diff --git a/src/rsprocess/assert.rs b/src/rsprocess/assert.rs
index 7aaad49..0c50ef9 100644
--- a/src/rsprocess/assert.rs
+++ b/src/rsprocess/assert.rs
@@ -20,16 +20,7 @@ pub enum Tree {
     For(Variable, Range, Box<Tree>),
 }
 
-#[derive(Debug, Clone)]
-pub struct Variable {
-    data: String,
-}
-
-impl Variable {
-    pub fn from(data: String) -> Self {
-	Self { data }
-    }
-}
+pub type Variable = String;
 
 #[derive(Debug, Clone)]
 pub enum AssignmentVariable {
@@ -37,6 +28,15 @@ pub enum AssignmentVariable {
     QualifiedVar(Variable, QualifierRestricted)
 }
 
+impl fmt::Display for AssignmentVariable {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+	match self {
+	    Self::Var(v) => write!(f, "{v}"),
+	    Self::QualifiedVar(v, q) => write!(f, "{v}.{q}"),
+	}
+    }
+}
+
 #[derive(Debug, Clone)]
 pub enum Expression {
     True,
@@ -66,7 +66,36 @@ pub enum Unary {
     Empty,
     Length,
     ToStr,
-    Qualifier(Qualifier)
+    Qualifier(Qualifier),
+}
+
+impl Unary {
+    fn associated_types_unary(&self) -> Vec<AssertionTypes> {
+	match self {
+	    Unary::Not          => vec![AssertionTypes::Boolean],
+	    Unary::Rand         => vec![AssertionTypes::Integer],
+	    Unary::Empty        => vec![AssertionTypes::Set],
+	    Unary::Length       => vec![AssertionTypes::String,
+					AssertionTypes::Set],
+	    Unary::ToStr        => vec![AssertionTypes::Boolean,
+					AssertionTypes::Integer,
+					AssertionTypes::Element],
+	    Unary::Qualifier(_) => vec![AssertionTypes::Set]
+	}
+    }
+}
+
+impl fmt::Display for Unary {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+	match self {
+	    Unary::Not => write!(f, "not"),
+	    Unary::Rand => write!(f, "rand"),
+	    Unary::Empty => write!(f, ".empty"),
+	    Unary::Length => write!(f, ".length"),
+	    Unary::ToStr => write!(f, ".tostr"),
+	    Unary::Qualifier(q) => write!(f, ".{q}"),
+	}
+    }
 }
 
 
@@ -81,6 +110,27 @@ pub enum QualifierRestricted {
     Products,
 }
 
+impl fmt::Display for QualifierRestricted {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+	match self {
+	    QualifierRestricted::Entities =>
+		write!(f, "Entities"),
+	    QualifierRestricted::Context =>
+		write!(f, "Context"),
+	    QualifierRestricted::Reactants =>
+		write!(f, "Reactants"),
+	    QualifierRestricted::ReactantsAbsent =>
+		write!(f, "ReactantsAbsent"),
+	    QualifierRestricted::Inhibitors =>
+		write!(f, "Inhibitors"),
+	    QualifierRestricted::InhibitorsPresent =>
+		write!(f, "InhibitorsPresent"),
+	    QualifierRestricted::Products =>
+		write!(f, "Products"),
+	}
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub enum Qualifier {
     AvailableEntities,
@@ -89,6 +139,18 @@ pub enum Qualifier {
     Restricted(QualifierRestricted),
 }
 
+impl fmt::Display for Qualifier {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+	match self {
+	    Self::AvailableEntities => write!(f, "AvailableEntities"),
+	    Self::AllReactants => write!(f, "AllReactants"),
+	    Self::AllInhibitors => write!(f, "AllInhibitors"),
+	    Self::Restricted(q) => write!(f, "{q}"),
+	}
+    }
+}
+
+
 #[derive(Debug, Clone, Copy)]
 pub enum Binary {
     And,
@@ -113,3 +175,387 @@ pub enum Binary {
     Max,
     CommonSubStr,
 }
+
+impl Binary {
+    fn associate(
+	&self,
+	t1: &AssertionTypes,
+	t2: &AssertionTypes
+    ) -> Result<AssertionTypes, String> {
+	//
+	match self {
+	    Self::And |
+	    Self::Or => {
+		if let (AssertionTypes::Boolean, AssertionTypes::Boolean) =
+		    (t1, t2)
+		{
+		    return Ok(AssertionTypes::Boolean)
+		}
+	    },
+	    Self::Xor => {
+		match (t1, t2) {
+		    (AssertionTypes::Boolean, AssertionTypes::Boolean) =>
+			return Ok(AssertionTypes::Boolean),
+		    (AssertionTypes::Set, AssertionTypes::Set) =>
+			return Ok(AssertionTypes::Set),
+		    _ => {}
+		}
+	    }
+	    Self::Less |
+	    Self::LessEq |
+	    Self::More |
+	    Self::MoreEq =>
+		match (t1, t2) {
+		    (AssertionTypes::Integer, AssertionTypes::Integer) |
+		    (AssertionTypes::Set,     AssertionTypes::Set    ) =>
+			return Ok(AssertionTypes::Boolean),
+		    _ => {}
+		}
+	    Self::Eq |
+	    Self::NotEq => {
+		match (t1, t2) {
+		    (AssertionTypes::Integer, AssertionTypes::Integer) |
+		    (AssertionTypes::Boolean, AssertionTypes::Boolean) |
+		    (AssertionTypes::Element, AssertionTypes::Element) |
+		    (AssertionTypes::Label,   AssertionTypes::Label)   |
+		    (AssertionTypes::String,  AssertionTypes::String)  |
+		    (AssertionTypes::Set,     AssertionTypes::Set    ) =>
+			return Ok(AssertionTypes::Boolean),
+		    _ => {}
+		}
+	    },
+	    Self::Plus |
+	    Self::Minus |
+	    Self::Times => {
+		match (t1, t2) {
+		    (AssertionTypes::Integer, AssertionTypes::Integer) =>
+			return Ok(AssertionTypes::Integer),
+		    (AssertionTypes::Set,     AssertionTypes::Set    ) =>
+			return Ok(AssertionTypes::Set),
+		    _ => {}
+		}
+	    },
+	    Self::Exponential |
+	    Self::Quotient |
+	    Self::Reminder => {
+		if let (AssertionTypes::Integer, AssertionTypes::Integer) =
+		    (t1, t2) {
+		    return Ok(AssertionTypes::Integer)
+		}
+	    },
+	    Self::Concat => {
+		if let (AssertionTypes::String, AssertionTypes::String) =
+		    (t1, t2) {
+		    return Ok(AssertionTypes::String)
+		}
+	    },
+	    Self::SubStr => {
+		if let (AssertionTypes::String, AssertionTypes::String) =
+		    (t1, t2) {
+		    return Ok(AssertionTypes::Boolean)
+		}
+	    },
+	    Self::Min |
+	    Self::Max => {
+		if let (AssertionTypes::Integer, AssertionTypes::Integer) =
+		    (t1, t2) {
+		    return Ok(AssertionTypes::Integer)
+		}
+	    },
+	    Self::CommonSubStr => {
+		if let (AssertionTypes::String, AssertionTypes::String) =
+		    (t1, t2) {
+		    return Ok(AssertionTypes::String)
+		}
+	    },
+	}
+	Err(format!("Expressions have incompatible types: {t1} and {t2} with \
+		     operation {self}."))
+    }
+}
+
+impl fmt::Display for Binary {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+	match self {
+	    Self::And => write!(f, "&&"),
+	    Self::Or => write!(f, "||"),
+	    Self::Xor => write!(f, "^^"),
+	    Self::Less => write!(f, "<"),
+	    Self::LessEq => write!(f, "<="),
+	    Self::More => write!(f, ">"),
+	    Self::MoreEq => write!(f, ">="),
+	    Self::Eq => write!(f, "=="),
+	    Self::NotEq => write!(f, "!="),
+	    Self::Plus => write!(f, "+"),
+	    Self::Minus => write!(f, "-"),
+	    Self::Times => write!(f, "*"),
+	    Self::Exponential => write!(f, "^"),
+	    Self::Quotient => write!(f, "/"),
+	    Self::Reminder => write!(f, "%"),
+	    Self::Concat => write!(f, "::"),
+	    Self::SubStr => write!(f, "substr"),
+	    Self::Min => write!(f, "min"),
+	    Self::Max => write!(f, "max"),
+	    Self::CommonSubStr => write!(f, "commonsubstr"),
+	}
+    }
+}
+
+
+use core::fmt;
+// -----------------------------------------------------------------------------
+use std::collections::HashMap;
+
+struct Context {
+    data: HashMap<Variable, AssertionTypes>,
+    ty_return: Option<AssertionTypes>
+}
+
+impl Context {
+    pub fn new() -> Self {
+	Context {
+	    data: HashMap::new(),
+	    ty_return: None
+	}
+    }
+
+    pub fn assign(
+	&mut self,
+	v: &AssignmentVariable,
+	ty: AssertionTypes
+    ) -> Result<(), String> {
+	match v {
+	    AssignmentVariable::Var(v) => {
+		self.data.insert(v.clone(), ty);
+		Ok(())
+	    },
+	    AssignmentVariable::QualifiedVar(v, q) => {
+		if let Some(AssertionTypes::Label) = self.data.get(v) {
+		    Ok(())
+		} else {
+		    Err(format!("Variable {v} does not have type label while \
+				 trying to assign to qualification {q}"))
+		}
+	    }
+	}
+    }
+
+    pub fn return_type(
+	&mut self,
+	ty: AssertionTypes
+    ) -> Result<(), String> {
+	if let Some(ty_return) = self.ty_return {
+	    if ty_return == ty {
+		Ok(())
+	    } else {
+		Err(format!("Return statements don't agree: {ty_return} and \
+			     {ty} found."))
+	    }
+	} else {
+	    self.ty_return = Some(ty);
+	    Ok(())
+	}
+    }
+
+    pub fn assign_range(
+	&mut self,
+	v: &Variable,
+	ty: AssertionTypes
+    ) -> Result<(), String> {
+	match ty {
+	    AssertionTypes::RangeSet => {
+		self.data.insert(v.clone(), AssertionTypes::Element);
+		Ok(())
+	    },
+	    AssertionTypes::RangeInteger => {
+		self.data.insert(v.clone(), AssertionTypes::Integer);
+		Ok(())
+	    },
+	    _ => {
+		Err(format!("Range has incorrect type {ty}."))
+	    }
+	}
+    }
+
+    pub fn get(
+	&self,
+	v: &AssignmentVariable,
+    ) -> Result<AssertionTypes, String> {
+	match v {
+	    AssignmentVariable::Var(v) => {
+		if let Some(ty) = self.data.get(v) {
+		    Ok(*ty)
+		} else {
+		    Err(format!("Could not find variable {v}."))
+		}
+	    },
+	    AssignmentVariable::QualifiedVar(var, _) => {
+		if let Some(ty) = self.data.get(var) {
+		    if *ty == AssertionTypes::Label {
+			Ok(AssertionTypes::Set)
+		    } else {
+			Err(format!("Variable {var} is not a label."))
+		    }
+		} else {
+		    Err(format!("Could not find variable {v}."))
+		}
+	    }
+	}
+
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq)]
+enum AssertionTypes {
+    Boolean,
+    Integer,
+    String,
+    Label,
+    Set,
+    Element,
+    NoType,
+    RangeInteger,
+    RangeSet,
+}
+
+impl fmt::Display for AssertionTypes {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+	match self {
+	    AssertionTypes::Boolean => write!(f, "boolean"),
+	    AssertionTypes::Integer => write!(f, "integer"),
+	    AssertionTypes::String => write!(f, "string"),
+	    AssertionTypes::Label => write!(f, "label"),
+	    AssertionTypes::Set => write!(f, "set"),
+	    AssertionTypes::Element => write!(f, "element"),
+	    AssertionTypes::NoType => write!(f, "no type"),
+	    AssertionTypes::RangeInteger => write!(f, "range of integers"),
+	    AssertionTypes::RangeSet => write!(f, "range of set"),
+	}
+    }
+}
+
+fn typecheck(tree: &Tree, c: &mut Context) -> Result<AssertionTypes, String> {
+    match tree {
+	Tree::Concat(t1, t2) => {
+	    typecheck(t1, c)?;
+	    typecheck(t2, c)
+	},
+	Tree::If(exp, t) => {
+	    match typecheck_expression(exp, c)? {
+		AssertionTypes::Boolean => {},
+		_ => {return Err("Expression in if statement doesn't return a \
+				  boolean.".to_string())}
+	    };
+	    typecheck(t, c)
+	},
+	Tree::IfElse(exp, t1, t2) => {
+	    match typecheck_expression(exp, c)? {
+		AssertionTypes::Boolean => {},
+		_ => {return Err("Expression in if statement doesn't return a \
+				  boolean.".into())}
+	    };
+	    let type_t1 = typecheck(t1, c)?;
+	    let type_t2 = typecheck(t2, c)?;
+	    if type_t1 == type_t2 {
+		Ok(type_t1)
+	    } else {
+		Err("Branches of if statement do not match.".into())
+	    }
+	},
+	Tree::Assignment(assignvar, exp) => {
+	    let type_exp = typecheck_expression(exp, c)?;
+	    c.assign(assignvar, type_exp)?;
+	    Ok(AssertionTypes::NoType)
+	},
+	Tree::Return(exp) => {
+	    let type_exp = typecheck_expression(exp, c)?;
+	    c.return_type(type_exp)?;
+	    Ok(AssertionTypes::NoType)
+	},
+	Tree::For(var, range, t) => {
+	    let type_range = typecheck_range(range, c)?;
+	    c.assign_range(var, type_range)?;
+	    typecheck(t, c)
+	},
+    }
+}
+
+fn typecheck_expression(
+    exp: &Expression,
+    c: &Context
+) -> Result<AssertionTypes, String> {
+    match exp {
+	Expression::True |
+	Expression::False => Ok(AssertionTypes::Boolean),
+	Expression::Integer(_) => Ok(AssertionTypes::Integer),
+	Expression::Label(_) => Ok(AssertionTypes::Label),
+	Expression::Set(_) => Ok(AssertionTypes::Set),
+	Expression::Element(_) => Ok(AssertionTypes::Element),
+	Expression::String(_) => Ok(AssertionTypes::String),
+	Expression::Var(v) => c.get(v),
+
+	Expression::Unary(u, exp) => {
+	    if let Unary::Qualifier(_) = u {
+		let type_exp = typecheck_expression(exp, c)?;
+		if type_exp == AssertionTypes::Label {
+		    Ok(AssertionTypes::Set)
+		} else {
+		    Err("Trying to get the field of a label, but value is not \
+			 a label.".into())
+		}
+	    } else {
+		let type_exp = typecheck_expression(exp, c)?;
+		let possible_types = u.associated_types_unary();
+		if possible_types.contains(&type_exp) {
+		    Ok(type_exp)
+		} else {
+		    Err(format!("Cannot use operation {u} on type {type_exp}"))
+		}
+	    }
+	},
+
+	Expression::Binary(b, exp1, exp2) => {
+	    let type_exp1 = typecheck_expression(exp1, c)?;
+	    let type_exp2 = typecheck_expression(exp2, c)?;
+	    b.associate(&type_exp1, &type_exp2)
+	}
+    }
+}
+
+
+fn typecheck_range(
+    range: &Range,
+    c: &mut Context
+) -> Result<AssertionTypes, String> {
+    match range {
+	Range::IterateInRange(exp1, exp2) => {
+	    let type_exp1 = typecheck_expression(exp1, c)?;
+	    let type_exp2 = typecheck_expression(exp2, c)?;
+	    if let (AssertionTypes::Integer, AssertionTypes::Integer) =
+		(type_exp1, type_exp2)
+	    {
+		Ok(AssertionTypes::RangeInteger)
+	    } else {
+		Err(format!("Expressions in range are not integers, but are: \
+			     {type_exp1} and {type_exp2}."))
+	    }
+	},
+	Range::IterateOverSet(exp) => {
+	    let type_exp = typecheck_expression(exp, c)?;
+	    if let AssertionTypes::Set = type_exp {
+		Ok(AssertionTypes::RangeSet)
+	    } else {
+		Err(format!("Expressions in range is not a set, but is: \
+			     {type_exp}."))
+	    }
+	}
+    }
+}
+
+impl RSassert {
+    pub fn typecheck(&self) -> Result<(), String> {
+	let mut context = Context::new();
+	typecheck(&self.tree, &mut context)?;
+	Ok(())
+    }
+}
diff --git a/src/rsprocess/grammar.lalrpop b/src/rsprocess/grammar.lalrpop
index 1b8a5d7..01679c9 100644
--- a/src/rsprocess/grammar.lalrpop
+++ b/src/rsprocess/grammar.lalrpop
@@ -211,7 +211,7 @@ AssertAssignmentVar: assert::AssignmentVariable = {
 }
 
 AssertVariable: assert::Variable = {
-    <v: Literal> => assert::Variable::from(v)
+    <v: Literal> => v
 }
 
 AssertExpression: assert::Expression = {