diff --git a/TODO.md b/TODO.md
index f26a9b4..81cd2db 100644
--- a/TODO.md
+++ b/TODO.md
@@ -31,3 +31,5 @@
   - Functions don't seem to be a thing on typing level yet?
   - Related to the FIXME in phasm_type3?
   - Type constuctor should also be able to constuct placeholders - somehow.
+
+- Read https://bytecodealliance.org/articles/multi-value-all-the-wasm
diff --git a/phasm/codestyle.py b/phasm/codestyle.py
index 3b7e6f4..940ecfa 100644
--- a/phasm/codestyle.py
+++ b/phasm/codestyle.py
@@ -105,10 +105,6 @@ 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 b758715..4eaf1b2 100644
--- a/phasm/compiler.py
+++ b/phasm/compiler.py
@@ -4,12 +4,12 @@ This module contains the code to convert parsed Ourlang into WebAssembly code
 import struct
 from typing import List
 
-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
 from .stdlib.types import TYPE_INFO_CONSTRUCTED, TYPE_INFO_MAP
-from .type3.functions import TypeVariable
+from .type3.functions import FunctionArgument, TypeVariable
 from .type3.routers import NoRouteForTypeException, TypeApplicationRouter
 from .type3.typeclasses import Type3ClassMethod
 from .type3.types import (
@@ -273,6 +273,10 @@ def expression(wgn: WasmGenerator, mod: ourlang.Module, inp: ourlang.Expression)
                 type_var_map[type_var] = arg_expr.type3
                 continue
 
+            if isinstance(type_var, FunctionArgument):
+                # Fixed type, not part of the lookup requirements
+                continue
+
             raise NotImplementedError(type_var, arg_expr.type3)
 
         router = prelude.PRELUDE_TYPE_CLASS_INSTANCE_METHODS[inp.operator]
@@ -298,6 +302,10 @@ def expression(wgn: WasmGenerator, mod: ourlang.Module, inp: ourlang.Expression)
                     type_var_map[type_var] = arg_expr.type3
                     continue
 
+                if isinstance(type_var, FunctionArgument):
+                    # Fixed type, not part of the lookup requirements
+                    continue
+
                 raise NotImplementedError(type_var, arg_expr.type3)
 
             router = prelude.PRELUDE_TYPE_CLASS_INSTANCE_METHODS[inp.function]
@@ -359,90 +367,8 @@ def expression(wgn: WasmGenerator, mod: ourlang.Module, inp: ourlang.Expression)
         )))
         return
 
-    if isinstance(inp, ourlang.Fold):
-        expression_fold(wgn, mod, inp)
-        return
-
     raise NotImplementedError(expression, inp)
 
-def expression_fold(wgn: WasmGenerator, mod: ourlang.Module, 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, mod, inp.base)
-    wgn.local.set(acu_var)
-
-    wgn.add_statement('nop', comment='adr = adr(iter)')
-    expression(wgn, mod, 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, mod: ourlang.Module, inp: ourlang.StatementReturn) -> None:
     """
     Compile: Return statement
diff --git a/phasm/ourlang.py b/phasm/ourlang.py
index 6d828d9..2bd3cac 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
@@ -219,36 +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 4ff906c..e72946d 100644
--- a/phasm/parser.py
+++ b/phasm/parser.py
@@ -14,7 +14,6 @@ from .ourlang import (
     ConstantStruct,
     ConstantTuple,
     Expression,
-    Fold,
     Function,
     FunctionCall,
     FunctionParam,
@@ -467,7 +466,7 @@ 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?
 
@@ -480,28 +479,6 @@ 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]),
-            )
         elif node.func.id in our_locals:
             func = our_locals[node.func.id]
         else:
diff --git a/phasm/prelude/__init__.py b/phasm/prelude/__init__.py
index a440e54..5c5afd8 100644
--- a/phasm/prelude/__init__.py
+++ b/phasm/prelude/__init__.py
@@ -553,12 +553,21 @@ instance_type_class(Promotable, f32, f64, methods={
 
 Foldable = Type3Class('Foldable', (t, ), methods={
     'sum': [t(a), a],
+    'foldl': [[b, a, 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, dynamic_array, methods={
+    'sum': stdtypes.dynamic_array_sum,
+    'foldl': stdtypes.dynamic_array_foldl,
+    'foldr': stdtypes.dynamic_array_foldr,
+})
 instance_type_class(Foldable, static_array, methods={
     'sum': stdtypes.static_array_sum,
+    'foldl': stdtypes.static_array_foldl,
+    'foldr': stdtypes.static_array_foldr,
 })
 
 bytes_ = dynamic_array(u8)
diff --git a/phasm/stdlib/types.py b/phasm/stdlib/types.py
index 0f41d9c..680228d 100644
--- a/phasm/stdlib/types.py
+++ b/phasm/stdlib/types.py
@@ -1059,12 +1059,16 @@ def dynamic_array_sized_len(g: Generator, tv_map: TypeVariableLookup) -> None:
     # The length is stored in the first 4 bytes
     g.i32.load()
 
-def static_array_sized_len(g: Generator, tv_map: TypeVariableLookup) -> None:
-    assert len(tv_map) == 1
-    sa_type, sa_len = next(iter(tv_map.values()))
-    assert isinstance(sa_type, Type3)
-    assert isinstance(sa_len, IntType3)
+def static_array_sized_len(g: Generator, tvl: TypeVariableLookup) -> None:
+    tv_map, tc_map = tvl
 
+    tvn_map = {
+        x.name: y
+        for x, y in tv_map.items()
+    }
+
+    sa_len = tvn_map['a*']
+    assert isinstance(sa_len, IntType3)
     g.i32.const(sa_len.value)
 
 ## ###
@@ -1137,9 +1141,23 @@ def f32_f64_demote(g: Generator, tv_map: TypeVariableLookup) -> None:
     del tv_map
     g.f32.demote_f64()
 
-def static_array_sum(g: Generator, tv_map: TypeVariableLookup) -> None:
-    assert len(tv_map) == 1
-    sa_type, sa_len = next(iter(tv_map.values()))
+## ###
+## Foldable
+
+def dynamic_array_sum(g: Generator, tvl: TypeVariableLookup) -> None:
+    raise NotImplementedError('dynamic_array_sum')
+
+def static_array_sum(g: Generator, tvl: TypeVariableLookup) -> None:
+    tv_map, tc_map = tvl
+
+    tvn_map = {
+        x.name: y
+        for x, y in tv_map.items()
+    }
+
+    sa_type = tvn_map['a']
+    sa_len = tvn_map['a*']
+
     assert isinstance(sa_type, Type3)
     assert isinstance(sa_len, IntType3)
 
@@ -1171,7 +1189,7 @@ def static_array_sum(g: Generator, tv_map: TypeVariableLookup) -> None:
     g.nop(comment=f'Start sum for {sa_type.name}[{sa_len.value}]')
     g.local.set(sum_adr)
 
-    # stop = adr + ar_len * sa_type_alloc_size
+    # stop = adr + ar_len * sa_type_info.alloc_size
     # Stack: []
     g.nop(comment='Calculate address at which to stop looping')
     g.local.get(sum_adr)
@@ -1186,7 +1204,7 @@ def static_array_sum(g: Generator, tv_map: TypeVariableLookup) -> None:
     g.add_statement(sa_type_info.wasm_load_func)
 
     # Since we did the first one, increase adr
-    # adr = adr + sa_type_alloc_size
+    # adr = adr + sa_type_info.alloc_size
     # Stack: [sum] -> [sum]
     g.local.get(sum_adr)
     g.i32.const(sa_type_info.alloc_size)
@@ -1194,15 +1212,15 @@ def static_array_sum(g: Generator, tv_map: TypeVariableLookup) -> None:
     g.local.set(sum_adr)
 
     if sa_len.value > 1:
-        with g.loop(params=[sa_type_info.wasm_type().to_wat()], result=sa_type_info.wasm_type().to_wat()):
+        with g.loop(params=[sa_type_info.wasm_type], result=sa_type_info.wasm_type):
             # sum = sum + *adr
             # Stack: [sum] -> [sum + *adr]
             g.nop(comment='Add array value')
             g.local.get(sum_adr)
             g.add_statement(sa_type_info.wasm_load_func)
-            sa_type_add_gen(g, {})
+            sa_type_add_gen(g, ({}, {}, ))
 
-            # adr = adr + sa_type_alloc_size
+            # adr = adr + sa_type_info.alloc_size
             # Stack: [sum] -> [sum]
             g.nop(comment='Calculate address of the next value')
             g.local.get(sum_adr)
@@ -1219,3 +1237,294 @@ 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 dynamic_array_foldl(g: Generator, tvl: TypeVariableLookup) -> None:
+    tv_map, tc_map = tvl
+
+    tvn_map = {
+        x.name: y
+        for x, y in tv_map.items()
+    }
+
+    sa_type = tvn_map['a']
+    res_type = tvn_map['b']
+
+    assert isinstance(sa_type, Type3)
+    assert isinstance(res_type, Type3)
+
+    sa_type_info = TYPE_INFO_MAP.get(sa_type.name, TYPE_INFO_CONSTRUCTED)
+    res_type_info = TYPE_INFO_MAP.get(res_type.name, TYPE_INFO_CONSTRUCTED)
+
+    # Definitions
+    fold_adr = g.temp_var(i32('fold_adr'))
+    fold_stop = g.temp_var(i32('fold_stop'))
+    fold_init = g.temp_var_t(res_type_info.wasm_type, 'fold_init')
+    fold_func = g.temp_var(i32('fold_func'))
+    fold_len = g.temp_var(i32('fold_len'))
+
+    with g.block(params=['i32', res_type_info.wasm_type, 'i32'], result=res_type_info.wasm_type, comment=f'foldl a={sa_type.name} b={res_type.name}'):
+        # Stack: [fn*, b, sa*] -> [fn*, b]
+        g.local.tee(fold_adr)  # Store address, but also keep it for loading the length
+        g.i32.load()  # Load the length
+        g.local.set(fold_len)  # Store the length
+
+        # Stack: [fn*, b] -> [fn*]
+        g.local.set(fold_init)
+        # Stack: [fn*] -> []
+        g.local.set(fold_func)
+
+        # Stack: [] -> [b]
+        g.nop(comment='No applications if array is empty')
+        g.local.get(fold_init)
+        g.local.get(fold_len)
+        g.i32.eqz() # If the array is empty
+        g.br_if(0)  # Then the base value is the result
+
+        # Stack: [b] -> [b]  ; fold_adr=fold_adr + 4
+        g.nop(comment='Skip the header')
+        g.local.get(fold_adr)
+        g.i32.const(4)
+        g.i32.add()
+        g.local.set(fold_adr)
+
+        # Stack: [b] -> [b]
+        g.nop(comment='Apply the first function call')
+        g.local.get(fold_adr)
+        g.add_statement(sa_type_info.wasm_load_func)
+        g.local.get(fold_func)
+        g.call_indirect([res_type_info.wasm_type, sa_type_info.wasm_type], res_type_info.wasm_type)
+
+        # Stack: [b] -> [b]
+        g.nop(comment='No loop if there is only one item')
+        g.local.get(fold_len)
+        g.i32.const(1)
+        g.i32.eq()
+        g.br_if(0)  # just one value, don't need to loop
+
+        # Stack: [b] -> [b]  ; fold_stop=fold_adr + (sa_len.value * sa_type_info.alloc_size)
+        g.nop(comment='Calculate address at which to stop looping')
+        g.local.get(fold_adr)
+        g.local.get(fold_len)
+        g.i32.const(sa_type_info.alloc_size)
+        g.i32.mul()
+        g.i32.add()
+        g.local.set(fold_stop)
+
+        # Stack: [b] -> [b]  ; fold_adr = fold_adr + sa_type_info.alloc_size
+        g.nop(comment='Calculate address of the next value')
+        g.local.get(fold_adr)
+        g.i32.const(sa_type_info.alloc_size)
+        g.i32.add()
+        g.local.set(fold_adr)
+
+        with g.loop(params=[res_type_info.wasm_type], result=res_type_info.wasm_type):
+            # Stack: [b] -> [b]
+            g.nop(comment='Apply function call')
+            g.local.get(fold_adr)
+            g.add_statement(sa_type_info.wasm_load_func)
+            g.local.get(fold_func)
+            g.call_indirect([res_type_info.wasm_type, sa_type_info.wasm_type], res_type_info.wasm_type)
+
+            # Stack: [b] -> [b]  ; fold_adr = fold_adr + sa_type_info.alloc_size
+            g.nop(comment='Calculate address of the next value')
+            g.local.get(fold_adr)
+            g.i32.const(sa_type_info.alloc_size)
+            g.i32.add()
+            g.local.tee(fold_adr)
+
+            # loop if adr > stop
+            # Stack: [b] -> [b]
+            g.nop(comment='Check if address exceeds array bounds')
+            g.local.get(fold_stop)
+            g.i32.lt_u()
+            g.br_if(0)
+
+    # Stack: [b]
+
+def static_array_foldl(g: Generator, tvl: TypeVariableLookup) -> None:
+    tv_map, tc_map = tvl
+
+    tvn_map = {
+        x.name: y
+        for x, y in tv_map.items()
+    }
+
+    sa_type = tvn_map['a']
+    sa_len = tvn_map['a*']
+    res_type = tvn_map['b']
+
+    assert isinstance(sa_type, Type3)
+    assert isinstance(sa_len, IntType3)
+    assert isinstance(res_type, Type3)
+
+    sa_type_info = TYPE_INFO_MAP.get(sa_type.name, TYPE_INFO_CONSTRUCTED)
+    res_type_info = TYPE_INFO_MAP.get(res_type.name, TYPE_INFO_CONSTRUCTED)
+
+    # Definitions
+    fold_adr = g.temp_var(i32('fold_adr'))
+    fold_stop = g.temp_var(i32('fold_stop'))
+    fold_init = g.temp_var_t(res_type_info.wasm_type, 'fold_init')
+    fold_func = g.temp_var(i32('fold_func'))
+
+    with g.block(params=['i32', res_type_info.wasm_type, 'i32'], result=res_type_info.wasm_type, comment=f'foldl a={sa_type.name} a*={sa_len.value} b={res_type.name}'):
+        # Stack: [fn*, b, sa*] -> [fn*, b]
+        g.local.set(fold_adr)
+        # Stack: [fn*, b] -> [fn*]
+        g.local.set(fold_init)
+        # Stack: [fn*] -> []
+        g.local.set(fold_func)
+
+        if sa_len.value < 1:
+            g.local.get(fold_init)
+            return
+
+        # Stack: [] -> [b]
+        g.nop(comment='Apply the first function call')
+        g.local.get(fold_init)
+        g.local.get(fold_adr)
+        g.add_statement(sa_type_info.wasm_load_func)
+        g.local.get(fold_func)
+        g.call_indirect([res_type_info.wasm_type, sa_type_info.wasm_type], res_type_info.wasm_type)
+
+        if sa_len.value > 1:
+            # Stack: [b] -> [b]  ; fold_stop=fold_adr + (sa_len.value * sa_type_info.alloc_size)
+            g.nop(comment='Calculate address at which to stop looping')
+            g.local.get(fold_adr)
+            g.i32.const(sa_len.value * sa_type_info.alloc_size)
+            g.i32.add()
+            g.local.set(fold_stop)
+
+            # Stack: [b] -> [b]  ; fold_adr = fold_adr + sa_type_info.alloc_size
+            g.nop(comment='Calculate address of the next value')
+            g.local.get(fold_adr)
+            g.i32.const(sa_type_info.alloc_size)
+            g.i32.add()
+            g.local.set(fold_adr)
+
+            with g.loop(params=[res_type_info.wasm_type], result=res_type_info.wasm_type):
+                # Stack: [b] -> [b]
+                g.nop(comment='Apply function call')
+                g.local.get(fold_adr)
+                g.add_statement(sa_type_info.wasm_load_func)
+                g.local.get(fold_func)
+                g.call_indirect([res_type_info.wasm_type, sa_type_info.wasm_type], res_type_info.wasm_type)
+
+                # Stack: [b] -> [b]  ; fold_adr = fold_adr + sa_type_info.alloc_size
+                g.nop(comment='Calculate address of the next value')
+                g.local.get(fold_adr)
+                g.i32.const(sa_type_info.alloc_size)
+                g.i32.add()
+                g.local.tee(fold_adr)
+
+                # loop if adr > stop
+                # Stack: [b] -> [b]
+                g.nop(comment='Check if address exceeds array bounds')
+                g.local.get(fold_stop)
+                g.i32.lt_u()
+                g.br_if(0)
+        # else: just one value, don't need to loop
+
+    # Stack: [b]
+
+def dynamic_array_foldr(g: Generator, tvl: TypeVariableLookup) -> None:
+    raise NotImplementedError('dynamic_array_foldr')
+
+def static_array_foldr(g: Generator, tvl: TypeVariableLookup) -> None:
+    tv_map, tc_map = tvl
+
+    tvn_map = {
+        x.name: y
+        for x, y in tv_map.items()
+    }
+
+    sa_type = tvn_map['a']
+    sa_len = tvn_map['a*']
+    res_type = tvn_map['b']
+
+    assert isinstance(sa_type, Type3)
+    assert isinstance(sa_len, IntType3)
+    assert isinstance(res_type, Type3)
+
+    sa_type_info = TYPE_INFO_MAP.get(sa_type.name, TYPE_INFO_CONSTRUCTED)
+    res_type_info = TYPE_INFO_MAP.get(res_type.name, TYPE_INFO_CONSTRUCTED)
+
+    # Definitions
+    fold_adr = g.temp_var(i32('fold_adr'))
+    fold_stop = g.temp_var(i32('fold_stop'))
+    fold_tmp = g.temp_var_t(res_type_info.wasm_type, 'fold_tmp')
+    fold_func = g.temp_var(i32('fold_func'))
+
+    with g.block(params=['i32', res_type_info.wasm_type, 'i32'], result=res_type_info.wasm_type, comment=f'foldr a={sa_type.name} a*={sa_len.value} b={res_type.name}'):
+        # Stack: [fn*, b, sa*] -> [fn*, b]  ; fold_adr=fn*, fold_tmp=b, fold_func=fn*
+        g.local.set(fold_adr)
+        # Stack: [fn*, b] -> [fn*]
+        g.local.set(fold_tmp)
+        # Stack: [fn*] -> []
+        g.local.set(fold_func)
+
+        if sa_len.value < 1:
+            g.local.get(fold_tmp)
+            return
+
+        # Stack: [] -> []  ; fold_stop=fold_adr
+        g.nop(comment='Calculate address at which to stop looping')
+        g.local.get(fold_adr)
+        g.local.set(fold_stop)
+
+        # Stack: [] -> []  ; fold_adr=fold_adr + (sa_len.value - 1) * sa_type_info.alloc_size
+        g.nop(comment='Calculate address at which to stop looping')
+        g.local.get(fold_adr)
+        g.i32.const((sa_len.value - 1) * sa_type_info.alloc_size)
+        g.i32.add()
+        g.local.set(fold_adr)
+
+        # Stack: [] -> [b]
+        g.nop(comment='Get the init value and first array value as starting point')
+        g.local.get(fold_adr)
+        g.add_statement(sa_type_info.wasm_load_func)
+        g.local.get(fold_tmp)
+        g.local.get(fold_func)
+        g.call_indirect([sa_type_info.wasm_type, res_type_info.wasm_type], res_type_info.wasm_type)
+
+        if sa_len.value > 1:
+            # Stack: [b] -> [b]  ; fold_adr = fold_adr - sa_type_info.alloc_size
+            g.nop(comment='Calculate address of the next value')
+            g.local.get(fold_adr)
+            g.i32.const(sa_type_info.alloc_size)
+            g.i32.sub()
+            g.local.set(fold_adr)
+
+            with g.loop(params=[res_type_info.wasm_type], result=res_type_info.wasm_type):
+                g.nop(comment='Apply function call')
+
+                # Stack [b] since we don't have proper stack switching opcodes
+                # Stack: [b] -> []
+                g.local.set(fold_tmp)
+
+                # Stack: [] -> [a]
+                g.local.get(fold_adr)
+                g.add_statement(sa_type_info.wasm_load_func)
+
+                # Stack [a] -> [a, b]
+                g.local.get(fold_tmp)
+
+                # Stack [a, b] -> [b]
+                g.local.get(fold_func)
+                g.call_indirect([sa_type_info.wasm_type, res_type_info.wasm_type], res_type_info.wasm_type)
+
+                # Stack: [b] -> [b]  ; fold_adr = fold_adr - sa_type_info.alloc_size
+                g.nop(comment='Calculate address of the next value')
+                g.local.get(fold_adr)
+                g.i32.const(sa_type_info.alloc_size)
+                g.i32.sub()
+                g.local.tee(fold_adr)
+
+                # loop if adr >= stop
+                # Stack: [b] -> [b]
+                g.nop(comment='Check if address exceeds array bounds')
+                g.local.get(fold_stop)
+                g.i32.ge_u()
+                g.br_if(0)
+        # else: just one value, don't need to loop
+
+    # Stack: [b]
diff --git a/phasm/type3/constraints.py b/phasm/type3/constraints.py
index 6046724..3ed8ccf 100644
--- a/phasm/type3/constraints.py
+++ b/phasm/type3/constraints.py
@@ -183,7 +183,7 @@ 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
@@ -200,12 +200,16 @@ class SameTypeArgumentConstraint(ConstraintBase):
             # So we can let the MustImplementTypeClassConstraint handle it.
             return None
 
+        if isinstance(tc_typ.application, TypeApplication_Type):
+            return [SameTypeConstraint(
+                tc_typ.application.arguments[0],
+                self.arg_var,
+                comment=self.comment,
+            )]
+
         # 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 [SameTypeConstraint(
                 tc_typ.application.arguments[0],
                 self.arg_var,
@@ -346,14 +350,10 @@ class MustImplementTypeClassConstraint(ConstraintBase):
     __slots__ = ('context', 'type_class3', 'types', )
 
     context: Context
-    type_class3: Union[str, Type3Class]
+    type_class3: Type3Class
     types: list[Type3OrPlaceholder]
 
-    DATA = {
-        'dynamic_array': {'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
@@ -381,13 +381,9 @@ 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 b0183d3..f28b88f 100644
--- a/phasm/type3/constraintsgenerator.py
+++ b/phasm/type3/constraintsgenerator.py
@@ -202,6 +202,10 @@ 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
 
@@ -265,19 +269,6 @@ 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/routers.py b/phasm/type3/routers.py
index 90b8fa4..26fa6cc 100644
--- a/phasm/type3/routers.py
+++ b/phasm/type3/routers.py
@@ -3,6 +3,7 @@ from typing import Any, Callable
 from .functions import (
     TypeConstructorVariable,
     TypeVariable,
+    TypeVariableApplication_Nullary,
     TypeVariableApplication_Unary,
 )
 from .typeclasses import Type3ClassArgs
@@ -60,7 +61,10 @@ class TypeApplicationRouter[S, R]:
 
         raise NoRouteForTypeException(arg0, typ)
 
-TypeVariableLookup = dict[TypeVariable, tuple[KindArgument, ...]]
+TypeVariableLookup = tuple[
+    dict[TypeVariable, KindArgument],
+    dict[TypeConstructorVariable, TypeConstructor_Base[Any]],
+]
 
 class TypeClassArgsRouter[S, R]:
     """
@@ -95,11 +99,12 @@ class TypeClassArgsRouter[S, R]:
 
     def __call__(self, arg0: S, tv_map: dict[TypeVariable, Type3]) -> R:
         key: list[Type3 | TypeConstructor_Base[Any]] = []
-        arguments: TypeVariableLookup = {}
+        arguments: TypeVariableLookup = (dict(tv_map), {}, )
 
         for tc_arg in self.args:
             if isinstance(tc_arg, TypeVariable):
                 key.append(tv_map[tc_arg])
+                arguments[0][tc_arg] = tv_map[tc_arg]
                 continue
 
             for tvar, typ in tv_map.items():
@@ -108,16 +113,24 @@ class TypeClassArgsRouter[S, R]:
                     continue
 
                 key.append(typ.application.constructor)
+                arguments[1][tc_arg] = typ.application.constructor
 
                 if isinstance(tvar.application, TypeVariableApplication_Unary):
                     if isinstance(typ.application, TypeApplication_Type):
-                        arguments[tvar.application.arguments] = typ.application.arguments
+                        da_type, = typ.application.arguments
+                        sa_type_tv = tvar.application.arguments
+                        arguments[0][sa_type_tv] = da_type
                         continue
 
                     # FIXME: This feels sketchy. Shouldn't the type variable
                     # have the exact same number as arguments?
                     if isinstance(typ.application, TypeApplication_TypeInt):
-                        arguments[tvar.application.arguments] = typ.application.arguments
+                        sa_type, sa_len = typ.application.arguments
+                        sa_type_tv = tvar.application.arguments
+                        sa_len_tv = TypeVariable(sa_type_tv.name + '*', TypeVariableApplication_Nullary(None, None))
+
+                        arguments[0][sa_type_tv] = sa_type
+                        arguments[0][sa_len_tv] = sa_len
                         continue
 
                 raise NotImplementedError(tvar.application, typ.application)
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/wasmgenerator.py b/phasm/wasmgenerator.py
index 9bee38e..2b6ead0 100644
--- a/phasm/wasmgenerator.py
+++ b/phasm/wasmgenerator.py
@@ -170,23 +170,32 @@ class Generator_Local:
         self.generator.add_statement('local.tee', variable.name_ref, comment=comment)
 
 class GeneratorBlock:
-    def __init__(self, generator: 'Generator', name: str, params: Iterable[str] = (), result: str | None = None) -> None:
+    def __init__(
+            self,
+            generator: 'Generator',
+            name: str,
+            params: Iterable[str | Type[wasm.WasmType]] = (),
+            result: str | Type[wasm.WasmType] | None = None,
+            comment: str | None = None,
+        ) -> None:
         self.generator = generator
         self.name = name
         self.params = params
         self.result = result
+        self.comment = comment
 
     def __enter__(self) -> None:
         stmt = self.name
         if self.params:
             stmt = f'{stmt} ' + ' '.join(
-                f'(param {typ})'
+                f'(param {typ})' if isinstance(typ, str) else f'(param {typ().to_wat()})'
                 for typ in self.params
             )
         if self.result:
-            stmt = f'{stmt} (result {self.result})'
+            result = self.result if isinstance(self.result, str) else self.result().to_wat()
+            stmt = f'{stmt} (result {result})'
 
-        self.generator.add_statement(stmt)
+        self.generator.add_statement(stmt, comment=self.comment)
 
     def __exit__(self, exc_type: Any, exc_value: Any, traceback: Any) -> None:
         if not exc_type:
@@ -208,7 +217,7 @@ class Generator:
         # 2.4.5 Control Instructions
         self.nop = functools.partial(self.add_statement, 'nop')
         self.unreachable = functools.partial(self.add_statement, 'unreachable')
-        # block
+        self.block = functools.partial(GeneratorBlock, self, 'block')
         self.loop = functools.partial(GeneratorBlock, self, 'loop')
         self.if_ = functools.partial(GeneratorBlock, self, 'if')
         # br
@@ -224,6 +233,16 @@ class Generator:
     def call(self, function: wasm.Function) -> None:
         self.add_statement('call', f'${function.name}')
 
+    def call_indirect(self, params: Iterable[Type[wasm.WasmType]], result: Type[wasm.WasmType]) -> None:
+        param_str = ' '.join(
+            x().to_wat()
+            for x in params
+        )
+
+        result_str = result().to_wat()
+
+        self.add_statement(f'call_indirect (param {param_str}) (result {result_str})')
+
     def add_statement(self, name: str, *args: str, comment: Optional[str] = None) -> None:
         self.statements.append(wasm.Statement(name, *args, comment=comment))
 
@@ -234,6 +253,28 @@ class Generator:
 
         return var.__class__(varname)
 
+    def temp_var_t(self, typ: Type[wasm.WasmType], name: str) -> VarType_Base:
+        idx = 0
+        while (varname := f'__{name}_tmp_var_{idx}__') in self.locals:
+            idx += 1
+
+        if typ is wasm.WasmTypeInt32:
+            return VarType_u8(varname)
+
+        if typ is wasm.WasmTypeInt32:
+            return VarType_i32(varname)
+
+        if typ is wasm.WasmTypeInt64:
+            return VarType_i64(varname)
+
+        if typ is wasm.WasmTypeFloat32:
+            return VarType_f32(varname)
+
+        if typ is wasm.WasmTypeFloat64:
+            return VarType_f64(varname)
+
+        raise NotImplementedError(typ)
+
     def temp_var_i32(self, infix: str) -> VarType_i32:
         return self.temp_var(VarType_i32(infix))
 
diff --git a/tests/integration/helpers.py b/tests/integration/helpers.py
index dad1009..e9b5338 100644
--- a/tests/integration/helpers.py
+++ b/tests/integration/helpers.py
@@ -1,3 +1,4 @@
+import os
 import struct
 import sys
 from typing import Any, Generator, Iterable, List, TextIO, Union
@@ -36,26 +37,39 @@ class Suite:
     def __init__(self, code_py: str) -> None:
         self.code_py = code_py
 
-    def run_code(self, *args: Any, runtime: str = 'wasmtime', func_name: str = 'testEntry', imports: runners.Imports = None, do_format_check: bool = True) -> Any:
+    def run_code(
+            self,
+            *args: Any,
+            runtime: str = 'wasmtime',
+            func_name: str = 'testEntry',
+            imports: runners.Imports = None,
+            do_format_check: bool = True,
+            verbose: bool | None = None,
+        ) -> Any:
         """
         Compiles the given python code into wasm and
         then runs it
 
         Returned is an object with the results set
         """
+        if verbose is None:
+            verbose = bool(os.environ.get('VERBOSE'))
+
         class_ = RUNNER_CLASS_MAP[runtime]
 
         runner = class_(self.code_py)
 
-        write_header(sys.stderr, 'Phasm')
-        runner.dump_phasm_code(sys.stderr)
+        if verbose:
+            write_header(sys.stderr, 'Phasm')
+            runner.dump_phasm_code(sys.stderr)
 
-        runner.parse()
+        runner.parse(verbose=verbose)
         runner.compile_ast()
         runner.compile_wat()
 
-        write_header(sys.stderr, 'Assembly')
-        runner.dump_wasm_wat(sys.stderr)
+        if verbose:
+            write_header(sys.stderr, 'Assembly')
+            runner.dump_wasm_wat(sys.stderr)
 
         runner.interpreter_setup()
         runner.interpreter_load(imports)
@@ -70,8 +84,9 @@ class Suite:
 
         wasm_args: List[Union[float, int]] = []
         if args:
-            write_header(sys.stderr, 'Memory (pre alloc)')
-            runner.interpreter_dump_memory(sys.stderr)
+            if verbose:
+                write_header(sys.stderr, 'Memory (pre alloc)')
+                runner.interpreter_dump_memory(sys.stderr)
 
             for arg, arg_typ in zip(args, func_args, strict=True):
                 if arg_typ in (prelude.u8, prelude.u32, prelude.u64, ):
@@ -95,8 +110,9 @@ class Suite:
                 except NoRouteForTypeException:
                     raise NotImplementedError(arg_typ, arg)
 
-        write_header(sys.stderr, 'Memory (pre run)')
-        runner.interpreter_dump_memory(sys.stderr)
+        if verbose:
+            write_header(sys.stderr, 'Memory (pre run)')
+            runner.interpreter_dump_memory(sys.stderr)
 
         result = SuiteResult()
         result.returned_value = runner.call(func_name, *wasm_args)
@@ -107,8 +123,9 @@ class Suite:
             result.returned_value,
         )
 
-        write_header(sys.stderr, 'Memory (post run)')
-        runner.interpreter_dump_memory(sys.stderr)
+        if verbose:
+            write_header(sys.stderr, 'Memory (post run)')
+            runner.interpreter_dump_memory(sys.stderr)
 
         return result
 
diff --git a/tests/integration/runners.py b/tests/integration/runners.py
index 6afeacf..3957233 100644
--- a/tests/integration/runners.py
+++ b/tests/integration/runners.py
@@ -32,12 +32,12 @@ class RunnerBase:
         """
         _dump_code(textio, self.phasm_code)
 
-    def parse(self) -> None:
+    def parse(self, verbose: bool = True) -> None:
         """
         Parses the Phasm code into an AST
         """
         self.phasm_ast = phasm_parse(self.phasm_code)
-        phasm_type3(self.phasm_ast, verbose=True)
+        phasm_type3(self.phasm_ast, verbose=verbose)
 
     def compile_ast(self) -> None:
         """
@@ -120,6 +120,8 @@ class RunnerWasmtime(RunnerBase):
 
             if vartype is int:
                 params.append(wasmtime.ValType.i32())
+            elif vartype is float:
+                params.append(wasmtime.ValType.f32())
             else:
                 raise NotImplementedError
 
@@ -128,6 +130,8 @@ class RunnerWasmtime(RunnerBase):
             pass # No return value
         elif func.__annotations__['return'] is int:
             results.append(wasmtime.ValType.i32())
+        elif func.__annotations__['return'] is float:
+            results.append(wasmtime.ValType.f32())
         else:
             raise NotImplementedError('Return type', func.__annotations__['return'])
 
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_typeclasses/test_foldable.py b/tests/integration/test_typeclasses/test_foldable.py
index 34320b0..c4e1c53 100644
--- a/tests/integration/test_typeclasses/test_foldable.py
+++ b/tests/integration/test_typeclasses/test_foldable.py
@@ -9,10 +9,13 @@ from .test_natnum import FLOAT_TYPES, INT_TYPES
 @pytest.mark.integration_test
 @pytest.mark.parametrize('length', [1, 5, 13])
 @pytest.mark.parametrize('a_type', INT_TYPES + FLOAT_TYPES)
-def test_foldable_sum(length, a_type):
+@pytest.mark.parametrize('static', [True, False])
+def test_foldable_sum(length, a_type, static):
+    typ_arg = str(length) if static else '...'
+
     code_py = f"""
 @exported
-def testEntry(x: {a_type}[{length}]) -> {a_type}:
+def testEntry(x: {a_type}[{typ_arg}]) -> {a_type}:
     return sum(x)
 """
 
@@ -36,6 +39,145 @@ 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
+@pytest.mark.parametrize('length', [1, 5, 13])
+@pytest.mark.parametrize('direction', ['foldl', 'foldr'])
+def test_foldable_foldl_foldr_size(direction, length):
+    code_py = f"""
+def u64_add(l: u64, r: u64) -> u64:
+    return l + r
+
+@exported
+def testEntry(b: u64[{length}]) -> u64:
+    return {direction}(u64_add, 100, b)
+"""
+    suite = Suite(code_py)
+
+    in_put = tuple(range(1, length + 1))
+
+    result = suite.run_code(in_put)
+    assert (100 + sum(in_put)) == result.returned_value
+
+@pytest.mark.integration_test
+@pytest.mark.parametrize('direction', ['foldl', 'foldr'])
+def test_foldable_foldl_foldr_compounded_type(direction):
+    code_py = f"""
+def combine_foldl(b: u64, a: (u32, u32, )) -> u64:
+    return extend(a[0] * a[1]) + b
+
+def combine_foldr(a: (u32, u32, ), b: u64) -> u64:
+    return extend(a[0] * a[1]) + b
+
+@exported
+def testEntry(b: (u32, u32, )[3]) -> u64:
+    return {direction}(combine_{direction}, 10000, b)
+"""
+    suite = Suite(code_py)
+
+    result = suite.run_code(((2, 5), (25, 4), (125, 8)))
+    assert 11110 == result.returned_value
+
+@pytest.mark.integration_test
+@pytest.mark.parametrize('in_put, direction, exp_result', [
+    ([], 'foldl', 0, ),
+    ([], 'foldr', 0, ),
+
+    ([1], 'foldl', -1, ),
+    ([1], 'foldr', 1, ),
+
+    ([1,2], 'foldl', -3, ),
+    ([1,2], 'foldr', -1, ),
+
+    ([1,2,3], 'foldl', -6, ),
+    ([1,2,3], 'foldr', 2, ),
+
+    ([1,2,3,4], 'foldl', -10, ),
+    ([1,2,3,4], 'foldr', -2, ),
+
+    ([1,2,3,4,5], 'foldl', -15, ),
+    ([1,2,3,4,5], 'foldr', 3, ),
+
+    ([1,2,3,4,5,6], 'foldl', -21, ),
+    ([1,2,3,4,5,6], 'foldr', -3, ),
+
+    ([1,2,3,4,5,6,7], 'foldl', -28, ),
+    ([1,2,3,4,5,6,7], 'foldr', 4, ),
+
+    ([1,2,3,4,5,6,7,8], 'foldl', -36, ),
+    ([1,2,3,4,5,6,7,8], 'foldr', -4, ),
+])
+@pytest.mark.parametrize('static', [True, False])
+def test_foldable_foldl_foldr_result(direction, in_put, exp_result, static):
+    typ_arg = str(len(in_put)) if static else '...'
+
+    # 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[{typ_arg}]) -> i32:
+    return {direction}(i32_sub, 0, b)
+"""
+    suite = Suite(code_py)
+
+    result = suite.run_code(tuple(in_put))
+    assert exp_result == result.returned_value
+
+@pytest.mark.integration_test
+def test_foldable_foldl_bytes():
+    code_py = """
+def u8_or(l: u8, r: u8) -> u8:
+    return l | r
+
+@exported
+def testEntry(b: bytes) -> u8:
+    return foldl(u8_or, 0, b)
+"""
+    suite = Suite(code_py)
+
+    result = suite.run_code(b'')
+    assert 0 == 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('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'Callable\[i64, i64, i64\] must be Callable\[i32, i64, i64\] instead'):
+        Suite(code_py).run_code()
 
 @pytest.mark.integration_test
 def test_foldable_invalid_return_type():