diff --git a/phasm/codestyle.py b/phasm/codestyle.py
index fc40868..10fd276 100644
--- a/phasm/codestyle.py
+++ b/phasm/codestyle.py
@@ -102,10 +102,6 @@ def expression(inp: ourlang.Expression) -> str:
     if isinstance(inp, ourlang.AccessStructMember):
         return f'{expression(inp.varref)}.{inp.member}'
 
-    if isinstance(inp, ourlang.Fold):
-        fold_name = 'foldl' if ourlang.Fold.Direction.LEFT == inp.dir else 'foldr'
-        return f'{fold_name}({inp.func.name}, {expression(inp.base)}, {expression(inp.iter)})'
-
     raise NotImplementedError(expression, inp)
 
 def statement(inp: ourlang.Statement) -> Statements:
diff --git a/phasm/compiler.py b/phasm/compiler.py
index d098499..656b269 100644
--- a/phasm/compiler.py
+++ b/phasm/compiler.py
@@ -4,7 +4,7 @@ This module contains the code to convert parsed Ourlang into WebAssembly code
 import struct
 from typing import List, Optional
 
-from . import codestyle, ourlang, prelude, wasm
+from . import ourlang, prelude, wasm
 from .runtime import calculate_alloc_size, calculate_member_offset
 from .stdlib import alloc as stdlib_alloc
 from .stdlib import types as stdlib_types
@@ -376,90 +376,8 @@ def expression(wgn: WasmGenerator, inp: ourlang.Expression) -> None:
         )))
         return
 
-    if isinstance(inp, ourlang.Fold):
-        expression_fold(wgn, inp)
-        return
-
     raise NotImplementedError(expression, inp)
 
-def expression_fold(wgn: WasmGenerator, inp: ourlang.Fold) -> None:
-    """
-    Compile: Fold expression
-    """
-    assert inp.type3 is not None, TYPE3_ASSERTION_ERROR
-
-    if inp.iter.type3 is not prelude.bytes_:
-        raise NotImplementedError(expression_fold, inp, inp.iter.type3)
-
-    wgn.add_statement('nop', comment='acu :: u8')
-    acu_var = wgn.temp_var_u8(f'fold_{codestyle.type3(inp.type3)}_acu')
-    wgn.add_statement('nop', comment='adr :: bytes*')
-    adr_var = wgn.temp_var_i32('fold_i32_adr')
-    wgn.add_statement('nop', comment='len :: i32')
-    len_var = wgn.temp_var_i32('fold_i32_len')
-
-    wgn.add_statement('nop', comment='acu = base')
-    expression(wgn, inp.base)
-    wgn.local.set(acu_var)
-
-    wgn.add_statement('nop', comment='adr = adr(iter)')
-    expression(wgn, inp.iter)
-    wgn.local.set(adr_var)
-
-    wgn.add_statement('nop', comment='len = len(iter)')
-    wgn.local.get(adr_var)
-    wgn.i32.load()
-    wgn.local.set(len_var)
-
-    wgn.add_statement('nop', comment='i = 0')
-    idx_var = wgn.temp_var_i32(f'fold_{codestyle.type3(inp.type3)}_idx')
-    wgn.i32.const(0)
-    wgn.local.set(idx_var)
-
-    wgn.add_statement('nop', comment='if i < len')
-    wgn.local.get(idx_var)
-    wgn.local.get(len_var)
-    wgn.i32.lt_u()
-    with wgn.if_():
-        # From here on, adr_var is the address of byte we're referencing
-        # This is akin to calling stdlib_types.__subscript_bytes__
-        # But since we already know we are inside of bounds,
-        # can just bypass it and load the memory directly.
-        wgn.local.get(adr_var)
-        wgn.i32.const(3) # Bytes header -1, since we do a +1 every loop
-        wgn.i32.add()
-        wgn.local.set(adr_var)
-
-        wgn.add_statement('nop', comment='while True')
-        with wgn.loop():
-            wgn.add_statement('nop', comment='acu = func(acu, iter[i])')
-            wgn.local.get(acu_var)
-
-            # Get the next byte, write back the address
-            wgn.local.get(adr_var)
-            wgn.i32.const(1)
-            wgn.i32.add()
-            wgn.local.tee(adr_var)
-            wgn.i32.load8_u()
-
-            wgn.add_statement('call', f'${inp.func.name}')
-            wgn.local.set(acu_var)
-
-            wgn.add_statement('nop', comment='i = i + 1')
-            wgn.local.get(idx_var)
-            wgn.i32.const(1)
-            wgn.i32.add()
-            wgn.local.set(idx_var)
-
-            wgn.add_statement('nop', comment='if i >= len: break')
-            wgn.local.get(idx_var)
-            wgn.local.get(len_var)
-            wgn.i32.lt_u()
-            wgn.br_if(0)
-
-    # return acu
-    wgn.local.get(acu_var)
-
 def statement_return(wgn: WasmGenerator, inp: ourlang.StatementReturn) -> None:
     """
     Compile: Return statement
diff --git a/phasm/ourlang.py b/phasm/ourlang.py
index df97b23..29c1ce4 100644
--- a/phasm/ourlang.py
+++ b/phasm/ourlang.py
@@ -1,7 +1,6 @@
 """
 Contains the syntax tree for ourlang
 """
-import enum
 from typing import Dict, Iterable, List, Optional, Union
 
 from . import prelude
@@ -161,6 +160,18 @@ class FunctionCall(Expression):
         self.function = function
         self.arguments = []
 
+class FunctionReference(Expression):
+    """
+    An function reference expression within a statement
+    """
+    __slots__ = ('function', )
+
+    function: 'Function'
+
+    def __init__(self, function: 'Function') -> None:
+        super().__init__()
+        self.function = function
+
 class TupleInstantiation(Expression):
     """
     Instantiation a tuple
@@ -207,36 +218,6 @@ class AccessStructMember(Expression):
         self.struct_type3 = struct_type3
         self.member = member
 
-class Fold(Expression):
-    """
-    A (left or right) fold
-    """
-    class Direction(enum.Enum):
-        """
-        Which direction to fold in
-        """
-        LEFT = 0
-        RIGHT = 1
-
-    dir: Direction
-    func: 'Function'
-    base: Expression
-    iter: Expression
-
-    def __init__(
-            self,
-            dir_: Direction,
-            func: 'Function',
-            base: Expression,
-            iter_: Expression,
-        ) -> None:
-        super().__init__()
-
-        self.dir = dir_
-        self.func = func
-        self.base = base
-        self.iter = iter_
-
 class Statement:
     """
     A statement within a function
diff --git a/phasm/parser.py b/phasm/parser.py
index 9944dc2..2346a71 100644
--- a/phasm/parser.py
+++ b/phasm/parser.py
@@ -14,10 +14,10 @@ from .ourlang import (
     ConstantStruct,
     ConstantTuple,
     Expression,
-    Fold,
     Function,
     FunctionCall,
     FunctionParam,
+    FunctionReference,
     Module,
     ModuleConstantDef,
     ModuleDataBlock,
@@ -446,6 +446,9 @@ class OurVisitor:
                 cdef = module.constant_defs[node.id]
                 return VariableReference(cdef)
 
+            if node.id in module.functions:
+                return FunctionReference(module.functions[node.id])
+
             _raise_static_error(node, f'Undefined variable {node.id}')
 
         if isinstance(node, ast.Tuple):
@@ -462,7 +465,7 @@ class OurVisitor:
 
         raise NotImplementedError(f'{node} as expr in FunctionDef')
 
-    def visit_Module_FunctionDef_Call(self, module: Module, function: Function, our_locals: OurLocals, node: ast.Call) -> Union[Fold, FunctionCall]:
+    def visit_Module_FunctionDef_Call(self, module: Module, function: Function, our_locals: OurLocals, node: ast.Call) -> Union[FunctionCall]:
         if node.keywords:
             _raise_static_error(node, 'Keyword calling not supported') # Yet?
 
@@ -475,28 +478,6 @@ class OurVisitor:
 
         if node.func.id in PRELUDE_METHODS:
             func = PRELUDE_METHODS[node.func.id]
-        elif node.func.id == 'foldl':
-            if 3 != len(node.args):
-                _raise_static_error(node, f'Function {node.func.id} requires 3 arguments but {len(node.args)} are given')
-
-            # TODO: This is not generic, you cannot return a function
-            subnode = node.args[0]
-            if not isinstance(subnode, ast.Name):
-                raise NotImplementedError(f'Calling methods that are not a name {subnode}')
-            if not isinstance(subnode.ctx, ast.Load):
-                _raise_static_error(subnode, 'Must be load context')
-            if subnode.id not in module.functions:
-                _raise_static_error(subnode, 'Reference to undefined function')
-            func = module.functions[subnode.id]
-            if 2 != len(func.posonlyargs):
-                _raise_static_error(node, f'Function {node.func.id} requires a function with 2 arguments but a function with {len(func.posonlyargs)} args is given')
-
-            return Fold(
-                Fold.Direction.LEFT,
-                func,
-                self.visit_Module_FunctionDef_expr(module, function, our_locals, node.args[1]),
-                self.visit_Module_FunctionDef_expr(module, function, our_locals, node.args[2]),
-            )
         else:
             if node.func.id not in module.functions:
                 _raise_static_error(node, 'Call to undefined function')
diff --git a/phasm/prelude/__init__.py b/phasm/prelude/__init__.py
index e269590..3e9672d 100644
--- a/phasm/prelude/__init__.py
+++ b/phasm/prelude/__init__.py
@@ -20,6 +20,7 @@ from ..type3.types import (
     Type3,
     TypeApplication_Nullary,
     TypeConstructor_Base,
+    TypeConstructor_Function,
     TypeConstructor_StaticArray,
     TypeConstructor_Struct,
     TypeConstructor_Tuple,
@@ -186,6 +187,16 @@ It should be applied with zero or more arguments. It has a compile time
 determined length, and each argument can be different.
 """
 
+def fn_on_create(args: tuple[Type3, ...], typ: Type3) -> None:
+    pass # ? instance_type_class(InternalPassAsPointer, typ)
+
+function = TypeConstructor_Function('function', on_create=fn_on_create)
+"""
+This is a function.
+
+It should be applied with one or more arguments. The last argument is the 'return' type.
+"""
+
 def st_on_create(args: tuple[tuple[str, Type3], ...], typ: Type3) -> None:
     instance_type_class(InternalPassAsPointer, typ)
 
@@ -574,12 +585,16 @@ instance_type_class(Promotable, f32, f64, methods={
 
 Foldable = Type3Class('Foldable', (t, ), methods={
     'sum': [t(a), a],
+    'foldl': [[a, b, b], b, t(a), b],
+    'foldr': [[a, b, b], b, t(a), b],
 }, operators={}, additional_context={
     'sum': [Constraint_TypeClassInstanceExists(NatNum, (a, ))],
 })
 
 instance_type_class(Foldable, static_array, methods={
     'sum': stdtypes.static_array_sum,
+    'foldl': stdtypes.static_array_foldl,
+    'foldr': stdtypes.static_array_foldr,
 })
 
 PRELUDE_TYPE_CLASSES = {
diff --git a/phasm/stdlib/types.py b/phasm/stdlib/types.py
index 425dfa0..3e2eb06 100644
--- a/phasm/stdlib/types.py
+++ b/phasm/stdlib/types.py
@@ -1185,3 +1185,9 @@ def static_array_sum(g: Generator, tv_map: TypeVariableLookup) -> None:
 
     g.nop(comment=f'Completed sum for {sa_type.name}[{sa_len.value}]')
     # End result: [sum]
+
+def static_array_foldl(g: Generator, tv_map: TypeVariableLookup) -> None:
+    raise NotImplementedError(tv_map)
+
+def static_array_foldr(g: Generator, tv_map: TypeVariableLookup) -> None:
+    raise NotImplementedError(tv_map)
diff --git a/phasm/type3/constraints.py b/phasm/type3/constraints.py
index 4a9541b..4372d50 100644
--- a/phasm/type3/constraints.py
+++ b/phasm/type3/constraints.py
@@ -6,6 +6,7 @@ These need to be resolved before the program can be compiled.
 from typing import Any, Dict, Iterable, List, Optional, Tuple, Union
 
 from .. import ourlang, prelude
+from .functions import FunctionArgument, TypeVariable
 from .placeholders import PlaceholderForType, Type3OrPlaceholder
 from .routers import NoRouteForTypeException, TypeApplicationRouter
 from .typeclasses import Type3Class
@@ -158,7 +159,7 @@ class SameTypeConstraint(ConstraintBase):
         return (
             ' == '.join('{t' + str(idx) + '}' for idx in range(len(self.type_list))),
             {
-                't' + str(idx): typ
+                't' + str(idx): typ.name if isinstance(typ, ourlang.Function) else typ
                 for idx, typ in enumerate(self.type_list)
             },
         )
@@ -181,7 +182,7 @@ class SameTypeArgumentConstraint(ConstraintBase):
         self.arg_var = arg_var
 
     def check(self) -> CheckResult:
-        if self.tc_var.resolve_as is None or self.arg_var.resolve_as is None:
+        if self.tc_var.resolve_as is None:
             return RequireTypeSubstitutes()
 
         tc_typ = self.tc_var.resolve_as
@@ -201,13 +202,84 @@ class SameTypeArgumentConstraint(ConstraintBase):
         # FIXME: This feels sketchy. Shouldn't the type variable
         # have the exact same number as arguments?
         if isinstance(tc_typ.application, TypeApplication_TypeInt):
-            if tc_typ.application.arguments[0] == arg_typ:
-                return None
-
-            return Error(f'{tc_typ.application.arguments[0]:s} must be {arg_typ:s} instead')
+            return [SameTypeConstraint(
+                tc_typ.application.arguments[0],
+                self.arg_var,
+                comment=self.comment,
+            )]
 
         raise NotImplementedError(tc_typ, arg_typ)
 
+    def human_readable(self) -> HumanReadableRet:
+        return (
+            '{tc_var}` == {arg_var}',
+            {
+                'tc_var': self.tc_var if self.tc_var.resolve_as is None else self.tc_var,
+                'arg_var': self.arg_var if self.arg_var.resolve_as is None else self.arg_var,
+            },
+        )
+
+class SameFunctionArgumentConstraint(ConstraintBase):
+    __slots__ = ('type3', 'func_arg', 'type_var_map', )
+
+    type3: PlaceholderForType
+    func_arg: FunctionArgument
+    type_var_map: dict[TypeVariable, PlaceholderForType]
+
+    def __init__(self, type3: PlaceholderForType, func_arg: FunctionArgument, type_var_map: dict[TypeVariable, PlaceholderForType], *, comment: str) -> None:
+        super().__init__(comment=comment)
+
+        self.type3 = type3
+        self.func_arg = func_arg
+        self.type_var_map = type_var_map
+
+    def check(self) -> CheckResult:
+        if self.type3.resolve_as is None:
+            return RequireTypeSubstitutes()
+
+        typ = self.type3.resolve_as
+
+        if isinstance(typ.application, TypeApplication_Nullary):
+            return Error(f'{typ:s} must be a function instead')
+
+        if not isinstance(typ.application, TypeApplication_TypeStar):
+            return Error(f'{typ:s} must be a function instead')
+
+        type_var_map = {
+            x: y.resolve_as
+            for x, y in self.type_var_map.items()
+            if y.resolve_as is not None
+        }
+
+        exp_type_arg_list = [
+            tv if isinstance(tv, Type3) else type_var_map[tv]
+            for tv in self.func_arg.args
+            if isinstance(tv, Type3) or tv in type_var_map
+        ]
+
+        print('self.func_arg.args', self.func_arg.args)
+        print('exp_type_arg_list', exp_type_arg_list)
+
+        if len(exp_type_arg_list) != len(self.func_arg.args):
+            return RequireTypeSubstitutes()
+
+        return [
+            SameTypeConstraint(
+                typ,
+                prelude.function(*exp_type_arg_list),
+                comment=self.comment,
+            )
+        ]
+
+    def human_readable(self) -> HumanReadableRet:
+        return (
+            '{type3} == {func_arg}',
+            {
+                'type3': self.type3,
+                'func_arg': self.func_arg.name,
+            },
+        )
+
 class TupleMatchConstraint(ConstraintBase):
     __slots__ = ('exp_type', 'args', )
 
@@ -264,14 +336,10 @@ class MustImplementTypeClassConstraint(ConstraintBase):
     __slots__ = ('context', 'type_class3', 'types', )
 
     context: Context
-    type_class3: Union[str, Type3Class]
+    type_class3: Type3Class
     types: list[Type3OrPlaceholder]
 
-    DATA = {
-        'bytes': {'Foldable'},
-    }
-
-    def __init__(self, context: Context, type_class3: Union[str, Type3Class], typ_list: list[Type3OrPlaceholder], comment: Optional[str] = None) -> None:
+    def __init__(self, context: Context, type_class3: Type3Class, typ_list: list[Type3OrPlaceholder], comment: Optional[str] = None) -> None:
         super().__init__(comment=comment)
 
         self.context = context
@@ -299,13 +367,9 @@ class MustImplementTypeClassConstraint(ConstraintBase):
 
         assert len(typ_list) == len(self.types)
 
-        if isinstance(self.type_class3, Type3Class):
-            key = (self.type_class3, tuple(typ_list), )
-            if key in self.context.type_class_instances_existing:
-                return None
-        else:
-            if self.type_class3 in self.__class__.DATA.get(typ_list[0].name, set()):
-                return None
+        key = (self.type_class3, tuple(typ_list), )
+        if key in self.context.type_class_instances_existing:
+            return None
 
         typ_cls_name = self.type_class3 if isinstance(self.type_class3, str) else self.type_class3.name
         typ_name_list = ' '.join(x.name for x in typ_list)
diff --git a/phasm/type3/constraintsgenerator.py b/phasm/type3/constraintsgenerator.py
index 01c1549..7f174d5 100644
--- a/phasm/type3/constraintsgenerator.py
+++ b/phasm/type3/constraintsgenerator.py
@@ -12,12 +12,14 @@ from .constraints import (
     Context,
     LiteralFitsConstraint,
     MustImplementTypeClassConstraint,
+    SameFunctionArgumentConstraint,
     SameTypeArgumentConstraint,
     SameTypeConstraint,
     TupleMatchConstraint,
 )
 from .functions import (
     Constraint_TypeClassInstanceExists,
+    FunctionArgument,
     FunctionSignature,
     TypeVariable,
     TypeVariableApplication_Unary,
@@ -111,6 +113,33 @@ def _expression_function_call(
 
         raise NotImplementedError(constraint)
 
+    func_var_map = {
+        x: PlaceholderForType([])
+        for x in signature.args
+        if isinstance(x, FunctionArgument)
+    }
+
+    # If some of the function arguments are functions,
+    # we need to deal with those separately.
+    for sig_arg in signature.args:
+        if not isinstance(sig_arg, FunctionArgument):
+            continue
+
+        # Ensure that for all type variables in the function
+        # there are also type variables available
+        for func_arg in sig_arg.args:
+            if isinstance(func_arg, Type3):
+                continue
+
+            type_var_map.setdefault(func_arg, PlaceholderForType([]))
+
+        yield SameFunctionArgumentConstraint(
+            func_var_map[sig_arg],
+            sig_arg,
+            type_var_map,
+            comment=f'Ensure `{sig_arg.name}` matches in {signature}',
+        )
+
     # If some of the function arguments are type constructors,
     # we need to deal with those separately.
     # That is, given `foo :: t a -> a` we need to ensure
@@ -120,6 +149,9 @@ def _expression_function_call(
             # Not a type variable at all
             continue
 
+        if isinstance(sig_arg, FunctionArgument):
+            continue
+
         if sig_arg.application.constructor is None:
             # Not a type variable for a type constructor
             continue
@@ -150,9 +182,20 @@ def _expression_function_call(
             yield SameTypeConstraint(sig_part, arg_placeholders[arg_expr], comment=comment)
             continue
 
+        if isinstance(sig_part, FunctionArgument):
+            yield SameTypeConstraint(func_var_map[sig_part], arg_placeholders[arg_expr], comment=comment)
+            continue
+
         raise NotImplementedError(sig_part)
     return
 
+def expression_function_reference(ctx: Context, inp: ourlang.FunctionReference, phft: PlaceholderForType) -> ConstraintGenerator:
+    yield SameTypeConstraint(
+        prelude.function(*(x.type3 for x in inp.function.posonlyargs), inp.function.returns_type3),
+        phft,
+        comment=f'typeOf("{inp.function.name}") == typeOf({inp.function.name})',
+    )
+
 def expression(ctx: Context, inp: ourlang.Expression, phft: PlaceholderForType) -> ConstraintGenerator:
     if isinstance(inp, ourlang.Constant):
         yield from constant(ctx, inp, phft)
@@ -171,6 +214,10 @@ def expression(ctx: Context, inp: ourlang.Expression, phft: PlaceholderForType)
         yield from expression_function_call(ctx, inp, phft)
         return
 
+    if isinstance(inp, ourlang.FunctionReference):
+        yield from expression_function_reference(ctx, inp, phft)
+        return
+
     if isinstance(inp, ourlang.TupleInstantiation):
         r_type = []
         for arg in inp.elements:
@@ -209,19 +256,6 @@ def expression(ctx: Context, inp: ourlang.Expression, phft: PlaceholderForType)
             comment=f'The type of a struct member reference is the same as the type of struct member {inp.struct_type3.name}.{inp.member}')
         return
 
-    if isinstance(inp, ourlang.Fold):
-        base_phft = PlaceholderForType([inp.base])
-        iter_phft = PlaceholderForType([inp.iter])
-
-        yield from expression(ctx, inp.base, base_phft)
-        yield from expression(ctx, inp.iter, iter_phft)
-
-        yield SameTypeConstraint(inp.func.posonlyargs[0].type3, inp.func.returns_type3, base_phft, phft,
-            comment='foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b')
-        yield MustImplementTypeClassConstraint(ctx, 'Foldable', [iter_phft])
-
-        return
-
     raise NotImplementedError(expression, inp)
 
 def statement_return(ctx: Context, fun: ourlang.Function, inp: ourlang.StatementReturn) -> ConstraintGenerator:
diff --git a/phasm/type3/functions.py b/phasm/type3/functions.py
index 1b91948..a49f80b 100644
--- a/phasm/type3/functions.py
+++ b/phasm/type3/functions.py
@@ -1,4 +1,6 @@
-from typing import TYPE_CHECKING, Any, Hashable, Iterable, List, Union
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any, Hashable, Iterable, List
 
 if TYPE_CHECKING:
     from .typeclasses import Type3Class
@@ -155,15 +157,29 @@ class TypeVariableContext:
     def __repr__(self) -> str:
         return f'TypeVariableContext({self.constraints!r})'
 
+class FunctionArgument:
+    __slots__ = ('args', 'name', )
+
+    args: list[Type3 | TypeVariable]
+    name: str
+
+    def __init__(self, args: list[Type3 | TypeVariable]) -> None:
+        self.args = args
+
+        self.name = '(' + ' -> '.join(x.name for x in args) + ')'
+
 class FunctionSignature:
     __slots__ = ('context', 'args', )
 
     context: TypeVariableContext
-    args: List[Union['Type3', TypeVariable]]
+    args: List[Type3 | TypeVariable | FunctionArgument]
 
-    def __init__(self, context: TypeVariableContext, args: Iterable[Union['Type3', TypeVariable]]) -> None:
+    def __init__(self, context: TypeVariableContext, args: Iterable[Type3 | TypeVariable | list[Type3 | TypeVariable]]) -> None:
         self.context = context.__copy__()
-        self.args = list(args)
+        self.args = list(
+            FunctionArgument(x) if isinstance(x, list) else x
+            for x in args
+        )
 
     def __str__(self) -> str:
         return str(self.context) + ' -> '.join(x.name for x in self.args)
diff --git a/phasm/type3/typeclasses.py b/phasm/type3/typeclasses.py
index 83d87be..02c6ad0 100644
--- a/phasm/type3/typeclasses.py
+++ b/phasm/type3/typeclasses.py
@@ -1,4 +1,4 @@
-from typing import Dict, Iterable, List, Mapping, Optional, Union
+from typing import Dict, Iterable, List, Mapping, Optional
 
 from .functions import (
     Constraint_TypeClassInstanceExists,
@@ -42,8 +42,8 @@ class Type3Class:
             self,
             name: str,
             args: Type3ClassArgs,
-            methods: Mapping[str, Iterable[Union[Type3, TypeVariable]]],
-            operators: Mapping[str, Iterable[Union[Type3, TypeVariable]]],
+            methods: Mapping[str, Iterable[Type3 | TypeVariable | list[Type3 | TypeVariable]]],
+            operators: Mapping[str, Iterable[Type3 | TypeVariable | list[Type3 | TypeVariable]]],
             inherited_classes: Optional[List['Type3Class']] = None,
             additional_context: Optional[Mapping[str, Iterable[ConstraintBase]]] = None,
         ) -> None:
@@ -71,19 +71,23 @@ class Type3Class:
         return self.name
 
 def _create_signature(
-        method_arg_list: Iterable[Type3 | TypeVariable],
+        method_arg_list: Iterable[Type3 | TypeVariable | list[Type3 | TypeVariable]],
         type_class3: Type3Class,
     ) -> FunctionSignature:
     context = TypeVariableContext()
     if not isinstance(type_class3.args[0], TypeConstructorVariable):
         context.constraints.append(Constraint_TypeClassInstanceExists(type_class3, type_class3.args))
 
-    signature_args: list[Type3 | TypeVariable] = []
+    signature_args: list[Type3 | TypeVariable | list[Type3 | TypeVariable]] = []
     for method_arg in method_arg_list:
         if isinstance(method_arg, Type3):
             signature_args.append(method_arg)
             continue
 
+        if isinstance(method_arg, list):
+            signature_args.append(method_arg)
+            continue
+
         if isinstance(method_arg, TypeVariable):
             type_constructor = method_arg.application.constructor
             if type_constructor is None:
diff --git a/phasm/type3/types.py b/phasm/type3/types.py
index 7a1b835..a949fb5 100644
--- a/phasm/type3/types.py
+++ b/phasm/type3/types.py
@@ -239,6 +239,10 @@ class TypeConstructor_Tuple(TypeConstructor_TypeStar):
     def make_name(self, key: Tuple[Type3, ...]) -> str:
         return '(' + ', '.join(x.name for x in key) + ', )'
 
+class TypeConstructor_Function(TypeConstructor_TypeStar):
+    def make_name(self, key: Tuple[Type3, ...]) -> str:
+        return '(' + ' -> '.join(x.name for x in key) + ')'
+
 class TypeConstructor_Struct(TypeConstructor_Base[tuple[tuple[str, Type3], ...]]):
     """
     Constructs struct types
diff --git a/tests/integration/test_lang/test_builtins.py b/tests/integration/test_lang/test_builtins.py
deleted file mode 100644
index 13400fd..0000000
--- a/tests/integration/test_lang/test_builtins.py
+++ /dev/null
@@ -1,61 +0,0 @@
-import pytest
-
-from ..helpers import Suite
-
-
-@pytest.mark.integration_test
-def test_foldl_1():
-    code_py = """
-def u8_or(l: u8, r: u8) -> u8:
-    return l | r
-
-@exported
-def testEntry(b: bytes) -> u8:
-    return foldl(u8_or, 128, b)
-"""
-    suite = Suite(code_py)
-
-    result = suite.run_code(b'')
-    assert 128 == result.returned_value
-
-    result = suite.run_code(b'\x80')
-    assert 128 == result.returned_value
-
-    result = suite.run_code(b'\x80\x40')
-    assert 192 == result.returned_value
-
-    result = suite.run_code(b'\x80\x40\x20\x10')
-    assert 240 == result.returned_value
-
-    result = suite.run_code(b'\x80\x40\x20\x10\x08\x04\x02\x01')
-    assert 255 == result.returned_value
-
-@pytest.mark.integration_test
-def test_foldl_2():
-    code_py = """
-def xor(l: u8, r: u8) -> u8:
-    return l ^ r
-
-@exported
-def testEntry(a: bytes, b: bytes) -> u8:
-    return foldl(xor, 0, a) ^ foldl(xor, 0, b)
-"""
-    suite = Suite(code_py)
-
-    result = suite.run_code(b'\x55\x0F', b'\x33\x80')
-    assert 233 == result.returned_value
-
-@pytest.mark.integration_test
-def test_foldl_3():
-    code_py = """
-def xor(l: u32, r: u8) -> u32:
-    return l ^ extend(r)
-
-@exported
-def testEntry(a: bytes) -> u32:
-    return foldl(xor, 0, a)
-"""
-    suite = Suite(code_py)
-
-    result = suite.run_code(b'\x55\x0F\x33\x80')
-    assert 233 == result.returned_value
diff --git a/tests/integration/test_lang/test_foldable.py b/tests/integration/test_lang/test_foldable.py
index c03d6e7..38e08ce 100644
--- a/tests/integration/test_lang/test_foldable.py
+++ b/tests/integration/test_lang/test_foldable.py
@@ -36,6 +36,80 @@ def testEntry(x: Foo[4]) -> Foo:
     with pytest.raises(Type3Exception, match='Missing type class instantation: NatNum Foo'):
         Suite(code_py).run_code()
 
+@pytest.mark.integration_test
+def test_foldable_foldl_size():
+    code_py = """
+def u8_or(l: u8, r: u8) -> u8:
+    return l | r
+
+@exported
+def testEntry(b: bytes) -> u8:
+    return foldl(u8_or, 128, b)
+"""
+    suite = Suite(code_py)
+
+    result = suite.run_code(b'')
+    assert 128 == result.returned_value
+
+    result = suite.run_code(b'\x80')
+    assert 128 == result.returned_value
+
+    result = suite.run_code(b'\x80\x40')
+    assert 192 == result.returned_value
+
+    result = suite.run_code(b'\x80\x40\x20\x10')
+    assert 240 == result.returned_value
+
+    result = suite.run_code(b'\x80\x40\x20\x10\x08\x04\x02\x01')
+    assert 255 == result.returned_value
+
+@pytest.mark.integration_test
+@pytest.mark.parametrize('direction, exp_result', [
+    ('foldl', -55, ),
+    ('foldr', -5, ),
+])
+def test_foldable_foldl_foldr(direction, exp_result):
+    # See https://stackoverflow.com/a/13280185
+    code_py = f"""
+def i32_sub(l: i32, r: i32) -> i32:
+    return l - r
+
+@exported
+def testEntry(b: i32[10]) -> i32:
+    return {direction}(i32_sub, 0, b)
+"""
+    suite = Suite(code_py)
+
+    result = suite.run_code(tuple(range(1, 10)))
+    assert exp_result == result.returned_value
+
+@pytest.mark.integration_test
+@pytest.mark.parametrize('in_typ', ['i8', 'i8[3]'])
+def test_foldable_argument_must_be_a_function(in_typ):
+    code_py = f"""
+@exported
+def testEntry(x: {in_typ}, y: i32, z: i64[3]) -> i32:
+    return foldl(x, y, z)
+"""
+
+    r_in_typ = in_typ.replace('[', '\\[').replace(']', '\\]')
+
+    with pytest.raises(Type3Exception, match=f'{r_in_typ} must be a function instead'):
+        Suite(code_py).run_code()
+
+@pytest.mark.integration_test
+def test_foldable_argument_must_be_right_function():
+    code_py = """
+def foo(l: i32, r: i64) -> i64:
+    return extend(l) + r
+
+@exported
+def testEntry(i: i64, l: i64[3]) -> i64:
+    return foldr(foo, i, l)
+"""
+
+    with pytest.raises(Type3Exception, match=r'\(i64 -> i64 -> i64\) must be \(i32 -> i64 -> i64\) instead'):
+        Suite(code_py).run_code()
 
 @pytest.mark.integration_test
 def test_foldable_invalid_return_type():