3 files changed, 103 insertions, 3 deletions

diff --git a/dhall/src/lib.rs b/dhall/src/lib.rs
index 0be6db3..25196ba 100644
--- a/dhall/src/lib.rs
+++ b/dhall/src/lib.rs
@@ -30,7 +30,7 @@ pub type ParsedExpr = Expr;
 pub type DecodedExpr = Expr;
 pub type ResolvedExpr = Expr;
 pub type NormalizedExpr = Expr;
-pub use crate::simple::{SValKind, SimpleValue};
+pub use crate::simple::{STyKind, SValKind, SimpleType, SimpleValue};
 
 #[derive(Debug, Clone)]
 pub struct Parsed(ParsedExpr, ImportLocation);
@@ -146,6 +146,10 @@ impl Normalized {
     pub fn to_simple_value(&self) -> Result<SimpleValue, Expr> {
         self.0.to_simple_value().ok_or_else(|| self.to_expr())
     }
+    /// Converts a value into a SimpleType.
+    pub fn to_simple_type(&self) -> Result<SimpleType, Expr> {
+        self.0.to_simple_type().ok_or_else(|| self.to_expr())
+    }
     /// Converts a value back to the corresponding Hir expression.
     pub(crate) fn to_hir(&self) -> Hir {
         self.0.to_hir_noenv()
diff --git a/dhall/src/semantics/nze/nir.rs b/dhall/src/semantics/nze/nir.rs
index 9e3644c..4615b39 100644
--- a/dhall/src/semantics/nze/nir.rs
+++ b/dhall/src/semantics/nze/nir.rs
@@ -11,7 +11,7 @@ use crate::syntax::{
     Span,
 };
 use crate::{NormalizedExpr, ToExprOptions};
-use crate::{SValKind, SimpleValue};
+use crate::{STyKind, SValKind, SimpleType, SimpleValue};
 
 /// Stores a possibly unevaluated value. Gets (partially) normalized on-demand, sharing computation
 /// automatically. Uses a Rc<RefCell> to share computation.
@@ -176,6 +176,52 @@ impl Nir {
             _ => return None,
         }))
     }
+    pub(crate) fn to_simple_type(&self) -> Option<SimpleType> {
+        Some(SimpleType::new(match self.kind() {
+            NirKind::AppliedBuiltin(BuiltinClosure { b, args, .. })
+                if args.is_empty() =>
+            {
+                match b {
+                    Builtin::Bool => STyKind::Bool,
+                    Builtin::Natural => STyKind::Natural,
+                    Builtin::Integer => STyKind::Integer,
+                    Builtin::Double => STyKind::Double,
+                    Builtin::Text => STyKind::Text,
+                    _ => return None,
+                }
+            }
+            NirKind::AppliedBuiltin(BuiltinClosure {
+                b: Builtin::Optional,
+                args,
+                ..
+            }) if args.len() == 1 => {
+                STyKind::Optional(args[0].to_simple_type()?)
+            }
+            NirKind::AppliedBuiltin(BuiltinClosure {
+                b: Builtin::List,
+                args,
+                ..
+            }) if args.len() == 1 => STyKind::List(args[0].to_simple_type()?),
+            NirKind::RecordType(kts) => STyKind::Record(
+                kts.iter()
+                    .map(|(k, v)| Some((k.into(), v.to_simple_type()?)))
+                    .collect::<Option<_>>()?,
+            ),
+            NirKind::UnionType(kts) => STyKind::Union(
+                kts.iter()
+                    .map(|(k, v)| {
+                        Some((
+                            k.into(),
+                            v.as_ref()
+                                .map(|v| Ok(v.to_simple_type()?))
+                                .transpose()?,
+                        ))
+                    })
+                    .collect::<Option<_>>()?,
+            ),
+            _ => return None,
+        }))
+    }
 
     pub(crate) fn normalize(&self) {
         self.0.normalize()
diff --git a/dhall/src/simple.rs b/dhall/src/simple.rs
index 6457291..9d3846e 100644
--- a/dhall/src/simple.rs
+++ b/dhall/src/simple.rs
@@ -1,6 +1,7 @@
 use std::collections::BTreeMap;
 
-use crate::syntax::LitKind;
+use crate::syntax::{Builtin, LitKind};
+use crate::Normalized;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct SimpleValue {
@@ -17,6 +18,24 @@ pub enum SValKind {
     Text(String),
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SimpleType {
+    kind: Box<STyKind>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum STyKind {
+    Bool,
+    Natural,
+    Integer,
+    Double,
+    Text,
+    Optional(SimpleType),
+    List(SimpleType),
+    Record(BTreeMap<String, SimpleType>),
+    Union(BTreeMap<String, Option<SimpleType>>),
+}
+
 impl SimpleValue {
     pub(crate) fn new(kind: SValKind) -> Self {
         SimpleValue {
@@ -27,3 +46,34 @@ impl SimpleValue {
         self.kind.as_ref()
     }
 }
+
+impl SimpleType {
+    pub fn new(kind: STyKind) -> Self {
+        SimpleType {
+            kind: Box::new(kind),
+        }
+    }
+    pub fn kind(&self) -> &STyKind {
+        self.kind.as_ref()
+    }
+    pub fn into_normalized(self) -> Normalized {
+        match *self.kind {
+            STyKind::Bool => Normalized::make_builtin_type(Builtin::Bool),
+            STyKind::Natural => Normalized::make_builtin_type(Builtin::Natural),
+            STyKind::Integer => Normalized::make_builtin_type(Builtin::Integer),
+            STyKind::Double => Normalized::make_builtin_type(Builtin::Double),
+            STyKind::Text => Normalized::make_builtin_type(Builtin::Text),
+            STyKind::Optional(t) => {
+                Normalized::make_optional_type(t.into_normalized())
+            }
+            STyKind::List(t) => Normalized::make_list_type(t.into_normalized()),
+            STyKind::Record(kts) => Normalized::make_record_type(
+                kts.into_iter().map(|(k, t)| (k, t.into_normalized())),
+            ),
+            STyKind::Union(kts) => Normalized::make_union_type(
+                kts.into_iter()
+                    .map(|(k, t)| (k, t.map(|t| t.into_normalized()))),
+            ),
+        }
+    }
+}