1 files changed, 149 insertions, 0 deletions
diff --git a/dhall/src/api/traits/static_type.rs b/dhall/src/api/traits/static_type.rs
new file mode 100644
index 0000000..8b141a0
--- /dev/null
+++ b/dhall/src/api/traits/static_type.rs
@@ -0,0 +1,149 @@
+use crate::phase::*;
+use dhall_proc_macros as dhall;
+use dhall_syntax::*;
+
+/// A value that has a statically-known Dhall type.
+///
+/// This trait is strictly more general than [SimpleStaticType].
+/// The reason is that it allows an arbitrary [Type] to be returned
+/// instead of just a [SimpleType].
+///
+/// For now the only interesting impl is [SimpleType] itself, who
+/// has a statically-known type which is not itself a [SimpleType].
+pub trait StaticType {
+    fn get_static_type() -> Type;
+}
+
+/// A Rust type that can be represented as a Dhall type.
+///
+/// A typical example is `Option<bool>`,
+/// represented by the dhall expression `Optional Bool`.
+///
+/// This trait can and should be automatically derived.
+///
+/// The representation needs to be independent of the value.
+/// For this reason, something like `HashMap<String, bool>` cannot implement
+/// [SimpleStaticType] because each different value would
+/// have a different Dhall record type.
+///
+/// The `Simple` in `SimpleStaticType` indicates that the returned type is
+/// a [SimpleType].
+pub trait SimpleStaticType {
+    fn get_simple_static_type() -> SimpleType;
+}
+
+fn mktype(x: SubExpr<X, X>) -> SimpleType {
+    x.into()
+}
+
+impl<T: SimpleStaticType> StaticType for T {
+    fn get_static_type() -> Type {
+        crate::phase::Normalized::from_thunk_and_type(
+            crate::phase::normalize::Thunk::from_normalized_expr(
+                T::get_simple_static_type().into(),
+            ),
+            Type::const_type(),
+        )
+        .to_type()
+    }
+}
+
+impl StaticType for SimpleType {
+    /// By definition, a [SimpleType] has type `Type`.
+    /// This returns the Dhall expression `Type`
+    fn get_static_type() -> Type {
+        Type::const_type()
+    }
+}
+
+impl SimpleStaticType for bool {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Bool))
+    }
+}
+
+impl SimpleStaticType for Natural {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Natural))
+    }
+}
+
+impl SimpleStaticType for u32 {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Natural))
+    }
+}
+
+impl SimpleStaticType for u64 {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Natural))
+    }
+}
+
+impl SimpleStaticType for Integer {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Integer))
+    }
+}
+
+impl SimpleStaticType for i32 {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Integer))
+    }
+}
+
+impl SimpleStaticType for i64 {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Integer))
+    }
+}
+
+impl SimpleStaticType for String {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!(Text))
+    }
+}
+
+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::subexpr!({ _1: ta, _2: tb }))
+    }
+}
+
+impl<T: SimpleStaticType> SimpleStaticType for Option<T> {
+    fn get_simple_static_type() -> SimpleType {
+        let t: SubExpr<_, _> = T::get_simple_static_type().into();
+        mktype(dhall::subexpr!(Optional 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::subexpr!(List t))
+    }
+}
+
+impl<'a, T: SimpleStaticType> SimpleStaticType for &'a T {
+    fn get_simple_static_type() -> SimpleType {
+        T::get_simple_static_type()
+    }
+}
+
+impl<T> SimpleStaticType for std::marker::PhantomData<T> {
+    fn get_simple_static_type() -> SimpleType {
+        mktype(dhall::subexpr!({}))
+    }
+}
+
+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::subexpr!(< Ok: tt | Err: te>))
+    }
+}