diff --git a/rust/nix-expr/src/eval_state.rs b/rust/nix-expr/src/eval_state.rs
index 633eb57..7510cfa 100644
--- a/rust/nix-expr/src/eval_state.rs
+++ b/rust/nix-expr/src/eval_state.rs
@@ -1,4 +1,4 @@
-use crate::value::{Int, Value, ValueType};
+use crate::value::{Int, Value, ValueType, ValueTypeOrThunk};
 use anyhow::Context as _;
 use anyhow::{bail, Result};
 use lazy_static::lazy_static;
@@ -133,19 +133,24 @@ impl EvalState {
         }
         self.context.check_err()
     }
-    pub fn value_is_thunk(&self, value: &Value) -> bool {
-        let r = unsafe {
-            raw::get_type(self.context.ptr(), value.raw_ptr()) == raw::ValueType_NIX_TYPE_THUNK
-        };
-        self.context.check_err().unwrap();
-        r
-    }
-    pub fn value_type(&self, value: &Value) -> Result<ValueType> {
-        if self.value_is_thunk(value) {
-            self.force(value)?;
-        }
+    pub fn value_type_unforced(&self, value: &Value) -> ValueTypeOrThunk {
         let r = unsafe { raw::get_type(self.context.ptr(), value.raw_ptr()) };
-        Ok(ValueType::from_raw(r))
+        self.context.check_err().unwrap();
+        ValueTypeOrThunk::from_raw(r)
+    }
+    pub fn value_type_forced(&self, value: &Value) -> Result<ValueType> {
+        match self.value_type_unforced(value) {
+            ValueTypeOrThunk::ValueType(a) => Ok(a),
+            ValueTypeOrThunk::Thunk => {
+                self.force(value)?;
+                match self.value_type_unforced(value) {
+                    ValueTypeOrThunk::ValueType(a) => Ok(a),
+                    ValueTypeOrThunk::Thunk => {
+                        panic!("values should not be thunks after having been forced.")
+                    }
+                }
+            }
+        }
     }
     pub fn require_int(&self, v: &Value) -> Result<Int> {
         let t = self.value_type(v).unwrap();
diff --git a/rust/nix-expr/src/value.rs b/rust/nix-expr/src/value.rs
index 46371a2..dfc160e 100644
--- a/rust/nix-expr/src/value.rs
+++ b/rust/nix-expr/src/value.rs
@@ -19,26 +19,31 @@ pub enum ValueType {
     Null,
     Path,
     String,
-    Thunk,
     Unknown,
 }
 
-impl ValueType {
-    pub(crate) fn from_raw(raw: raw::ValueType) -> ValueType {
+#[derive(Eq, PartialEq, Debug)]
+pub enum ValueTypeOrThunk {
+    ValueType(ValueType),
+    Thunk,
+}
+
+impl ValueTypeOrThunk {
+    pub(crate) fn from_raw(raw: raw::ValueType) -> ValueTypeOrThunk {
         match raw {
-            raw::ValueType_NIX_TYPE_ATTRS => ValueType::AttrSet,
-            raw::ValueType_NIX_TYPE_BOOL => ValueType::Bool,
-            raw::ValueType_NIX_TYPE_EXTERNAL => ValueType::External,
-            raw::ValueType_NIX_TYPE_FLOAT => ValueType::Float,
-            raw::ValueType_NIX_TYPE_FUNCTION => ValueType::Function,
-            raw::ValueType_NIX_TYPE_INT => ValueType::Int,
-            raw::ValueType_NIX_TYPE_LIST => ValueType::List,
-            raw::ValueType_NIX_TYPE_NULL => ValueType::Null,
-            raw::ValueType_NIX_TYPE_PATH => ValueType::Path,
-            raw::ValueType_NIX_TYPE_STRING => ValueType::String,
-            raw::ValueType_NIX_TYPE_THUNK => ValueType::Thunk,
+            raw::ValueType_NIX_TYPE_ATTRS => ValueTypeOrThunk::ValueType(ValueType::AttrSet),
+            raw::ValueType_NIX_TYPE_BOOL => ValueTypeOrThunk::ValueType(ValueType::Bool),
+            raw::ValueType_NIX_TYPE_EXTERNAL => ValueTypeOrThunk::ValueType(ValueType::External),
+            raw::ValueType_NIX_TYPE_FLOAT => ValueTypeOrThunk::ValueType(ValueType::Float),
+            raw::ValueType_NIX_TYPE_FUNCTION => ValueTypeOrThunk::ValueType(ValueType::Function),
+            raw::ValueType_NIX_TYPE_INT => ValueTypeOrThunk::ValueType(ValueType::Int),
+            raw::ValueType_NIX_TYPE_LIST => ValueTypeOrThunk::ValueType(ValueType::List),
+            raw::ValueType_NIX_TYPE_NULL => ValueTypeOrThunk::ValueType(ValueType::Null),
+            raw::ValueType_NIX_TYPE_PATH => ValueTypeOrThunk::ValueType(ValueType::Path),
+            raw::ValueType_NIX_TYPE_STRING => ValueTypeOrThunk::ValueType(ValueType::String),
+            raw::ValueType_NIX_TYPE_THUNK => ValueTypeOrThunk::Thunk,
             // This would happen if a new type of value is added in Nix.
-            _ => ValueType::Unknown,
+            _ => ValueTypeOrThunk::ValueType(ValueType::Unknown),
         }
     }
 }