3 files changed, 71 insertions, 2 deletions

diff --git a/dhall/src/lib.rs b/dhall/src/lib.rs
index 24e4377..0be6db3 100644
--- a/dhall/src/lib.rs
+++ b/dhall/src/lib.rs
@@ -9,6 +9,7 @@ mod tests;
 
 pub mod error;
 pub mod semantics;
+pub mod simple;
 pub mod syntax;
 
 use std::fmt::Display;
@@ -29,6 +30,7 @@ pub type ParsedExpr = Expr;
 pub type DecodedExpr = Expr;
 pub type ResolvedExpr = Expr;
 pub type NormalizedExpr = Expr;
+pub use crate::simple::{SValKind, SimpleValue};
 
 #[derive(Debug, Clone)]
 pub struct Parsed(ParsedExpr, ImportLocation);
@@ -53,7 +55,7 @@ pub struct Typed {
 pub struct Normalized(Nir);
 
 /// Controls conversion from `Nir` to `Expr`
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Default)]
 pub(crate) struct ToExprOptions {
     /// Whether to convert all variables to `_`
     pub(crate) alpha: bool,
@@ -138,7 +140,11 @@ impl Normalized {
 
     /// Converts a value back to the corresponding AST expression.
     pub fn to_expr(&self) -> NormalizedExpr {
-        self.0.to_expr(ToExprOptions { alpha: false })
+        self.0.to_expr(ToExprOptions::default())
+    }
+    /// Converts a value into a SimpleValue.
+    pub fn to_simple_value(&self) -> Result<SimpleValue, Expr> {
+        self.0.to_simple_value().ok_or_else(|| self.to_expr())
     }
     /// Converts a value back to the corresponding Hir expression.
     pub(crate) fn to_hir(&self) -> Hir {
diff --git a/dhall/src/semantics/nze/nir.rs b/dhall/src/semantics/nze/nir.rs
index 32ef590..9e3644c 100644
--- a/dhall/src/semantics/nze/nir.rs
+++ b/dhall/src/semantics/nze/nir.rs
@@ -11,6 +11,7 @@ use crate::syntax::{
     Span,
 };
 use crate::{NormalizedExpr, ToExprOptions};
+use crate::{SValKind, SimpleValue};
 
 /// Stores a possibly unevaluated value. Gets (partially) normalized on-demand, sharing computation
 /// automatically. Uses a Rc<RefCell> to share computation.
@@ -142,6 +143,39 @@ impl Nir {
     pub(crate) fn to_expr_tyenv(&self, tyenv: &TyEnv) -> NormalizedExpr {
         self.to_hir(tyenv.as_varenv()).to_expr_tyenv(tyenv)
     }
+    pub(crate) fn to_simple_value(&self) -> Option<SimpleValue> {
+        Some(SimpleValue::new(match self.kind() {
+            NirKind::Lit(lit) => SValKind::Lit(lit.clone()),
+            NirKind::TextLit(x) => SValKind::Text(
+                x.as_text()
+                    .expect("Normal form should ensure the text is a string"),
+            ),
+            NirKind::EmptyOptionalLit(_) => SValKind::Optional(None),
+            NirKind::NEOptionalLit(x) => {
+                SValKind::Optional(Some(x.to_simple_value()?))
+            }
+            NirKind::EmptyListLit(_) => SValKind::List(vec![]),
+            NirKind::NEListLit(xs) => SValKind::List(
+                xs.iter()
+                    .map(|v| v.to_simple_value())
+                    .collect::<Option<_>>()?,
+            ),
+            NirKind::RecordLit(kvs) => SValKind::Record(
+                kvs.iter()
+                    .map(|(k, v)| Some((k.into(), v.to_simple_value()?)))
+                    .collect::<Option<_>>()?,
+            ),
+            NirKind::UnionLit(field, x, _) => {
+                SValKind::Union(field.into(), Some(x.to_simple_value()?))
+            }
+            NirKind::UnionConstructor(field, ty)
+                if ty.get(field).map(|f| f.is_some()) == Some(false) =>
+            {
+                SValKind::Union(field.into(), None)
+            }
+            _ => return None,
+        }))
+    }
 
     pub(crate) fn normalize(&self) {
         self.0.normalize()
diff --git a/dhall/src/simple.rs b/dhall/src/simple.rs
new file mode 100644
index 0000000..6457291
--- /dev/null
+++ b/dhall/src/simple.rs
@@ -0,0 +1,29 @@
+use std::collections::BTreeMap;
+
+use crate::syntax::LitKind;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SimpleValue {
+    kind: Box<SValKind>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum SValKind {
+    Lit(LitKind),
+    Optional(Option<SimpleValue>),
+    List(Vec<SimpleValue>),
+    Record(BTreeMap<String, SimpleValue>),
+    Union(String, Option<SimpleValue>),
+    Text(String),
+}
+
+impl SimpleValue {
+    pub(crate) fn new(kind: SValKind) -> Self {
+        SimpleValue {
+            kind: Box::new(kind),
+        }
+    }
+    pub fn kind(&self) -> &SValKind {
+        self.kind.as_ref()
+    }
+}