(case {} {} {})", cond, cases.iter().map(|(cond, body)| format!("({} {})", cond, body)).collect::<Vec<_>>().join(" "), default)
-            },
-            IRKind::Unary { ref op, ref right } => {
-                write!(f, "({} {})", op, right)
-            },
-            IRKind::Binary { ref op, ref left, ref right } => {
-                write!(f, "({} {} {})", op, left, right)
-            },
-            IRKind::Value { ref value } => {
-                write!(f, "{}", value)
-            },
-            IRKind::Vector { ref values } => {
-                write!(f, "[{}]", values.iter().map(|expr| expr.to_string()).collect::<Vec<_>>().join(" "))
-            },
-            IRKind::Return { ref value } => {
-                write!(f, "(return {})", value)
-            }
-            #[allow(unreachable_patterns)]
-            _ => { dbg!(self); unreachable!() }
-        }
-    }
-}
-
 #[derive(Debug)]
 pub struct LoweringError {
     pub span: Range<usize>,
     pub message: String,
-    pub note: Option<String>,
 }
 
 pub fn ast_to_ir(ast: Vec<(Expr, Range<usize>)>) -> (Vec<IR>, Vec<LoweringError>) {
@@ -153,13 +124,31 @@ macro_rules! if_err_return {
     };
 }
 
+#[macro_export]
+macro_rules! return_ok {
+    ($value:expr) => {
+        return (Some($value), None)
+    };
+}
+
+#[macro_export]
+macro_rules! return_err {
+    ($value:expr) => {
+        return (None, Some($value))
+    };
+}
+
 pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
     match expr {
         Expr::Unary { op, rhs } => {
             let rhs_ir = expr_to_ir(&rhs.0);
             if_err_return!(rhs_ir.1);
 
-            return (Some(IRKind::Unary { op: op.to_string(), right: Box::new(rhs_ir.0.unwrap()) }), None);
+            return_ok!(IRKind::Unary {
+                op: op.to_string(),
+                right: Box::new(rhs_ir.0.unwrap()),
+                span: rhs.1.clone()
+            });
         }
 
         Expr::Binary { lhs, op, rhs } => {
@@ -169,14 +158,19 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
             let rhs_ir = expr_to_ir(&rhs.0);
             if_err_return!(rhs_ir.1);
 
-            return (Some(IRKind::Binary { op: op.to_string(), left: Box::new(lhs_ir.0.unwrap()), right: Box::new(rhs_ir.0.unwrap()) }), None)
+            return_ok!(IRKind::Binary {
+                op: op.to_string(),
+                left: Box::new(lhs_ir.0.unwrap()),
+                right: Box::new(rhs_ir.0.unwrap()),
+                span: lhs.1.start..rhs.1.end
+            });
         },
 
         Expr::Call { name, args } => {
-            let name = match &name.0 {
+            let lname = match &name.0 {
                 Expr::Identifier(s) => s.clone(),
                 // Should never happen because the parser should have caught this
-                _ => return (None, Some(LoweringError { span: name.1.clone(), message: "Expected identifier".to_string(), note: None }))
+                _ => return_err!(LoweringError { span: name.1.clone(), message: "Expected identifier".to_string()})
             };
             let mut largs = Vec::new(); // `largs` stand for lowered args
             // Iterate over args
@@ -186,8 +180,12 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
                 if_err_return!(arg.1);
                 largs.push(arg.0.unwrap());
             }
-            let ir_kind = IRKind::Call { name, args: largs };
-            return (Some(ir_kind), None);
+
+            return_ok!(IRKind::Call {
+                name: lname,
+                args: largs,
+                span: name.1.start..args.1.end
+            });
         },
 
         Expr::Pipeline { lhs, rhs } => {
@@ -200,7 +198,7 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
                     let cname = match &name.0 {
                         Expr::Identifier(s) => s.clone(),
                         // Should never happen because the parser should have caught this
-                        _ => return (None, Some(LoweringError { span: name.1.clone(), message: "Expected identifier".to_string(), note: None }))
+                        _ => return_err!(LoweringError { span: name.1.clone(), message: "Expected identifier".to_string()})
                     };
 
                     // Get all the `Hole` indexes
@@ -213,11 +211,10 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
 
                     // If there is no `Hole` in the args then return early
                     if indexes.is_empty() {
-                        return (None, Some(LoweringError {
+                        return_err!(LoweringError {
                             span: rhs.1.clone(),
                             message: "Expected hole in piping".to_string(),
-                            note: None
-                        }));
+                        });
                     }
 
                     // Remove the `Hole` from the args
@@ -259,50 +256,54 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
 
                     // Match the call to the right IRKind
                     let ir_kind = match new_call.0.unwrap() {
-                        IRKind::Call { .. } => IRKind::Call { name: cname, args: largs },
-                        IRKind::Intrinsic { .. } => IRKind::Intrinsic { name: cname, args: largs },
+                        IRKind::Call { .. } => IRKind::Call {
+                            name: cname,
+                            args: largs,
+                            span: name.1.start..args.1.end
+                        },
+                        IRKind::Intrinsic { .. } => IRKind::Intrinsic {
+                            name: cname,
+                            args: largs,
+                            span: name.1.start..args.1.end
+                        },
                         _ => unreachable!()
                     };
 
-                    return (Some(ir_kind), None);
+                    return_ok!(ir_kind);
                 },
-                _ => return (None, Some(LoweringError {
+                _ => return_err!(LoweringError {
                     span: rhs.1.clone(),
-                    message: "Expected call".to_string(),
-                    note: None
-                })),
+                    message: "Expected call".to_string()
+                }),
             };
         },
 
         Expr::Let { public, name, type_hint, value, mutable } => {
-            let value = expr_to_ir(&value.0);
-            if_err_return!(value.1);
+            let lvalue = expr_to_ir(&value.0);
+            if_err_return!(lvalue.1);
 
-            let value = value.0.unwrap();
-            let ir_kind = IRKind::Define {
+            return_ok!(IRKind::Define {
                 public: *public,
-                name: name.clone(),
-                type_hint: gen_type_hint(type_hint),
-                value: Box::new(value),
-                mutable: *mutable
-            };
-
-            return (Some(ir_kind), None);
+                name: name.0.clone(),
+                type_hint: gen_type_hint(&type_hint.0),
+                value: Box::new(lvalue.0.unwrap()),
+                mutable: *mutable,
+                span: value.1.clone()
+            });
         },
 
         Expr::Intrinsic { name, args } => {
-            let name = match &name.0 {
+            let lname = match &name.0 {
                 Expr::Identifier(s) => {
                     if INTRINSICS.contains(&s.as_str()) { s.clone() }
                     else {
-                        return (None, Some(LoweringError {
+                        return_err!(LoweringError {
                             span: name.1.clone(),
                             message: format!("Unknown intrinsic: `{}`", s),
-                            note: Some(format!("Did you mean: {}?", closet_intrinsic(s.to_string())))
-                        }));
+                        });
                     }
                 }
-                _ => return (None, Some(LoweringError { span: name.1.clone(), message: "Expected identifier".to_string(), note: None }))
+                _ => return_err!(LoweringError { span: name.1.clone(), message: "Expected identifier".to_string()})
             };
 
             let mut largs = Vec::new();
@@ -314,73 +315,80 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
                 if let IRKind::Value{ .. } = larg.0.clone().unwrap() {
                     largs.push(larg.0.clone().unwrap());
                 } else {
-                    return (None, Some(LoweringError { span: arg.1.clone(), message: "Expected string".to_string(), note: None }))
+                    return_err!(LoweringError { span: arg.1.clone(), message: "Expected string".to_string()});
                 }
             }
 
-            let ir_kind = IRKind::Intrinsic { name, args: largs };
-            return (Some(ir_kind), None);
+            return_ok!(IRKind::Intrinsic {
+                name: lname,
+                args: largs,
+                span: name.1.start..args.1.end
+            });
         },
 
         Expr::Fun { public, name, type_hint, args, body } => {
             // Iterate each argument and give it a type hint
-            let args = args.0.iter().map(|arg| (arg.0.0.clone(), gen_type_hint(&arg.1.0))).collect::<Vec<_>>();
+            let largs = args.0.iter().map(|arg| (arg.0.0.clone(), gen_type_hint(&arg.1.0))).collect::<Vec<_>>();
 
-            let body = expr_to_ir(&body.0);
-            if_err_return!(body.1);
+            let lbody = expr_to_ir(&body.0);
+            if_err_return!(lbody.1);
 
-            let body = body.0.unwrap();
-            let ir_kind = IRKind::Fun {
+            return_ok!(IRKind::Fun {
                 public: *public,
-                name: name.clone(),
-                return_type_hint: gen_type_hint(type_hint),
-                args,
-                body: Box::new(body)
-            };
-            return (Some(ir_kind), None);
+                name: name.0.clone(),
+                return_type_hint: gen_type_hint(&type_hint.0),
+                args: largs,
+                body: Box::new(lbody.0.unwrap()),
+                span: name.1.start..body.1.end
+            });
         },
 
         Expr::Return { expr } => {
-            let expr = expr_to_ir(&expr.0);
-            if_err_return!(expr.1);
+            let lexpr = expr_to_ir(&expr.0);
+            if_err_return!(lexpr.1);
 
-            let expr = expr.0.unwrap();
-            let ir_kind = IRKind::Return { value: Box::new(expr) };
-            return (Some(ir_kind), None);
+            return_ok!(IRKind::Return {
+                value: Box::new(lexpr.0.unwrap()),
+                span: expr.1.clone()
+            });
         },
 
         Expr::Do { body } => {
             let mut lbody = Vec::new();
-            for expr in body {
+
+            for expr in &body.0 {
                 let expr = expr_to_ir(&expr.0);
                 if_err_return!(expr.1);
                 lbody.push(expr.0.unwrap());
-            }
-            let ir_kind = IRKind::Do { body: lbody };
-            return (Some(ir_kind), None);
+            };
+
+            return_ok!(IRKind::Do {
+                body: lbody,
+                span: body.1.clone()
+            });
         },
 
         Expr::If { cond, body, else_body } => {
-            let cond = expr_to_ir(&cond.0);
-            if_err_return!(cond.1);
+            let lcond = expr_to_ir(&cond.0);
+            if_err_return!(lcond.1);
 
-            let body = expr_to_ir(&body.0);
-            if_err_return!(body.1);
+            let lbody = expr_to_ir(&body.0);
+            if_err_return!(lbody.1);
 
-            let else_body = expr_to_ir(&else_body.0);
-            if_err_return!(else_body.1);
+            let lelse_body = expr_to_ir(&else_body.0);
+            if_err_return!(lelse_body.1);
 
-            let ir_kind = IRKind::If {
-                cond: Box::new(cond.0.unwrap()),
-                body: Box::new(body.0.unwrap()),
-                else_body: Box::new(else_body.0.unwrap())
-            };
-            return (Some(ir_kind), None);
+            return_ok!(IRKind::If {
+                cond: Box::new(lcond.0.unwrap()),
+                body: Box::new(lbody.0.unwrap()),
+                else_body: Box::new(lelse_body.0.unwrap()),
+                span: cond.1.start..else_body.1.end
+            });
         },
 
         Expr::Case { expr, cases, default } => {
-            let expr = expr_to_ir(&expr.0);
-            if_err_return!(expr.1);
+            let lexpr = expr_to_ir(&expr.0);
+            if_err_return!(lexpr.1);
 
             let mut lcases = Vec::new();
             for case in &cases.0 {
@@ -395,23 +403,23 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
                 );
             }
 
-            let default = expr_to_ir(&default.0);
-            if_err_return!(default.1);
+            let ldefault = expr_to_ir(&default.0);
+            if_err_return!(ldefault.1);
 
-            let ir_kind = IRKind::Case {
-                cond: Box::new(expr.0.unwrap()),
+            return_ok!(IRKind::Case {
+                cond: Box::new(lexpr.0.unwrap()),
                 cases: lcases,
-                default: Box::new(default.0.unwrap())
-            };
-            return (Some(ir_kind), None);
+                default: Box::new(ldefault.0.unwrap()),
+                span: expr.1.start..default.1.end
+            });
         },
 
         // TODO: Handle primitive types error (e.g. overflow)
         // For now it just leaves the value as is and let the target compiler handle it
-        Expr::Int(value)        => (Some(IRKind::Value { value: Value::Int(*value) }), None),
-        Expr::Boolean(value)    => (Some(IRKind::Value { value: Value::Boolean(*value) }), None),
-        Expr::String(value)     => (Some(IRKind::Value { value: Value::String(value.clone()) }), None),
-        Expr::Identifier(value) => (Some(IRKind::Value { value: Value::Ident(value.clone()) }), None),
+        Expr::Int(value)        => return_ok!(IRKind::Value { value: Value::Int(*value) }),
+        Expr::Boolean(value)    => return_ok!(IRKind::Value { value: Value::Boolean(*value) }),
+        Expr::String(value)     => return_ok!(IRKind::Value { value: Value::String(value.clone()) }),
+        Expr::Identifier(value) => return_ok!(IRKind::Value { value: Value::Ident(value.clone()) }),
 
         Expr::Vector(values) => {
             let mut lvalues = Vec::new();
@@ -427,8 +435,7 @@ pub fn expr_to_ir(expr: &Expr) -> (Option<IRKind>, Option<LoweringError>) {
         // Probably will never happen because it is catched in parser
         Expr::Hole(start, end) => (None, Some(LoweringError {
             span: *start..*end,
-            message: "Hole can only be used in piping, it is not allowed here.".to_string(),
-            note: None
+            message: "Hole can only be used in piping, it is not allowed here.".to_string()
         })),
         _ => { dbg!(expr); todo!() }
     }
diff --git a/crates/main/Cargo.toml b/crates/main/Cargo.toml
index 9d207fe..d636802 100644
--- a/crates/main/Cargo.toml
+++ b/crates/main/Cargo.toml
@@ -18,6 +18,7 @@ lexer = { path = "../lexer" }
 parser = { path = "../parser" }
 
 # Diagnostics
+typecheck = { path = "../typecheck" }
 diagnostic = { path = "../diagnostic" }
 
 # Codegen
diff --git a/crates/main/src/main.rs b/crates/main/src/main.rs
index 1010ecf..90e5044 100644
--- a/crates/main/src/main.rs
+++ b/crates/main/src/main.rs
@@ -6,6 +6,7 @@ use lexer::lex;
 use parser::parse;
 use diagnostic::Diagnostics;
 use hir::ast_to_ir;
+use typecheck::check;
 use codegen::ts;
 
 pub mod args;
@@ -52,16 +53,24 @@ fn main() {
                 logif!(0, format!("Parsing took {}ms", start.elapsed().as_millis()));
             }
 
-            // TODO: S-Expr syntax for AST
-            // if print_ast { log(0, format!("{:#?}", ast)); }
-
             match ast {
                 Some(ast) => {
                     // Convert the AST to HIR
                     let (ir, lowering_error) = ast_to_ir(ast);
                     for err in lowering_error { diagnostics.add_lowering_error(err); }
 
-                    if print_ast { log(0, format!("IR\n{}", ir.iter().map(|x| format!("{}", x.kind)).collect::<Vec<String>>().join("\n\n"))); }
+                    if print_ast { log(0, format!("IR\n{:#?}", ir)); }
+
+                    // Typecheck the HIR
+                    match check(&ir) {
+                        Ok(_) => {},
+                        Err(err) => {
+                            diagnostics.add_typecheck_error(err);
+                            diagnostics.display(src);
+                            logif!(0, "Typechecking failed");
+                            std::process::exit(1);
+                        }
+                    }
 
                     // Report lowering errors if any
                     if diagnostics.has_error() {
diff --git a/crates/parser/src/lib.rs b/crates/parser/src/lib.rs
index 0c9ee5e..a7b2322 100644
--- a/crates/parser/src/lib.rs
+++ b/crates/parser/src/lib.rs
@@ -18,15 +18,15 @@ pub enum Expr {
 
     Let {
         public: bool,
-        name: String,
-        type_hint: String,
+        name: Spanned<String>,
+        type_hint: Spanned<String>,
         value: Box<Spanned<Self>>,
         mutable: bool,
     },
     Fun {
         public: bool,
-        name: String,
-        type_hint: String,
+        name: Spanned<String>,
+        type_hint: Spanned<String>,
         args: Spanned<Vec<(Spanned<String>, Spanned<String>)>>,
         body: Box<Spanned<Self>>
     },
@@ -43,7 +43,7 @@ pub enum Expr {
         default: Box<Spanned<Self>>
     },
     Do {
-        body: Vec<Spanned<Self>>
+        body: Spanned<Vec<Spanned<Self>>>
     },
 
     // Hole for positional argument(s) in piping
@@ -226,8 +226,8 @@ fn expr_parser() -> impl Parser<Token, Vec<Spanned<Expr>>, Error = Simple<Token>
                 (
                     Expr::Let {
                         public: public.is_some(),
-                        name: name.0.clone(),
-                        type_hint: type_hint.0,
+                        name: name.clone(),
+                        type_hint,
                         value: Box::new(value.clone()),
                         mutable: mutable.is_some(),
                     },
@@ -256,8 +256,8 @@ fn expr_parser() -> impl Parser<Token, Vec<Spanned<Expr>>, Error = Simple<Token>
                 (
                     Expr::Fun {
                         public: public.is_some(),
-                        name: name.0.clone(),
-                        type_hint: type_hint.0,
+                        name: name.clone(),
+                        type_hint,
                         args: (args, name.1.clone()),
                         body: Box::new(body.clone()),
                     },
@@ -286,7 +286,7 @@ fn expr_parser() -> impl Parser<Token, Vec<Spanned<Expr>>, Error = Simple<Token>
             .map_with_span(|body, span| {
                 (
                     Expr::Do {
-                        body: body.clone(),
+                        body: (body.clone(), span.clone()),
                     },
                     span,
                 )
diff --git a/crates/typecheck/Cargo.toml b/crates/typecheck/Cargo.toml
new file mode 100644
index 0000000..a94b23b
--- /dev/null
+++ b/crates/typecheck/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "typecheck"
+license = "MIT OR Apache-2.0"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+hir = { path = "../hir" }
diff --git a/crates/typecheck/src/lib.rs b/crates/typecheck/src/lib.rs
new file mode 100644
index 0000000..592e26b
--- /dev/null
+++ b/crates/typecheck/src/lib.rs
@@ -0,0 +1,34 @@
+use hir::{IR, IRKind};
+
+#[derive(Debug)]
+pub enum TypecheckErrorKind {
+    DefinitionTypeMismatch {
+        type_specified: String,
+        type_found: String,
+    },
+}
+
+impl std::fmt::Display for TypecheckErrorKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            TypecheckErrorKind::DefinitionTypeMismatch {
+                type_specified,
+                type_found,
+            } => write!(
+                f,
+                "expected type `{}`, found type `{}`",
+                type_specified, type_found
+            ),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct TypecheckError {
+    pub kind: TypecheckErrorKind,
+    pub span: std::ops::Range<usize>,
+}
+
+pub fn check(ir: &Vec<IR>) -> Result<(), TypecheckError> {
+    todo!();
+}
\ No newline at end of file