diff --git a/src/chalk/builtin.cr b/src/chalk/builtin.cr
index f9dea66..987cd94 100644
--- a/src/chalk/builtin.cr
+++ b/src/chalk/builtin.cr
@@ -13,6 +13,8 @@ module Chalk
 
       # Uses the given `Compiler::Emitter` to output code.
       abstract def generate!(codegen)
+      # Gets the `Compiler::FunctionType` of this function.
+      abstract def type
     end
 
     # A function to which a call is not generated. This function
@@ -32,6 +34,8 @@ module Chalk
       # the *params* are trees that are being passed as arguments.
       # See `Compiler::CodeGenerator#generate!` for what the other parameters mean.
       abstract def generate!(codegen, params, table, target, free)
+      # Gets the `Compiler::FunctionType` of this function.
+      abstract def type
     end
   end
 end
diff --git a/src/chalk/compiler.cr b/src/chalk/compiler.cr
index 60cad79..602cd8b 100644
--- a/src/chalk/compiler.cr
+++ b/src/chalk/compiler.cr
@@ -65,6 +65,7 @@ module Chalk
       # looking up identifiers in the symbol *table*, and appending the given *instruction*
       # at the end of the function to ensure correct program flow.
       private def create_code(tree : Trees::TreeFunction, table, instruction = Ir::ReturnInstruction.new)
+        tree.reduce(Trees::TypeChecker.new table, tree.type.return_type)
         optimizer = Optimizer.new
         generator = CodeGenerator.new table, tree
         @logger.debug("Generating code for #{tree.name}")
diff --git a/src/chalk/inline.cr b/src/chalk/inline.cr
index 154a6a6..ac3a36d 100644
--- a/src/chalk/inline.cr
+++ b/src/chalk/inline.cr
@@ -1,3 +1,6 @@
+require "./builtin"
+require "./type"
+
 module Chalk
   module Builtin
     # Inline function to draw sprite at address I.
@@ -14,6 +17,9 @@ module Chalk
         emitter.generate! params[1], table, free + 1, free + 2
         emitter.instructions << Ir::DrawInstruction.new free, free + 1, params[2].as(Trees::TreeLit).lit.to_i32
       end
+      def type
+          return Compiler::FunctionType.new([Compiler::Type::U8] * 3, Compiler::Type::U0)
+      end
     end
 
     # Inline function to await for a key and return it.
@@ -25,6 +31,9 @@ module Chalk
       def generate!(emitter, params, table, target, free)
         emitter.instructions << Ir::AwaitKeyInstruction.new target
       end
+      def type
+          return Compiler::FunctionType.new(([] of Compiler::Type), Compiler::Type::U8)
+      end
     end
 
     # Inline function to get font for a given value.
@@ -37,6 +46,9 @@ module Chalk
         emitter.generate! params[0], table, free, free + 1
         emitter.instructions << Ir::GetFontInstruction.new free
       end
+      def type
+          return Compiler::FunctionType.new([Compiler::Type::U8], Compiler::Type::U0)
+      end
     end
 
     # Inline function to set the delay timer.
@@ -49,6 +61,9 @@ module Chalk
         emitter.generate! params[0], table, free, free + 1
         emitter.instructions << Ir::SetDelayTimerInstruction.new free
       end
+      def type
+          return Compiler::FunctionType.new([Compiler::Type::U8], Compiler::Type::U0)
+      end
     end
 
     # Inline function to get the delay timer.
@@ -60,6 +75,9 @@ module Chalk
       def generate!(emitter, params, table, target, free)
         emitter.instructions << Ir::GetDelayTimerInstruction.new target
       end
+      def type
+          return Compiler::FunctionType.new(([] of Compiler::Type), Compiler::Type::U8)
+      end
     end
   end
 end
diff --git a/src/chalk/lexer.cr b/src/chalk/lexer.cr
index 0507da3..426b6c3 100644
--- a/src/chalk/lexer.cr
+++ b/src/chalk/lexer.cr
@@ -21,6 +21,7 @@ module Chalk
       KwInline
       KwFun
       KwU0
+      KwU4
       KwU8
       KwU12
       KwVar
@@ -70,6 +71,7 @@ module Chalk
         @lexer.add_pattern("inline", TokenType::KwInline.value)
         @lexer.add_pattern("fun", TokenType::KwFun.value)
         @lexer.add_pattern("u0", TokenType::KwU0.value)
+        @lexer.add_pattern("u4", TokenType::KwU4.value)
         @lexer.add_pattern("u8", TokenType::KwU8.value)
         @lexer.add_pattern("u12", TokenType::KwU12.value)
         @lexer.add_pattern("var", TokenType::KwVar.value)
diff --git a/src/chalk/parser.cr b/src/chalk/parser.cr
index 066829a..3544f08 100644
--- a/src/chalk/parser.cr
+++ b/src/chalk/parser.cr
@@ -8,7 +8,10 @@ module Chalk
 
       # Creates a parser for a type.
       private def create_type
-        either(type(Compiler::TokenType::KwU0), type(Compiler::TokenType::KwU8), type(Compiler::TokenType::KwU12))
+        either(type(Compiler::TokenType::KwU0),
+               type(Compiler::TokenType::KwU4),
+               type(Compiler::TokenType::KwU8),
+               type(Compiler::TokenType::KwU12))
       end
 
       # Creates a parser for an integer literal.
@@ -176,7 +179,13 @@ module Chalk
             params = arr[3..arr.size - 5].map &.as(Compiler::Token).string
             code = arr[arr.size - 1].as(Trees::Tree)
             type = arr[arr.size - 2].as(Compiler::Token).type
-            Trees::TreeFunction.new(name, params, code)
+            table = {
+                Compiler::TokenType::KwU0 => Compiler::Type::U0,
+                Compiler::TokenType::KwU4 => Compiler::Type::U4,
+                Compiler::TokenType::KwU8 => Compiler::Type::U8,
+                Compiler::TokenType::KwU12 => Compiler::Type::U12
+            }
+            Trees::TreeFunction.new(name, params, table[type], code)
           end
         return func
       end
diff --git a/src/chalk/tree.cr b/src/chalk/tree.cr
index a14f95c..db1b34f 100644
--- a/src/chalk/tree.cr
+++ b/src/chalk/tree.cr
@@ -18,6 +18,11 @@ module Chalk
       end
     end
 
+    class Reducer(T)
+      def reduce(tree, children)
+      end
+    end
+
     # The base class of a tree.
     class Tree
       def accept(v)
@@ -28,6 +33,10 @@ module Chalk
       def apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+          return r.reduce(self, [] of T)
+      end
     end
 
     # A tree that represents an ID.
@@ -66,6 +75,10 @@ module Chalk
         end
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+          return r.reduce(self, @params.map &.reduce(r))
+      end
     end
 
     # A tree that represents an operation on two values.
@@ -89,6 +102,10 @@ module Chalk
         @right = @right.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+          return r.reduce(self, [@left.reduce(r), @right.reduce(r)])
+      end
     end
 
     # A tree that represents a block of statements.
@@ -110,6 +127,9 @@ module Chalk
         end
         return t.transform(self)
       end
+      def reduce(r : Reducer(T)) forall T
+          return r.reduce(self, @children.map &.reduce(r))
+      end
     end
 
     # A tree that represents a function declaration.
@@ -117,8 +137,10 @@ module Chalk
       property name : String
       property params : Array(String)
       property block : Tree
+      getter type
 
-      def initialize(@name, @params, @block)
+      def initialize(@name, @params, return_type : Compiler::Type, @block)
+        @type = Compiler::FunctionType.new([Compiler::Type::U8] * @params.size, return_type)
       end
 
       def param_count
@@ -135,6 +157,10 @@ module Chalk
         @block = @block.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+          return r.reduce(self, [@block.reduce(r)])
+      end
     end
 
     # A tree that represents the declaration of
@@ -156,6 +182,10 @@ module Chalk
         @expr = @expr.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+        r.reduce(self, [@expr.reduce(r)])
+      end
     end
 
     # A tree that represents the assignment
@@ -177,6 +207,10 @@ module Chalk
         @expr = @expr.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+        r.reduce(self, [@expr.reduce(r)])
+      end
     end
 
     # A tree that represents an if statement.
@@ -202,6 +236,16 @@ module Chalk
         @otherwise = @otherwise.try &.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+          cond = @condition.reduce(r)
+          blk = @block.reduce(r)
+          if other = @otherwise
+              r.reduce(self, [cond, blk,other.reduce(r)])
+          else
+              r.reduce(self, [cond, blk])
+          end
+      end
     end
 
     # A tree that represents a while loop.
@@ -224,6 +268,10 @@ module Chalk
         @block = @block.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+          r.reduce(self, [@condition.reduce(r), @block.reduce(r)])
+      end
     end
 
     # A tree that represents a return statement.
@@ -243,6 +291,10 @@ module Chalk
         @rvalue = @rvalue.apply(t)
         return t.transform(self)
       end
+
+      def reduce(r : Reducer(T)) forall T
+        r.reduce(self, [@rvalue.reduce(r)])
+      end
     end
   end
 end
diff --git a/src/chalk/type.cr b/src/chalk/type.cr
new file mode 100644
index 0000000..0f8b44c
--- /dev/null
+++ b/src/chalk/type.cr
@@ -0,0 +1,35 @@
+module Chalk
+    module Compiler
+        # Possible types of values in chalk
+        enum Type
+            U0
+            U4
+            U8
+            U12
+
+            # Checks if one value can be cast to another.
+            def casts_to?(other)
+                return false if other == Type::U0 || self == Type::U0
+                return other.value >= self.value
+            end
+        end
+
+        # A type of a function.
+        class FunctionType
+            # Gets the types of the function's parameters.
+            getter param_types
+            # Gets the return type of the function.
+            getter return_type
+
+            def initialize(@param_types : Array(Type), @return_type : Type)
+            end
+
+            def to_s(io)
+                io << "("
+                io << param_types.map(&.to_s).join(", ")
+                io << ") -> "
+                return_type.to_s(io)
+            end
+        end
+    end
+end
diff --git a/src/chalk/typecheck.cr b/src/chalk/typecheck.cr
new file mode 100644
index 0000000..eb384ac
--- /dev/null
+++ b/src/chalk/typecheck.cr
@@ -0,0 +1,58 @@
+require "./type"
+
+module Chalk
+    module Trees
+        # Reducer to check types.
+        class TypeChecker < Reducer(Compiler::Type)
+            def initialize(@table : Compiler::Table, @return_type : Compiler::Type)
+            end
+
+            def reduce(t, children)
+                return Compiler::Type::U0
+            end
+
+            def reduce(t : TreeCall, children)
+                entry = @table[t.name]?
+                raise "Unknwon function" unless entry && entry.is_a?(Compiler::FunctionEntry)
+                type = entry.function.type
+                raise "Invalid parameters" if type.param_types.size != children.size
+                children.each_with_index do |child, i|
+                    raise "Incompatible parameter" if !child.casts_to?(type.param_types[i])
+                end
+                return entry.function.type.return_type
+            end
+
+            def reduce(t : TreeId, children)
+                return Compiler::Type::U8
+            end
+
+            def reduce(t : TreeLit, children)
+                max_12 = (2 ** 12) - 1
+                max_8 = (2 ** 8) - 1
+                max_4 = (2 ** 4) - 1
+                raise "Number too big" if t.lit > max_12
+                return Compiler::Type::U12 if t.lit > max_8
+                return Compiler::Type::U8 if t.lit > max_4
+                return Compiler::Type::U4
+            end
+
+            def reduce(t : TreeOp, children)
+                left = children[0]
+                right = children[1]
+                return left if right.casts_to?(left)
+                return right if left.casts_to?(right)
+                raise "Invalid operation"
+            end
+
+            def reduce(t : TreeAssign | TreeVar, children)
+                raise "Invalid assignment" if !children[0].casts_to?(Compiler::Type::U8)
+                return Compiler::Type::U0
+            end
+
+            def reduce(t : TreeReturn, children)
+                raise "Incompatible return type" if !children[0].casts_to?(@return_type)
+                return Compiler::Type::U0
+            end
+        end
+    end
+end