diff --git a/compiler/mono/src/layout_soa.rs b/compiler/mono/src/layout_soa.rs
new file mode 100644
index 0000000000..0d0d662a8a
--- /dev/null
+++ b/compiler/mono/src/layout_soa.rs
@@ -0,0 +1,228 @@
+use roc_builtins::bitcode::{FloatWidth, IntWidth};
+use roc_module::symbol::Symbol;
+use roc_types::subs::{Content, FlatType, Subs, Variable};
+
+#[derive(Clone, Copy)]
+struct Index<T> {
+    index: u32,
+    _marker: std::marker::PhantomData<T>,
+}
+
+impl<T> Index<T> {
+    pub const fn new(index: u32) -> Self {
+        Self {
+            index,
+            _marker: Default::default(),
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+struct Slice<T> {
+    start: u32,
+    length: u16,
+    _marker: std::marker::PhantomData<T>,
+}
+
+impl<T> Slice<T> {
+    pub const fn new(start: u32, length: u16) -> Self {
+        Self {
+            start,
+            length,
+            _marker: Default::default(),
+        }
+    }
+
+    pub const fn len(&self) -> usize {
+        self.length as _
+    }
+
+    pub const fn is_empty(&self) -> bool {
+        self.length == 0
+    }
+
+    pub const fn indices(&self) -> std::ops::Range<usize> {
+        self.start as usize..(self.start as usize + self.length as usize)
+    }
+}
+
+impl Slice<Layout> {
+    pub fn reserve(layouts: &mut Layouts, length: usize) -> Self {
+        let start = layouts.layouts.len() as u32;
+
+        let it = std::iter::repeat(Layout::Reserved).take(length);
+        layouts.layouts.extend(it);
+
+        Self {
+            start,
+            length: length as u16,
+            _marker: Default::default(),
+        }
+    }
+}
+
+impl Slice<Slice<Layout>> {
+    pub fn reserve(layouts: &mut Layouts, length: usize) -> Self {
+        let start = layouts.layout_slices.len() as u32;
+
+        let empty: Slice<Layout> = Slice::new(0, 0);
+        let it = std::iter::repeat(empty).take(length);
+        layouts.layout_slices.extend(it);
+
+        Self {
+            start,
+            length: length as u16,
+            _marker: Default::default(),
+        }
+    }
+}
+
+type SliceSlice = Slice<Slice<Layout>>;
+type LayoutIndex = Index<Layout>;
+type LambdaSetIndex = u32;
+type NullableUnionIndex = u32;
+type LayoutTuple = u32;
+type SymbolSlice = (u32, u16);
+
+static_assertions::assert_eq_size!([u8; 12], Layout);
+
+pub struct Layouts {
+    layouts: Vec<Layout>,
+    layout_slices: Vec<Slice<Layout>>,
+    function_layouts: Vec<(Slice<Layout>, LambdaSetIndex)>,
+    lambda_sets: Vec<LambdaSet>,
+    symbols: Vec<Symbol>,
+    nullable_unions: Vec<SliceSlice>,
+}
+
+struct FunctionLayout {
+    /// An index into the Layouts.function_layouts array:
+    ///
+    /// last element is the result, prior elements the arguments
+    /// arguments_and_result: Slice<Layout>,
+    /// lambda_set: LambdaSetIndex,
+    index: u32,
+}
+struct LambdaSet {
+    representation: LayoutIndex,
+    set_symbols: SymbolSlice,
+    set_layouts: SliceSlice,
+}
+
+#[derive(Clone, Copy)]
+enum Layout {
+    // theory: we can zero out memory to reserve space for many layouts
+    Reserved,
+
+    // Question: where to store signedness information?
+    Int(IntWidth),
+    Float(FloatWidth),
+    Decimal,
+
+    Str,
+    Dict(LayoutTuple),
+    Set(LayoutIndex),
+    List(LayoutIndex),
+
+    Struct(Slice<Layout>),
+
+    UnionNonRecursive(SliceSlice),
+    UnionRecursive(SliceSlice),
+    UnionNonNullableUnwrapped(Slice<Layout>),
+    UnionNullableWrapper {
+        data: NullableUnionIndex,
+        tag_id: u16,
+    },
+
+    UnionNullableUnwrappedTrue(Slice<Layout>),
+    UnionNullableUnwrappedFalse(Slice<Layout>),
+
+    RecursivePointer,
+}
+
+impl Layouts {
+    const VOID_INDEX: Index<Layout> = Index::new(0);
+    const UNIT_INDEX: Index<Layout> = Index::new(1);
+}
+
+enum LayoutError {
+    UnresolvedVariable(Variable),
+    TypeError(()),
+}
+
+impl Layout {
+    pub const UNIT: Self = Self::Struct(Slice::new(0, 0));
+    pub const VOID: Self = Self::UnionNonRecursive(Slice::new(0, 0));
+
+    pub const EMPTY_LIST: Self = Self::List(Layouts::VOID_INDEX);
+
+    fn from_var(layouts: &mut Layouts, subs: &Subs, var: Variable) -> Result<Layout, LayoutError> {
+        let content = &subs.get_ref(var).content;
+        Self::from_content(layouts, subs, content)
+    }
+
+    fn from_content(
+        layouts: &mut Layouts,
+        subs: &Subs,
+        content: &Content,
+    ) -> Result<Layout, LayoutError> {
+        match content {
+            Content::FlexVar(_) => todo!(),
+            Content::RigidVar(_) => todo!(),
+            Content::RecursionVar {
+                structure,
+                opt_name,
+            } => todo!(),
+            Content::Structure(flat_type) => Self::from_flat_type(layouts, subs, flat_type),
+            Content::Alias(_, _, actual) => {
+                // at this point we throw away alias information
+                Self::from_var(layouts, subs, *actual)
+            }
+            Content::Error => todo!(),
+        }
+    }
+
+    fn from_flat_type(
+        layouts: &mut Layouts,
+        subs: &Subs,
+        flat_type: &FlatType,
+    ) -> Result<Layout, LayoutError> {
+        use LayoutError::*;
+
+        match flat_type {
+            FlatType::Apply(Symbol::LIST_LIST, arguments) => {
+                debug_assert_eq!(arguments.len(), 1);
+
+                let element_var = subs.variables[arguments.slice.start as usize];
+                match Self::from_var(layouts, subs, element_var) {
+                    Ok(element_layout) => {
+                        let element_index = Index::new(layouts.layouts.len() as _);
+                        layouts.layouts.push(element_layout);
+
+                        Ok(Layout::List(element_index))
+                    }
+                    Err(UnresolvedVariable(_)) => Ok(Layout::EMPTY_LIST),
+                    Err(other) => Err(other),
+                }
+            }
+
+            FlatType::Apply(symbol, _) => {
+                unreachable!("Symbol {:?} does not have a layout", symbol)
+            }
+
+            FlatType::Func(_arguments, lambda_set, _result) => {
+                // in this case, a function (pointer) is represented by the environment it
+                // captures: the lambda set
+
+                Self::from_var(layouts, subs, *lambda_set)
+            }
+            FlatType::Record(_, _) => todo!(),
+            FlatType::TagUnion(_, _) => todo!(),
+            FlatType::FunctionOrTagUnion(_, _, _) => todo!(),
+            FlatType::RecursiveTagUnion(_, _, _) => todo!(),
+            FlatType::Erroneous(_) => todo!(),
+            FlatType::EmptyRecord => Ok(Layout::UNIT),
+            FlatType::EmptyTagUnion => Ok(Layout::VOID),
+        }
+    }
+}
diff --git a/compiler/mono/src/lib.rs b/compiler/mono/src/lib.rs
index b1986bc2f8..dfd693c55d 100644
--- a/compiler/mono/src/lib.rs
+++ b/compiler/mono/src/lib.rs
@@ -7,6 +7,7 @@ pub mod borrow;
 pub mod inc_dec;
 pub mod ir;
 pub mod layout;
+pub mod layout_soa;
 pub mod low_level;
 pub mod reset_reuse;
 pub mod tail_recursion;