5 files changed, 147 insertions, 61 deletions

diff --git a/dhall/src/expr.rs b/dhall/src/expr.rs
index 7baf628..5ff097b 100644
--- a/dhall/src/expr.rs
+++ b/dhall/src/expr.rs
@@ -36,6 +36,11 @@ derive_other_traits!(Typed);
 pub struct Normalized(pub(crate) SubExpr<X, X>, pub(crate) Type);
 derive_other_traits!(Normalized);
 
+/// An expression of type `Type` (like `Bool` or `Natural -> Text`, but not `Type`)
+#[derive(Debug, Clone, Eq)]
+pub struct SimpleType(pub(crate) SubExpr<X, X>);
+derive_other_traits!(SimpleType);
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Type(pub(crate) TypeInternal);
 
@@ -44,3 +49,19 @@ pub(crate) enum TypeInternal {
     Expr(Box<Normalized>),
     Untyped,
 }
+
+// Exposed for the macros
+#[doc(hidden)]
+impl From<SimpleType> for SubExpr<X, X> {
+    fn from(x: SimpleType) -> SubExpr<X, X> {
+        x.0
+    }
+}
+
+// Exposed for the macros
+#[doc(hidden)]
+impl From<SubExpr<X, X>> for SimpleType {
+    fn from(x: SubExpr<X, X>) -> SimpleType {
+        SimpleType(x)
+    }
+}
diff --git a/dhall/src/lib.rs b/dhall/src/lib.rs
index 5c5a641..3bfc46f 100644
--- a/dhall/src/lib.rs
+++ b/dhall/src/lib.rs
@@ -21,6 +21,7 @@ mod imports;
 mod normalize;
 mod traits;
 mod typecheck;
+pub use crate::traits::SimpleStaticType;
 pub use crate::traits::StaticType;
-pub use dhall_generator::StaticType;
+pub use dhall_generator::SimpleStaticType;
 pub mod expr;
diff --git a/dhall/src/traits.rs b/dhall/src/traits.rs
index 64e07d9..4925457 100644
--- a/dhall/src/traits.rs
+++ b/dhall/src/traits.rs
@@ -1,77 +1,108 @@
+use crate::expr::*;
 use dhall_core::*;
 use dhall_generator::*;
+use std::borrow::Cow;
 
 #[derive(Debug, Clone)]
 pub enum ConversionError {}
 
 pub trait StaticType {
-    fn get_type() -> DhallExpr;
-    // fn as_dhall(&self) -> DhallExpr;
-    // fn from_dhall(e: DhallExpr) -> Result<Self, DhallConversionError>;
+    fn get_static_type() -> Type;
 }
 
-impl StaticType for bool {
-    fn get_type() -> DhallExpr {
-        dhall_expr!(Bool)
+/// Trait for rust types that can be represented in dhall in
+/// a single way, independent of the value. A typical example is `Option<bool>`,
+/// represented by the dhall expression `Optional Bool`. A typical counterexample
+/// is `HashMap<Text, bool>` because dhall cannot represent records with a
+/// variable number of fields.
+pub trait SimpleStaticType {
+    fn get_simple_static_type() -> SimpleType;
+}
+
+fn mktype(x: SubExpr<X, X>) -> SimpleType {
+    SimpleType(x)
+}
+
+impl<T: SimpleStaticType> StaticType for T {
+    fn get_static_type() -> Type {
+        crate::expr::Normalized(
+            T::get_simple_static_type().into(),
+            Type::const_type(),
+        )
+        .into_type()
+    }
+}
+
+impl StaticType for SimpleType {
+    fn get_static_type() -> Type {
+        Type::const_type()
+    }
+}
+
+impl SimpleStaticType for bool {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall_expr!(Bool))
     }
 }
 
-impl StaticType for Natural {
-    fn get_type() -> DhallExpr {
-        dhall_expr!(Natural)
+impl SimpleStaticType for Natural {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall_expr!(Natural))
     }
 }
 
-impl StaticType for Integer {
-    fn get_type() -> DhallExpr {
-        dhall_expr!(Integer)
+impl SimpleStaticType for Integer {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall_expr!(Integer))
     }
 }
 
-impl StaticType for String {
-    fn get_type() -> DhallExpr {
-        dhall_expr!(Text)
+impl SimpleStaticType for String {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall_expr!(Text))
     }
 }
 
-impl<A: StaticType, B: StaticType> StaticType for (A, B) {
-    fn get_type() -> DhallExpr {
-        let ta = A::get_type();
-        let tb = B::get_type();
-        dhall_expr!({ _1: ta, _2: tb })
+impl<A: SimpleStaticType, B: SimpleStaticType> SimpleStaticType for (A, B) {
+    fn get_simple_static_type() -> SimpleType {
+        let ta: SubExpr<_, _> = A::get_simple_static_type().into();
+        let tb: SubExpr<_, _> = B::get_simple_static_type().into();
+        mktype(dhall_expr!({ _1: ta, _2: tb }))
     }
 }
 
-impl<T: StaticType> StaticType for Option<T> {
-    fn get_type() -> DhallExpr {
-        let t = T::get_type();
-        dhall_expr!(Optional t)
+impl<T: SimpleStaticType> SimpleStaticType for Option<T> {
+    fn get_simple_static_type() -> SimpleType {
+        let t: SubExpr<_, _> = T::get_simple_static_type().into();
+        mktype(dhall_expr!(Optional t))
     }
 }
 
-impl<T: StaticType> StaticType for Vec<T> {
-    fn get_type() -> DhallExpr {
-        let t = T::get_type();
-        dhall_expr!(List t)
+impl<T: SimpleStaticType> SimpleStaticType for Vec<T> {
+    fn get_simple_static_type() -> SimpleType {
+        let t: SubExpr<_, _> = T::get_simple_static_type().into();
+        mktype(dhall_expr!(List t))
     }
 }
 
-impl<'a, T: StaticType> StaticType for &'a T {
-    fn get_type() -> DhallExpr {
-        T::get_type()
+impl<'a, T: SimpleStaticType> SimpleStaticType for &'a T {
+    fn get_simple_static_type() -> SimpleType {
+        T::get_simple_static_type()
     }
 }
 
-impl<T> StaticType for std::marker::PhantomData<T> {
-    fn get_type() -> DhallExpr {
-        dhall_expr!({})
+impl<T> SimpleStaticType for std::marker::PhantomData<T> {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall_expr!({}))
     }
 }
 
-impl<T: StaticType, E: StaticType> StaticType for Result<T, E> {
-    fn get_type() -> DhallExpr {
-        let tt = T::get_type();
-        let te = E::get_type();
-        dhall_expr!(< Ok: tt | Err: te>)
+impl<T: SimpleStaticType, E: SimpleStaticType> SimpleStaticType
+    for std::result::Result<T, E>
+{
+    fn get_simple_static_type() -> SimpleType {
+        let tt: SubExpr<_, _> = T::get_simple_static_type().into();
+        let te: SubExpr<_, _> = E::get_simple_static_type().into();
+        mktype(dhall_expr!(< Ok: tt | Err: te>))
     }
 }
diff --git a/dhall/src/typecheck.rs b/dhall/src/typecheck.rs
index 998d3ca..f51f1b6 100644
--- a/dhall/src/typecheck.rs
+++ b/dhall/src/typecheck.rs
@@ -42,7 +42,7 @@ impl Normalized {
         &self.0
     }
     #[inline(always)]
-    fn into_expr(self) -> SubExpr<X, X> {
+    pub(crate) fn into_expr(self) -> SubExpr<X, X> {
         self.0
     }
     #[inline(always)]
@@ -50,7 +50,7 @@ impl Normalized {
         &self.1
     }
     #[inline(always)]
-    fn into_type(self) -> Type {
+    pub(crate) fn into_type(self) -> Type {
         crate::expr::Type(TypeInternal::Expr(Box::new(self)))
     }
     // Expose the outermost constructor
@@ -78,7 +78,7 @@ impl Type {
         }
     }
     #[inline(always)]
-    fn into_normalized(self) -> Result<Normalized, TypeError<X>> {
+    pub(crate) fn into_normalized(self) -> Result<Normalized, TypeError<X>> {
         use TypeInternal::*;
         match self.0 {
             Expr(e) => Ok(*e),
@@ -110,6 +110,21 @@ impl Type {
             Untyped => Untyped,
         })
     }
+
+    #[inline(always)]
+    pub fn const_sort() -> Self {
+        Normalized(rc(ExprF::Const(Const::Sort)), UNTYPE).into_type()
+    }
+    #[inline(always)]
+    pub fn const_kind() -> Self {
+        Normalized(rc(ExprF::Const(Const::Kind)), Type::const_sort())
+            .into_type()
+    }
+    #[inline(always)]
+    pub fn const_type() -> Self {
+        Normalized(rc(ExprF::Const(Const::Type)), Type::const_kind())
+            .into_type()
+    }
 }
 
 const UNTYPE: Type = Type(TypeInternal::Untyped);
diff --git a/dhall/tests/traits.rs b/dhall/tests/traits.rs
index ac6b5e6..1e9b3c2 100644
--- a/dhall/tests/traits.rs
+++ b/dhall/tests/traits.rs
@@ -1,53 +1,71 @@
 #![feature(proc_macro_hygiene)]
-use dhall::StaticType;
+use dhall::SimpleStaticType;
+use dhall_core::{SubExpr, X};
 use dhall_generator::dhall_expr;
 
 #[test]
-fn test_dhall_type() {
-    assert_eq!(bool::get_type(), dhall_expr!(Bool));
-    assert_eq!(String::get_type(), dhall_expr!(Text));
+fn test_static_type() {
+    fn mktype(x: SubExpr<X, X>) -> dhall::expr::SimpleType {
+        x.into()
+    }
+
+    assert_eq!(bool::get_simple_static_type(), mktype(dhall_expr!(Bool)));
+    assert_eq!(String::get_simple_static_type(), mktype(dhall_expr!(Text)));
     assert_eq!(
-        <(bool, Option<String>)>::get_type(),
-        dhall_expr!({ _1: Bool, _2: Optional Text })
+        <Option<bool>>::get_simple_static_type(),
+        mktype(dhall_expr!(Optional Bool))
+    );
+    assert_eq!(
+        <(bool, Option<String>)>::get_simple_static_type(),
+        mktype(dhall_expr!({ _1: Bool, _2: Optional Text }))
     );
 
-    #[derive(dhall::StaticType)]
+    #[derive(dhall::SimpleStaticType)]
     #[allow(dead_code)]
     struct A {
         field1: bool,
         field2: Option<bool>,
     }
     assert_eq!(
-        <A as dhall::StaticType>::get_type(),
-        dhall_expr!({ field1: Bool, field2: Optional Bool })
+        <A as dhall::SimpleStaticType>::get_simple_static_type(),
+        mktype(dhall_expr!({ field1: Bool, field2: Optional Bool }))
     );
 
-    #[derive(StaticType)]
+    #[derive(SimpleStaticType)]
     #[allow(dead_code)]
     struct B<'a, T: 'a> {
         field1: &'a T,
         field2: Option<T>,
     }
-    assert_eq!(<B<'static, bool>>::get_type(), A::get_type());
+    assert_eq!(
+        <B<'static, bool>>::get_simple_static_type(),
+        A::get_simple_static_type()
+    );
 
-    #[derive(StaticType)]
+    #[derive(SimpleStaticType)]
     #[allow(dead_code)]
     struct C<T>(T, Option<String>);
-    assert_eq!(<C<bool>>::get_type(), <(bool, Option<String>)>::get_type());
+    assert_eq!(
+        <C<bool>>::get_simple_static_type(),
+        <(bool, Option<String>)>::get_simple_static_type()
+    );
 
-    #[derive(StaticType)]
+    #[derive(SimpleStaticType)]
     #[allow(dead_code)]
     struct D();
     assert_eq!(
-        <C<D>>::get_type(),
-        dhall_expr!({ _1: {}, _2: Optional Text })
+        <C<D>>::get_simple_static_type(),
+        mktype(dhall_expr!({ _1: {}, _2: Optional Text }))
     );
 
-    #[derive(StaticType)]
+    #[derive(SimpleStaticType)]
     #[allow(dead_code)]
     enum E<T> {
         A(T),
         B(String),
     };
-    assert_eq!(<E<bool>>::get_type(), dhall_expr!(< A: Bool | B: Text >));
+    assert_eq!(
+        <E<bool>>::get_simple_static_type(),
+        mktype(dhall_expr!(< A: Bool | B: Text >))
+    );
 }