diff --git a/nix-js/src/context.rs b/nix-js/src/context.rs
index c687d96..0bba013 100644
--- a/nix-js/src/context.rs
+++ b/nix-js/src/context.rs
@@ -11,7 +11,7 @@ use crate::codegen::{CodegenContext, compile};
 use crate::error::{Error, Result, Source};
 use crate::ir::{
     Arg, ArgId, Bool, Builtin, Downgrade as _, DowngradeContext, ExprId, ExprRef, Ir, Null, SymId,
-    ToIr as _, synthetic_span,
+    Thunk, ToIr as _, synthetic_span,
 };
 use crate::runtime::{Runtime, RuntimeContext};
 use crate::store::{StoreBackend, StoreConfig};
@@ -351,6 +351,34 @@ impl DowngradeContext for DowngradeCtx<'_> {
         ExprId(self.ctx.irs.len() + self.irs.len() - 1)
     }
 
+    fn maybe_thunk(&mut self, id: ExprId) -> ExprId {
+        let ir = if id.0 < self.ctx.irs.len() {
+            self.ctx.irs.get(id.0).expect("unreachable")
+        } else {
+            self.irs
+                .get(id.0 - self.ctx.irs.len())
+                .expect("ExprId out of bounds")
+                .as_ref()
+                .expect("maybe_thunk called on an extracted expr")
+        };
+        match ir {
+            Ir::Builtin(_)
+            | Ir::Builtins(_)
+            | Ir::Int(_)
+            | Ir::Float(_)
+            | Ir::Bool(_)
+            | Ir::Null(_)
+            | Ir::Str(_) => id,
+            _ => self.new_expr(
+                Thunk {
+                    inner: id,
+                    span: ir.span(),
+                }
+                .to_ir(),
+            ),
+        }
+    }
+
     fn new_sym(&mut self, sym: String) -> SymId {
         self.ctx.symbols.get_or_intern(sym)
     }
diff --git a/nix-js/src/ir.rs b/nix-js/src/ir.rs
index 227a223..9f021b2 100644
--- a/nix-js/src/ir.rs
+++ b/nix-js/src/ir.rs
@@ -22,6 +22,7 @@ pub trait DowngradeContext {
 
     fn new_expr(&mut self, expr: Ir) -> ExprId;
     fn new_arg(&mut self, span: TextRange) -> ExprId;
+    fn maybe_thunk(&mut self, id: ExprId) -> ExprId;
 
     fn new_sym(&mut self, sym: String) -> SymId;
     fn get_sym(&self, id: SymId) -> &str;
diff --git a/nix-js/src/ir/downgrade.rs b/nix-js/src/ir/downgrade.rs
index 10c041a..160c4ad 100644
--- a/nix-js/src/ir/downgrade.rs
+++ b/nix-js/src/ir/downgrade.rs
@@ -121,7 +121,8 @@ impl<Ctx: DowngradeContext> Downgrade<Ctx> for ast::Str {
             .map(|part| match part {
                 ast::InterpolPart::Literal(lit) => Ok(ctx.new_expr(Str { val: lit, span }.to_ir())),
                 ast::InterpolPart::Interpolation(interpol) => {
-                    interpol.expr().unwrap().downgrade(ctx)
+                    let inner = interpol.expr().unwrap().downgrade(ctx)?;
+                    Ok(ctx.new_expr(Thunk { inner, span }.to_ir()))
                 }
             })
             .collect::<Result<Vec<_>>>()?;
@@ -210,7 +211,10 @@ impl<Ctx: DowngradeContext> Downgrade<Ctx> for ast::List {
     fn downgrade(self, ctx: &mut Ctx) -> Result<ExprId> {
         let items = self
             .items()
-            .map(|item| maybe_thunk(item, ctx))
+            .map(|item| {
+                let id = item.downgrade(ctx)?;
+                Ok(ctx.maybe_thunk(id))
+            })
             .collect::<Result<_>>()?;
         let span = self.syntax().text_range();
         Ok(ctx.new_expr(List { items, span }.to_ir()))
@@ -439,7 +443,8 @@ impl<Ctx: DowngradeContext> Downgrade<Ctx> for ast::Lambda {
 impl<Ctx: DowngradeContext> Downgrade<Ctx> for ast::Apply {
     fn downgrade(self, ctx: &mut Ctx) -> Result<ExprId> {
         let func = self.lambda().unwrap().downgrade(ctx)?;
-        let arg = maybe_thunk(self.argument().unwrap(), ctx)?;
+        let arg = self.argument().unwrap().downgrade(ctx)?;
+        let arg = ctx.maybe_thunk(arg);
         let span = self.syntax().text_range();
         Ok(ctx.new_expr(Call { func, arg, span }.to_ir()))
     }
diff --git a/nix-js/src/ir/utils.rs b/nix-js/src/ir/utils.rs
index 3ca0b68..5d9e150 100644
--- a/nix-js/src/ir/utils.rs
+++ b/nix-js/src/ir/utils.rs
@@ -13,57 +13,6 @@ use crate::value::format_symbol;
 
 use super::*;
 
-pub fn maybe_thunk(mut expr: ast::Expr, ctx: &mut impl DowngradeContext) -> Result<ExprId> {
-    use ast::Expr::*;
-    let expr = loop {
-        expr = match expr {
-            Paren(paren) => paren.expr().unwrap(),
-            Root(root) => root.expr().unwrap(),
-            expr => break expr,
-        }
-    };
-    match expr {
-        Error(error) => {
-            let span = error.syntax().text_range();
-            return Err(self::Error::downgrade_error(error.to_string())
-                .with_span(span)
-                .with_source(ctx.get_current_source()));
-        }
-        Ident(ident) => return ident.downgrade(ctx),
-        Literal(lit) => return lit.downgrade(ctx),
-        Str(str) => return str.downgrade(ctx),
-        Path(path) => return path.downgrade(ctx),
-
-        _ => (),
-    }
-    let id = match expr {
-        Apply(apply) => apply.downgrade(ctx),
-        Assert(assert) => assert.downgrade(ctx),
-        IfElse(ifelse) => ifelse.downgrade(ctx),
-        Select(select) => select.downgrade(ctx),
-        Lambda(lambda) => lambda.downgrade(ctx),
-        LegacyLet(let_) => let_.downgrade(ctx),
-        LetIn(letin) => letin.downgrade(ctx),
-        List(list) => list.downgrade(ctx),
-        BinOp(op) => op.downgrade(ctx),
-        AttrSet(attrs) => attrs.downgrade(ctx),
-        UnaryOp(op) => op.downgrade(ctx),
-        With(with) => with.downgrade(ctx),
-        HasAttr(has) => has.downgrade(ctx),
-
-        _ => unreachable!(),
-    }?;
-    Ok(ctx.new_expr(
-        Thunk {
-            inner: id,
-            // span: ctx.get_span(id),
-            // FIXME: span
-            span: synthetic_span(),
-        }
-        .to_ir(),
-    ))
-}
-
 /// Downgrades the entries of an attribute set.
 /// This handles `inherit` and `attrpath = value;` entries.
 pub fn downgrade_attrs(
@@ -231,7 +180,8 @@ pub fn downgrade_attrpathvalue(
     ctx: &mut impl DowngradeContext,
 ) -> Result<()> {
     let path = downgrade_attrpath(value.attrpath().unwrap(), ctx)?;
-    let value = maybe_thunk(value.value().unwrap(), ctx)?;
+    let value = value.value().unwrap().downgrade(ctx)?;
+    let value = ctx.maybe_thunk(value);
     attrs.insert(path, value, ctx)
 }