diff --git a/phasm/build/base.py b/phasm/build/base.py
index 316af75..e2e4c6a 100644
--- a/phasm/build/base.py
+++ b/phasm/build/base.py
@@ -45,6 +45,7 @@ class BuildBase[G]:
     __slots__ = (
         'dynamic_array_type5_constructor',
         'function_type5_constructor',
+        'io_type5_constructor',
         'static_array_type5_constructor',
         'tuple_type5_constructor_map',
 
@@ -83,6 +84,18 @@ class BuildBase[G]:
     See type5_make_function and type5_is_function.
     """
 
+    io_type5_constructor: type5typeexpr.TypeConstructor
+    """
+    Constructor for IO.
+
+    An IO function is a function that can have side effects.
+    It can do input or output. Other functions cannot have
+    side effects, and can only return a value based on the
+    input given.
+
+    See type5_make_io and type5_is_io.
+    """
+
     static_array_type5_constructor: type5typeexpr.TypeConstructor
     """
     Constructor for arrays of compiled time determined length.
@@ -207,6 +220,7 @@ class BuildBase[G]:
 
         self.dynamic_array_type5_constructor = type5typeexpr.TypeConstructor(kind=S >> S, name="dynamic_array")
         self.function_type5_constructor = type5typeexpr.TypeConstructor(kind=S >> (S >> S), name="function")
+        self.io_type5_constructor = type5typeexpr.TypeConstructor(kind=S >> S, name="IO")
         self.static_array_type5_constructor = type5typeexpr.TypeConstructor(kind=N >> (S >> S), name='static_array')
         self.tuple_type5_constructor_map = {}
 
@@ -344,6 +358,19 @@ class BuildBase[G]:
 
         return my_args + more_args
 
+    def type5_make_io(self, arg: type5typeexpr.TypeExpr) -> type5typeexpr.TypeApplication:
+        return type5typeexpr.TypeApplication(
+            constructor=self.io_type5_constructor,
+            argument=arg
+        )
+
+    def type5_is_io(self, typeexpr: type5typeexpr.TypeExpr | type5constrainedexpr.ConstrainedExpr) -> type5typeexpr.TypeExpr | None:
+        if not isinstance(typeexpr, type5typeexpr.TypeApplication):
+            return None
+        if typeexpr.constructor != self.io_type5_constructor:
+            return None
+        return typeexpr.argument
+
     def type5_make_tuple(self, args: Sequence[type5typeexpr.TypeExpr]) -> type5typeexpr.TypeApplication:
         if not args:
             raise TypeError("Tuples must at least one field")
diff --git a/phasm/build/default.py b/phasm/build/default.py
index 952f9ab..e173d6c 100644
--- a/phasm/build/default.py
+++ b/phasm/build/default.py
@@ -22,6 +22,7 @@ from .typeclasses import (
     fractional,
     integral,
     intnum,
+    monad,
     natnum,
     ord,
     promotable,
@@ -68,6 +69,7 @@ class BuildDefault(BuildBase[Generator]):
                             integral,
             foldable, subscriptable,
             sized,
+            monad,
         ]
 
         for tc in tc_list:
diff --git a/phasm/build/typeclasses/monad.py b/phasm/build/typeclasses/monad.py
new file mode 100644
index 0000000..4b0edd9
--- /dev/null
+++ b/phasm/build/typeclasses/monad.py
@@ -0,0 +1,26 @@
+"""
+The Monad type class is defined for type constructors that cause one thing to happen /after/ another.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+from ...type5.constrainedexpr import ConstrainedExpr
+from ...type5.kindexpr import Star
+from ...type5.typeexpr import TypeVariable
+from ...typeclass import TypeClass, TypeClassConstraint
+from ...wasmgenerator import Generator as WasmGenerator
+from ..base import BuildBase
+
+
+def load(build: BuildBase[Any]) -> None:
+    a = TypeVariable(kind=Star(), name='a')
+
+    Monad = TypeClass('Monad', (a, ), methods={}, operators={})
+
+    build.register_type_class(Monad)
+
+def wasm(build: BuildBase[WasmGenerator]) -> None:
+    Monad = build.type_classes['Monad']
+
+    build.instance_type_class(Monad, build.io_type5_constructor)
diff --git a/phasm/build/typerouter.py b/phasm/build/typerouter.py
index 44cfbf5..1312c78 100644
--- a/phasm/build/typerouter.py
+++ b/phasm/build/typerouter.py
@@ -35,6 +35,10 @@ class BuildTypeRouter[T](TypeRouter[T]):
         if fn_args is not None:
             return self.when_function(fn_args)
 
+        io_args = self.build.type5_is_io(typ)
+        if io_args is not None:
+            return self.when_io(io_args)
+
         sa_args = self.build.type5_is_static_array(typ)
         if sa_args is not None:
             sa_len, sa_typ = sa_args
@@ -58,6 +62,9 @@ class BuildTypeRouter[T](TypeRouter[T]):
     def when_function(self, fn_args: list[TypeExpr]) -> T:
         raise NotImplementedError
 
+    def when_io(self, io_arg: TypeExpr) -> T:
+        raise NotImplementedError
+
     def when_struct(self, typ: Record) -> T:
         raise NotImplementedError
 
@@ -93,6 +100,9 @@ class TypeName(BuildTypeRouter[str]):
     def when_function(self, fn_args: list[TypeExpr]) -> str:
         return 'Callable[' + ', '.join(map(self, fn_args)) + ']'
 
+    def when_io(self, io_arg: TypeExpr) -> str:
+        return 'IO[' + self(io_arg) + ']'
+
     def when_static_array(self, sa_len: int, sa_typ: TypeExpr) -> str:
         return f'{self(sa_typ)}[{sa_len}]'
 
diff --git a/phasm/codestyle.py b/phasm/codestyle.py
index 965b345..3a13a53 100644
--- a/phasm/codestyle.py
+++ b/phasm/codestyle.py
@@ -124,6 +124,10 @@ def statement(inp: ourlang.Statement) -> Statements:
         yield ''
         return
 
+    if isinstance(inp, ourlang.StatementCall):
+        yield expression(inp.call)
+        return
+
     raise NotImplementedError(statement, inp)
 
 def function(mod: ourlang.Module[Any], inp: ourlang.Function) -> str:
diff --git a/phasm/compiler.py b/phasm/compiler.py
index 3aafd1d..d1a37d7 100644
--- a/phasm/compiler.py
+++ b/phasm/compiler.py
@@ -36,6 +36,11 @@ def type5(mod: ourlang.Module[WasmGenerator], inp: TypeExpr) -> wasm.WasmType:
     Types are used for example in WebAssembly function parameters
     and return types.
     """
+    io_arg = mod.build.type5_is_io(inp)
+    if io_arg is not None:
+        # IO is type constructor that only exists on the typing layer
+        inp = io_arg
+
     typ_info = mod.build.type_info_map.get(inp.name)
     if typ_info is None:
         typ_info = mod.build.type_info_constructed
@@ -346,6 +351,9 @@ def statement_if(wgn: WasmGenerator, mod: ourlang.Module[WasmGenerator], fun: ou
             # for stat in inp.else_statements:
             #     statement(wgn, stat)
 
+def statement_call(wgn: WasmGenerator, mod: ourlang.Module[WasmGenerator], fun: ourlang.Function, inp: ourlang.StatementCall) -> None:
+    expression(wgn, mod, inp.call)
+
 def statement(wgn: WasmGenerator, mod: ourlang.Module[WasmGenerator], fun: ourlang.Function, inp: ourlang.Statement) -> None:
     """
     Compile: any statement
@@ -358,6 +366,10 @@ def statement(wgn: WasmGenerator, mod: ourlang.Module[WasmGenerator], fun: ourla
         statement_if(wgn, mod, fun, inp)
         return
 
+    if isinstance(inp, ourlang.StatementCall):
+        statement_call(wgn, mod, fun, inp)
+        return
+
     if isinstance(inp, ourlang.StatementPass):
         return
 
diff --git a/phasm/ourlang.py b/phasm/ourlang.py
index 698f5b0..f9b5c8e 100644
--- a/phasm/ourlang.py
+++ b/phasm/ourlang.py
@@ -295,6 +295,24 @@ class StatementReturn(Statement):
     def __repr__(self) -> str:
         return f'StatementReturn({repr(self.value)})'
 
+class StatementCall(Statement):
+    """
+    A function call within a function.
+
+    Executing is deferred to the given function until it completes.
+    """
+    __slots__ = ('call')
+
+    call: FunctionCall
+
+    def __init__(self, call: FunctionCall, sourceref: SourceRef) -> None:
+        super().__init__(sourceref=sourceref)
+
+        self.call = call
+
+    def __repr__(self) -> str:
+        return f'StatementCall({repr(self.call)})'
+
 class StatementIf(Statement):
     """
     An if statement within a function
diff --git a/phasm/parser.py b/phasm/parser.py
index 151747f..aab823a 100644
--- a/phasm/parser.py
+++ b/phasm/parser.py
@@ -26,6 +26,7 @@ from .ourlang import (
     ModuleDataBlock,
     SourceRef,
     Statement,
+    StatementCall,
     StatementIf,
     StatementPass,
     StatementReturn,
@@ -362,6 +363,9 @@ class OurVisitor[G]:
 
             return result
 
+        if isinstance(node, ast.Expr) and isinstance(node.value, ast.Call):
+            return StatementCall(self.visit_Module_FunctionDef_Call(module, function, our_locals, node.value), srf(module, node))
+
         if isinstance(node, ast.Pass):
             return StatementPass(srf(module, node))
 
@@ -485,7 +489,7 @@ class OurVisitor[G]:
 
         raise NotImplementedError(f'{node} as expr in FunctionDef')
 
-    def visit_Module_FunctionDef_Call(self, module: Module[G], function: Function, our_locals: OurLocals, node: ast.Call) -> Union[FunctionCall]:
+    def visit_Module_FunctionDef_Call(self, module: Module[G], function: Function, our_locals: OurLocals, node: ast.Call) -> FunctionCall:
         if node.keywords:
             _raise_static_error(node, 'Keyword calling not supported') # Yet?
 
@@ -648,6 +652,15 @@ class OurVisitor[G]:
                     for e in func_arg_types
                 ])
 
+            if isinstance(node.value, ast.Name) and node.value.id == 'IO':
+                assert isinstance(node.slice, ast.Name) or (isinstance(node.slice, ast.Tuple) and len(node.slice.elts) == 0)
+
+                return module.build.type5_make_io(
+                    self.visit_type5(module, node.slice)
+                )
+
+            # TODO: This u32[...] business is messing up the other type constructors
+
             if isinstance(node.slice, ast.Slice):
                 _raise_static_error(node, 'Must subscript using an index')
 
@@ -673,6 +686,9 @@ class OurVisitor[G]:
             if not isinstance(node.ctx, ast.Load):
                 _raise_static_error(node, 'Must be load context')
 
+            if not node.elts:
+                return module.build.unit_type5
+
             return module.build.type5_make_tuple(
                 [self.visit_type5(module, elt) for elt in node.elts],
             )
diff --git a/phasm/type5/fromast.py b/phasm/type5/fromast.py
index f5c4ae0..dc4741d 100644
--- a/phasm/type5/fromast.py
+++ b/phasm/type5/fromast.py
@@ -15,7 +15,7 @@ from .constraints import (
     TypeClassInstanceExistsConstraint,
     UnifyTypesConstraint,
 )
-from .kindexpr import KindExpr
+from .kindexpr import KindExpr, Star
 from .typeexpr import TypeApplication, TypeVariable, instantiate
 
 ConstraintGenerator = Generator[ConstraintBase, None, None]
@@ -212,12 +212,13 @@ def statement_return(ctx: Context, fun: ourlang.Function, inp: ourlang.Statement
     if fun.type5 is None:
         raise NotImplementedError("Deducing function type - you'll have to annotate it.")
 
-    if isinstance(fun.type5, TypeApplication):
-        args = ctx.build.type5_is_function(fun.type5)
-        assert args is not None
-        type5 = args[-1]
-    else:
-        type5 = fun.type5.expr if isinstance(fun.type5, ConstrainedExpr) else fun.type5
+    args = ctx.build.type5_is_function(fun.type5)
+    assert args is not None
+    type5 = args[-1]
+
+    # This is a hack to allow return statement in pure and non pure functions
+    if (io_arg := ctx.build.type5_is_io(type5)):
+        type5 = io_arg
 
     yield from expression(ctx, inp.value, phft)
     yield UnifyTypesConstraint(ctx, inp.sourceref, type5, phft)
@@ -235,6 +236,27 @@ def statement_if(ctx: Context, fun: ourlang.Function, inp: ourlang.StatementIf)
     for stmt in inp.else_statements:
         yield from statement(ctx, fun, stmt)
 
+def statement_call(ctx: Context, fun: ourlang.Function, inp: ourlang.StatementCall) -> ConstraintGenerator:
+
+    if fun.type5 is None:
+        raise NotImplementedError("Deducing function type - you'll have to annotate it.")
+
+    fn_args = ctx.build.type5_is_function(fun.type5)
+    assert fn_args is not None
+    fn_ret = fn_args[-1]
+
+    call_phft = ctx.make_placeholder(inp.call)
+
+    S = Star()
+
+    t_phft = ctx.make_placeholder(kind=S >> S)
+    a_phft = ctx.make_placeholder(kind=S)
+
+    yield from expression_function_call(ctx, inp.call, call_phft)
+    yield TypeClassInstanceExistsConstraint(ctx, inp.sourceref, 'Monad', [t_phft])
+    yield UnifyTypesConstraint(ctx, inp.sourceref, TypeApplication(constructor=t_phft, argument=a_phft), fn_ret)
+    yield UnifyTypesConstraint(ctx, inp.sourceref, TypeApplication(constructor=t_phft, argument=ctx.build.unit_type5), call_phft)
+
 def statement(ctx: Context, fun: ourlang.Function, inp: ourlang.Statement) -> ConstraintGenerator:
     if isinstance(inp, ourlang.StatementReturn):
         yield from statement_return(ctx, fun, inp)
@@ -244,12 +266,18 @@ def statement(ctx: Context, fun: ourlang.Function, inp: ourlang.Statement) -> Co
         yield from statement_if(ctx, fun, inp)
         return
 
+    if isinstance(inp, ourlang.StatementCall):
+        yield from statement_call(ctx, fun, inp)
+        return
+
     raise NotImplementedError(inp)
 
 def function(ctx: Context, inp: ourlang.Function) -> ConstraintGenerator:
     for stmt in inp.statements:
         yield from statement(ctx, inp, stmt)
 
+    # TODO: If function is imported or exported, it should be an IO[..] function
+
 def module_constant_def(ctx: Context, inp: ourlang.ModuleConstantDef) -> ConstraintGenerator:
     phft = ctx.make_placeholder(inp.constant)
 
diff --git a/tests/integration/memory.py b/tests/integration/memory.py
index 3f0f772..1aea951 100644
--- a/tests/integration/memory.py
+++ b/tests/integration/memory.py
@@ -139,6 +139,10 @@ class Extractor(BuildTypeRouter[ExtractorFunc]):
 
         return DynamicArrayExtractor(self.access, self(da_arg))
 
+    def when_io(self, io_arg: TypeExpr) -> ExtractorFunc:
+        # IO is a type only annotation, it is not related to allocation
+        return self(io_arg)
+
     def when_static_array(self, sa_len: int, sa_typ: TypeExpr) -> ExtractorFunc:
         return StaticArrayExtractor(self.access, sa_len, self(sa_typ))
 
diff --git a/tests/integration/test_typeclasses/test_io.py b/tests/integration/test_typeclasses/test_io.py
new file mode 100644
index 0000000..aaa0e53
--- /dev/null
+++ b/tests/integration/test_typeclasses/test_io.py
@@ -0,0 +1,38 @@
+import pytest
+
+from ..helpers import Suite
+
+@pytest.mark.integration_test
+def test_io_use_type_class():
+    code_py = f"""
+@exported
+def testEntry() -> IO[u32]:
+    return 4
+"""
+
+    result = Suite(code_py).run_code()
+    assert 4 == result.returned_value
+
+@pytest.mark.integration_test
+def test_io_call_io_function():
+    code_py = f"""
+@imported
+def log(val: u32) -> IO[()]:
+    pass
+
+@exported
+def testEntry() -> IO[u32]:
+    log(123)
+    return 4
+"""
+
+    log_history: list[Any] = []
+
+    def my_log(val: int) -> None:
+        log_history.append(val)
+
+    result = Suite(code_py).run_code(imports={
+        'log': my_log,
+    })
+    assert 4 == result.returned_value
+    assert [123] == log_history