Merge pull request #154 from Nadrieril/cleanup-api

Rewrite serde_dhall API

7 files changed, 1011 insertions, 406 deletions

diff --git a/serde_dhall/src/deserialize.rs b/serde_dhall/src/deserialize.rs
new file mode 100644
index 0000000..92be2e9
--- /dev/null
+++ b/serde_dhall/src/deserialize.rs
@@ -0,0 +1,143 @@
+use serde::de::value::{
+    MapAccessDeserializer, MapDeserializer, SeqDeserializer,
+};
+use std::borrow::Cow;
+
+use dhall::syntax::NumKind;
+
+use crate::value::SimpleValue;
+use crate::{Error, ErrorKind, Result, Value};
+
+pub trait Sealed {}
+
+/// A data structure that can be deserialized from a Dhall expression.
+///
+/// This is automatically implemented for any type that [serde] can deserialize.
+/// In fact, this trait cannot be implemented manually. To implement it for your type,
+/// use serde's derive mechanism.
+///
+/// # Example
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde::Deserialize;
+///
+/// // Use serde's derive
+/// #[derive(Deserialize)]
+/// struct Point {
+///     x: u64,
+///     y: u64,
+/// }
+///
+/// // Convert a Dhall string to a Point.
+/// let point: Point = serde_dhall::from_str("{ x = 1, y = 1 + 1 }").parse()?;
+/// # Ok(())
+/// # }
+/// ```
+///
+/// [serde]: https://serde.rs
+pub trait FromDhall: Sealed + Sized {
+    #[doc(hidden)]
+    fn from_dhall(v: &Value) -> Result<Self>;
+}
+
+impl<T> Sealed for T where T: serde::de::DeserializeOwned {}
+
+struct Deserializer<'a>(Cow<'a, SimpleValue>);
+
+impl<T> FromDhall for T
+where
+    T: serde::de::DeserializeOwned,
+{
+    fn from_dhall(v: &Value) -> Result<Self> {
+        let sval = v.to_simple_value().ok_or_else(|| {
+            Error(ErrorKind::Deserialize(format!(
+                "this cannot be deserialized into the serde data model: {}",
+                v
+            )))
+        })?;
+        T::deserialize(Deserializer(Cow::Owned(sval)))
+    }
+}
+
+impl<'de: 'a, 'a> serde::de::IntoDeserializer<'de, Error> for Deserializer<'a> {
+    type Deserializer = Deserializer<'a>;
+    fn into_deserializer(self) -> Self::Deserializer {
+        self
+    }
+}
+
+impl<'de: 'a, 'a> serde::Deserializer<'de> for Deserializer<'a> {
+    type Error = Error;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        use std::convert::TryInto;
+        use NumKind::*;
+        use SimpleValue::*;
+
+        let val = |x| Deserializer(Cow::Borrowed(x));
+        match self.0.as_ref() {
+            Num(Bool(x)) => visitor.visit_bool(*x),
+            Num(Natural(x)) => {
+                if let Ok(x64) = (*x).try_into() {
+                    visitor.visit_u64(x64)
+                } else if let Ok(x32) = (*x).try_into() {
+                    visitor.visit_u32(x32)
+                } else {
+                    unimplemented!()
+                }
+            }
+            Num(Integer(x)) => {
+                if let Ok(x64) = (*x).try_into() {
+                    visitor.visit_i64(x64)
+                } else if let Ok(x32) = (*x).try_into() {
+                    visitor.visit_i32(x32)
+                } else {
+                    unimplemented!()
+                }
+            }
+            Num(Double(x)) => visitor.visit_f64((*x).into()),
+            Text(x) => visitor.visit_str(x),
+            List(xs) => {
+                visitor.visit_seq(SeqDeserializer::new(xs.iter().map(val)))
+            }
+            Optional(None) => visitor.visit_none(),
+            Optional(Some(x)) => visitor.visit_some(val(x)),
+            Record(m) => visitor.visit_map(MapDeserializer::new(
+                m.iter().map(|(k, v)| (k.as_ref(), val(v))),
+            )),
+            Union(field_name, Some(x)) => visitor.visit_enum(
+                MapAccessDeserializer::new(MapDeserializer::new(
+                    Some((field_name.as_str(), val(x))).into_iter(),
+                )),
+            ),
+            Union(field_name, None) => visitor.visit_enum(
+                MapAccessDeserializer::new(MapDeserializer::new(
+                    Some((field_name.as_str(), ())).into_iter(),
+                )),
+            ),
+        }
+    }
+
+    fn deserialize_tuple<V>(self, _: usize, visitor: V) -> Result<V::Value>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        let val = |x| Deserializer(Cow::Borrowed(x));
+        match self.0.as_ref() {
+            // Blindly takes keys in sorted order.
+            SimpleValue::Record(m) => visitor
+                .visit_seq(SeqDeserializer::new(m.iter().map(|(_, v)| val(v)))),
+            _ => self.deserialize_any(visitor),
+        }
+    }
+
+    serde::forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
diff --git a/serde_dhall/src/error.rs b/serde_dhall/src/error.rs
new file mode 100644
index 0000000..896e8b9
--- /dev/null
+++ b/serde_dhall/src/error.rs
@@ -0,0 +1,40 @@
+use dhall::error::Error as DhallError;
+
+/// Alias for a `Result` with the error type `serde_dhall::Error`.
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Errors that can occur when deserializing Dhall data.
+#[derive(Debug)]
+pub struct Error(pub(crate) ErrorKind);
+
+#[derive(Debug)]
+pub(crate) enum ErrorKind {
+    Dhall(DhallError),
+    Deserialize(String),
+}
+
+impl From<ErrorKind> for Error {
+    fn from(kind: ErrorKind) -> Error {
+        Error(kind)
+    }
+}
+
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match &self.0 {
+            ErrorKind::Dhall(err) => write!(f, "{}", err),
+            ErrorKind::Deserialize(err) => write!(f, "{}", err),
+        }
+    }
+}
+
+impl std::error::Error for Error {}
+
+impl serde::de::Error for Error {
+    fn custom<T>(msg: T) -> Self
+    where
+        T: std::fmt::Display,
+    {
+        ErrorKind::Deserialize(msg.to_string()).into()
+    }
+}
diff --git a/serde_dhall/src/lib.rs b/serde_dhall/src/lib.rs
index 0a53420..c478b2a 100644
--- a/serde_dhall/src/lib.rs
+++ b/serde_dhall/src/lib.rs
@@ -1,4 +1,5 @@
 #![doc(html_root_url = "https://docs.rs/serde_dhall/0.4.0")]
+#![warn(missing_docs, missing_doc_code_examples)]
 //! [Dhall][dhall] is a programmable configuration language that provides a non-repetitive
 //! alternative to JSON and YAML.
 //!
@@ -16,20 +17,20 @@
 //!
 //! # Basic usage
 //!
-//! The main entrypoint of this library is the [`from_str`][from_str] function. It reads a string
+//! The main entrypoint of this library is the [`from_str`](fn.from_str.html) function. It reads a string
 //! containing a Dhall expression and deserializes it into any serde-compatible type.
 //!
 //! This could mean a common Rust type like `HashMap`:
 //!
 //! ```rust
-//! # fn main() -> serde_dhall::de::Result<()> {
+//! # fn main() -> serde_dhall::Result<()> {
 //! use std::collections::HashMap;
 //!
 //! // Some Dhall data
 //! let data = "{ x = 1, y = 1 + 1 } : { x: Natural, y: Natural }";
 //!
 //! // Deserialize it to a Rust type.
-//! let deserialized_map: HashMap<String, usize> = serde_dhall::from_str(data)?;
+//! let deserialized_map: HashMap<String, usize> = serde_dhall::from_str(data).parse()?;
 //!
 //! let mut expected_map = HashMap::new();
 //! expected_map.insert("x".to_string(), 1);
@@ -43,10 +44,10 @@
 //! or a custom datatype, using serde's `derive` mechanism:
 //!
 //! ```rust
-//! # fn main() -> serde_dhall::de::Result<()> {
+//! # fn main() -> serde_dhall::Result<()> {
 //! use serde::Deserialize;
 //!
-//! #[derive(Debug, Deserialize)]
+//! #[derive(Deserialize)]
 //! struct Point {
 //!     x: u64,
 //!     y: u64,
@@ -56,7 +57,7 @@
 //! let data = "{ x = 1, y = 1 + 1 } : { x: Natural, y: Natural }";
 //!
 //! // Convert the Dhall string to a Point.
-//! let point: Point = serde_dhall::from_str(data)?;
+//! let point: Point = serde_dhall::from_str(data).parse()?;
 //! assert_eq!(point.x, 1);
 //! assert_eq!(point.y, 2);
 //!
@@ -64,67 +65,82 @@
 //! # }
 //! ```
 //!
-//! # Type correspondence
-//!
-//! The following Dhall types correspond to the following Rust types:
-//!
-//! Dhall  | Rust
-//! -------|------
-//! `Bool`  | `bool`
-//! `Natural`  | `u64`, `u32`, ...
-//! `Integer`  | `i64`, `i32`, ...
-//! `Double`  | `f64`, `f32`, ...
-//! `Text`  | `String`
-//! `List T`  | `Vec<T>`
-//! `Optional T`  | `Option<T>`
-//! `{ x: T, y: U }`  | structs
-//! `{ _1: T, _2: U }`  | `(T, U)`, structs
-//! `{ x: T, y: T }`  | `HashMap<String, T>`, structs
-//! `< x: T \| y: U >`  | enums
-//! `T -> U`  | unsupported
-//! `Prelude.JSON.Type`  | unsupported
-//! `Prelude.Map.Type T U`  | unsupported
-//!
-//!
 //! # Replacing `serde_json` or `serde_yaml`
 //!
-//! If you used to consume JSON or YAML, you only need to replace [serde_json::from_str] or
-//! [serde_yaml::from_str] with [serde_dhall::from_str][from_str].
+//! If you used to consume JSON or YAML, you only need to replace [`serde_json::from_str`] or
+//! [`serde_yaml::from_str`] with [`serde_dhall::from_str(…).parse()`](fn.from_str.html).
 //!
-//! [serde_json::from_str]: https://docs.serde.rs/serde_json/de/fn.from_str.html
-//! [serde_yaml::from_str]: https://docs.serde.rs/serde_yaml/fn.from_str.html
+//! [`serde_json::from_str`]: https://docs.serde.rs/serde_json/fn.from_str.html
+//! [`serde_yaml::from_str`]: https://docs.serde.rs/serde_yaml/fn.from_str.html
 //!
 //!
 //! # Additional Dhall typechecking
 //!
 //! When deserializing, normal type checking is done to ensure that the returned value is a valid
-//! Dhall value, and that it can be deserialized into the required Rust type. However types are
-//! first-class in Dhall, and this library allows you to additionally check that some input data
+//! Dhall value. However types are
+//! first-class in Dhall, and this library allows you to additionally check that the input data
 //! matches a given Dhall type. That way, a type error will be caught on the Dhall side, and have
 //! pretty and explicit errors that point to the source file.
 //!
-//! There are two ways to typecheck a Dhall value: you can provide the type as Dhall text or you
-//! can let Rust infer it for you.
+//! There are two ways to typecheck a Dhall value in this way: you can provide the type manually or
+//! you can let Rust infer it for you.
 //!
-//! To provide a type written in Dhall, first parse it into a [`serde_dhall::Value`][Value], then
-//! pass it to [`from_str_check_type`][from_str_check_type].
+//! To let Rust infer the appropriate Dhall type, use the [StaticType](trait.StaticType.html)
+//! trait.
 //!
 //! ```rust
-//! # fn main() -> serde_dhall::de::Result<()> {
-//! use serde_dhall::Value;
+//! # fn main() -> serde_dhall::Result<()> {
+//! use serde::Deserialize;
+//! use serde_dhall::StaticType;
+//!
+//! #[derive(Deserialize, StaticType)]
+//! struct Point {
+//!     x: u64,
+//!     y: u64,
+//! }
+//!
+//! // Some Dhall data
+//! let data = "{ x = 1, y = 1 + 1 }";
+//!
+//! // Convert the Dhall string to a Point.
+//! let point = serde_dhall::from_str(data)
+//!     .static_type_annotation()
+//!     .parse::<Point>()?;
+//! assert_eq!(point.x, 1);
+//! assert_eq!(point.y, 2);
+//!
+//! // Invalid data fails the type validation
+//! let invalid_data = "{ x = 1, z = 0.3 }";
+//! assert!(
+//!     serde_dhall::from_str(invalid_data)
+//!         .static_type_annotation()
+//!         .parse::<Point>()
+//!         .is_err()
+//! );
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! To provide a type manually, you need a [`SimpleType`](enum.SimpleType.html) value. You
+//! can parse it from some Dhall text like you would parse any other value.
+//!
+//! ```rust
+//! # fn main() -> serde_dhall::Result<()> {
+//! use serde_dhall::SimpleType;
 //! use std::collections::HashMap;
 //!
 //! // Parse a Dhall type
 //! let point_type_str = "{ x: Natural, y: Natural }";
-//! let point_type: Value = serde_dhall::from_str(point_type_str)?;
+//! let point_type = serde_dhall::from_str(point_type_str).parse::<SimpleType>()?;
 //!
 //! // Some Dhall data
 //! let point_data = "{ x = 1, y = 1 + 1 }";
 //!
 //! // Deserialize the data to a Rust type. This checks that
 //! // the data matches the provided type.
-//! let deserialized_map: HashMap<String, usize> =
-//!         serde_dhall::from_str_check_type(point_data, &point_type)?;
+//! let deserialized_map = serde_dhall::from_str(point_data)
+//!     .type_annotation(&point_type)
+//!     .parse::<HashMap<String, usize>>()?;
 //!
 //! let mut expected_map = HashMap::new();
 //! expected_map.insert("x".to_string(), 1);
@@ -135,207 +151,35 @@
 //! # }
 //! ```
 //!
-//! You can also let Rust infer the appropriate Dhall type, using the [StaticType] trait.
-//!
-//! ```rust
-//! # fn main() -> serde_dhall::de::Result<()> {
-//! use serde::Deserialize;
-//! use serde_dhall::StaticType;
+//! # Controlling deserialization
 //!
-//! #[derive(Debug, Deserialize, StaticType)]
-//! struct Point {
-//!     x: u64,
-//!     y: u64,
-//! }
-//!
-//! // Some Dhall data
-//! let data = "{ x = 1, y = 1 + 1 }";
-//!
-//! // Convert the Dhall string to a Point.
-//! let point: Point = serde_dhall::from_str_auto_type(data)?;
-//! assert_eq!(point.x, 1);
-//! assert_eq!(point.y, 2);
-//!
-//! // Invalid data fails the type validation
-//! let invalid_data = "{ x = 1, z = 0.3 }";
-//! assert!(serde_dhall::from_str_auto_type::<Point>(invalid_data).is_err());
-//! # Ok(())
-//! # }
-//! ```
+//! If you need more control over the process of reading Dhall values, e.g. disabling
+//! imports, see the [`Deserializer`] methods.
 //!
+//! [`Deserializer`]: struct.Deserializer.html
 //! [dhall]: https://dhall-lang.org/
 //! [serde]: https://docs.serde.rs/serde/
 //! [serde::Deserialize]: https://docs.serde.rs/serde/trait.Deserialize.html
 
-mod serde;
+#[cfg(doctest)]
+mod test_readme {
+    doc_comment::doctest!("../../README.md");
+}
+
+mod deserialize;
+mod error;
+mod options;
 mod static_type;
+/// Dhall values
+mod value;
 
-#[doc(inline)]
-pub use de::{from_str, from_str_auto_type, from_str_check_type};
 #[doc(hidden)]
 pub use dhall_proc_macros::StaticType;
-pub use static_type::StaticType;
-#[doc(inline)]
-pub use value::Value;
-
-// A Dhall value.
-#[doc(hidden)]
-pub mod value {
-    use dhall::syntax::Builtin;
-    use dhall::{Normalized, NormalizedExpr, Parsed};
-
-    use super::de::{Error, Result};
-
-    /// A Dhall value
-    #[derive(Debug, Clone, PartialEq, Eq)]
-    pub struct Value(Normalized);
 
-    impl Value {
-        pub fn from_str(s: &str, ty: Option<&Value>) -> Result<Self> {
-            Value::from_str_using_dhall_error_type(s, ty).map_err(Error::Dhall)
-        }
-        fn from_str_using_dhall_error_type(
-            s: &str,
-            ty: Option<&Value>,
-        ) -> dhall::error::Result<Self> {
-            let resolved = Parsed::parse_str(s)?.resolve()?;
-            let typed = match ty {
-                None => resolved.typecheck()?,
-                Some(t) => resolved.typecheck_with(t.as_normalized())?,
-            };
-            Ok(Value(typed.normalize()))
-        }
-        pub(crate) fn to_expr(&self) -> NormalizedExpr {
-            self.0.to_expr()
-        }
-        pub(crate) fn as_normalized(&self) -> &Normalized {
-            &self.0
-        }
-
-        pub(crate) fn make_builtin_type(b: Builtin) -> Self {
-            Value(Normalized::make_builtin_type(b))
-        }
-        pub(crate) fn make_optional_type(t: Value) -> Self {
-            Value(Normalized::make_optional_type(t.0))
-        }
-        pub(crate) fn make_list_type(t: Value) -> Self {
-            Value(Normalized::make_list_type(t.0))
-        }
-        // Made public for the StaticType derive macro
-        #[doc(hidden)]
-        pub fn make_record_type(
-            kts: impl Iterator<Item = (String, Value)>,
-        ) -> Self {
-            Value(Normalized::make_record_type(kts.map(|(k, t)| (k, t.0))))
-        }
-        #[doc(hidden)]
-        pub fn make_union_type(
-            kts: impl Iterator<Item = (String, Option<Value>)>,
-        ) -> Self {
-            Value(Normalized::make_union_type(
-                kts.map(|(k, t)| (k, t.map(|t| t.0))),
-            ))
-        }
-    }
-
-    impl super::de::sealed::Sealed for Value {}
-
-    impl super::de::Deserialize for Value {
-        fn from_dhall(v: &Value) -> Result<Self> {
-            Ok(v.clone())
-        }
-    }
-}
-
-/// Deserialize Dhall data to a Rust data structure.
-pub mod de {
-    use super::StaticType;
-    use super::Value;
-    pub use error::{Error, Result};
-
-    mod error {
-        use dhall::error::Error as DhallError;
-
-        pub type Result<T> = std::result::Result<T, Error>;
-
-        #[derive(Debug)]
-        #[non_exhaustive]
-        pub enum Error {
-            Dhall(DhallError),
-            Deserialize(String),
-        }
-
-        impl std::fmt::Display for Error {
-            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-                match self {
-                    Error::Dhall(err) => write!(f, "{}", err),
-                    Error::Deserialize(err) => write!(f, "{}", err),
-                }
-            }
-        }
-
-        impl std::error::Error for Error {}
-
-        impl serde::de::Error for Error {
-            fn custom<T>(msg: T) -> Self
-            where
-                T: std::fmt::Display,
-            {
-                Error::Deserialize(msg.to_string())
-            }
-        }
-    }
-
-    pub(crate) mod sealed {
-        pub trait Sealed {}
-    }
-
-    /// A data structure that can be deserialized from a Dhall expression
-    ///
-    /// This is automatically implemented for any type that [serde][serde]
-    /// can deserialize.
-    ///
-    /// This trait cannot be implemented manually.
-    pub trait Deserialize: sealed::Sealed + Sized {
-        /// See [serde_dhall::from_str][crate::from_str]
-        fn from_dhall(v: &Value) -> Result<Self>;
-    }
-
-    /// Deserialize an instance of type `T` from a string of Dhall text.
-    ///
-    /// This will recursively resolve all imports in the expression, and
-    /// typecheck it before deserialization. Relative imports will be resolved relative to the
-    /// provided file. More control over this process is not yet available
-    /// but will be in a coming version of this crate.
-    pub fn from_str<T>(s: &str) -> Result<T>
-    where
-        T: Deserialize,
-    {
-        T::from_dhall(&Value::from_str(s, None)?)
-    }
-
-    /// Deserialize an instance of type `T` from a string of Dhall text,
-    /// additionally checking that it matches the supplied type.
-    ///
-    /// Like [from_str], but this additionally checks that
-    /// the type of the provided expression matches the supplied type.
-    pub fn from_str_check_type<T>(s: &str, ty: &Value) -> Result<T>
-    where
-        T: Deserialize,
-    {
-        T::from_dhall(&Value::from_str(s, Some(ty))?)
-    }
-
-    /// Deserialize an instance of type `T` from a string of Dhall text,
-    /// additionally checking that it matches the type of `T`.
-    ///
-    /// Like [from_str], but this additionally checks that
-    /// the type of the provided expression matches the output type `T`. The [StaticType] trait
-    /// captures Rust types that are valid Dhall types.
-    pub fn from_str_auto_type<T>(s: &str) -> Result<T>
-    where
-        T: Deserialize + StaticType,
-    {
-        from_str_check_type(s, &<T as StaticType>::static_type())
-    }
-}
+pub use deserialize::FromDhall;
+pub(crate) use deserialize::Sealed;
+pub(crate) use error::ErrorKind;
+pub use error::{Error, Result};
+pub use options::{from_file, from_str, Deserializer};
+pub use static_type::StaticType;
+pub use value::{SimpleType, Value};
diff --git a/serde_dhall/src/options.rs b/serde_dhall/src/options.rs
new file mode 100644
index 0000000..06a4368
--- /dev/null
+++ b/serde_dhall/src/options.rs
@@ -0,0 +1,368 @@
+use std::path::{Path, PathBuf};
+
+use dhall::Parsed;
+
+use crate::SimpleType;
+use crate::{Error, ErrorKind, FromDhall, Result, StaticType, Value};
+
+#[derive(Debug, Clone)]
+enum Source<'a> {
+    Str(&'a str),
+    File(PathBuf),
+    // Url(&'a str),
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct NoAnnot;
+#[derive(Debug, Clone, Copy)]
+pub struct ManualAnnot<'ty>(&'ty SimpleType);
+#[derive(Debug, Clone, Copy)]
+pub struct StaticAnnot;
+
+pub trait HasAnnot<A> {
+    fn get_annot(a: &A) -> Option<SimpleType>;
+}
+impl<T> HasAnnot<NoAnnot> for T {
+    fn get_annot(_: &NoAnnot) -> Option<SimpleType> {
+        None
+    }
+}
+impl<'ty, T> HasAnnot<ManualAnnot<'ty>> for T {
+    fn get_annot(a: &ManualAnnot<'ty>) -> Option<SimpleType> {
+        Some(a.0.clone())
+    }
+}
+impl<T: StaticType> HasAnnot<StaticAnnot> for T {
+    fn get_annot(_: &StaticAnnot) -> Option<SimpleType> {
+        Some(T::static_type())
+    }
+}
+
+/// Controls how a Dhall value is read.
+///
+/// This builder exposes the ability to configure how a value is deserialized and what operations
+/// are permitted during evaluation.
+///
+/// Generally speaking, when using [`Deserializer`], you'll create it with [`from_str`] or [`from_file`], then
+/// chain calls to methods to set each option, then call [`parse`]. This will give you a
+/// [`Result<T>`] where `T` is a deserializable type of your choice.
+///
+/// [`Deserializer`]: struct.Deserializer.html
+/// [`from_str`]: fn.from_str.html
+/// [`from_file`]: fn.from_file.html
+/// [`parse`]: struct.Deserializer.html#method.parse
+/// [`Result<T>`]: type.Result.html
+///
+/// # Examples
+///
+/// Reading from a file:
+///
+/// ```no_run
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde_dhall::from_file;
+///
+/// let data = from_file("foo.dhall").parse::<u64>()?;
+/// # Ok(())
+/// # }
+/// ```
+///
+/// Reading from a file and checking the value against a provided type:
+///
+/// ```no_run
+/// # fn main() -> serde_dhall::Result<()> {
+/// use std::collections::HashMap;
+/// use serde_dhall::{from_file, from_str};
+///
+/// let ty = from_str("{ x: Natural, y: Natural }").parse()?;
+/// let data = from_file("foo.dhall")
+///             .type_annotation(&ty)
+///             .parse::<HashMap<String, usize>>()?;
+/// # Ok(())
+/// # }
+/// ```
+#[derive(Debug, Clone)]
+pub struct Deserializer<'a, A> {
+    source: Source<'a>,
+    annot: A,
+    allow_imports: bool,
+    // allow_remote_imports: bool,
+    // use_cache: bool,
+}
+
+impl<'a> Deserializer<'a, NoAnnot> {
+    fn default_with_source(source: Source<'a>) -> Self {
+        Deserializer {
+            source,
+            annot: NoAnnot,
+            allow_imports: true,
+            // allow_remote_imports: true,
+            // use_cache: true,
+        }
+    }
+    fn from_str(s: &'a str) -> Self {
+        Self::default_with_source(Source::Str(s))
+    }
+    fn from_file<P: AsRef<Path>>(path: P) -> Self {
+        Self::default_with_source(Source::File(path.as_ref().to_owned()))
+    }
+    // fn from_url(url: &'a str) -> Self {
+    //     Self::default_with_source(Source::Url(url))
+    // }
+
+    /// Ensures that the parsed value matches the provided type.
+    ///
+    /// In many cases the Dhall type that corresponds to a Rust type can be inferred automatically.
+    /// See the [`StaticType`] trait and the [`static_type_annotation`] method for that.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// use std::collections::HashMap;
+    /// use serde::Deserialize;
+    /// use serde_dhall::{from_str, SimpleType};
+    ///
+    /// // Parse a Dhall type
+    /// let type_str = "{ x: Natural, y: Natural }";
+    /// let ty = from_str(type_str).parse::<SimpleType>()?;
+    ///
+    /// // Parse some Dhall data.
+    /// let data = "{ x = 1, y = 1 + 1 }";
+    /// let point = from_str(data)
+    ///     .type_annotation(&ty)
+    ///     .parse::<HashMap<String, usize>>()?;
+    /// assert_eq!(point.get("y"), Some(&2));
+    ///
+    /// // Invalid data fails the type validation; deserialization would have succeeded otherwise.
+    /// let invalid_data = "{ x = 1, z = 3 }";
+    /// assert!(
+    ///     from_str(invalid_data)
+    ///         .type_annotation(&ty)
+    ///         .parse::<HashMap<String, usize>>()
+    ///         .is_err()
+    /// );
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// [`static_type_annotation`]: struct.Deserializer.html#method.static_type_annotation
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn type_annotation<'ty>(
+        self,
+        ty: &'ty SimpleType,
+    ) -> Deserializer<'a, ManualAnnot<'ty>> {
+        Deserializer {
+            annot: ManualAnnot(ty),
+            source: self.source,
+            allow_imports: self.allow_imports,
+        }
+    }
+
+    /// Ensures that the parsed value matches the type of `T`.
+    ///
+    /// `T` must implement the [`StaticType`] trait. If it doesn't, you can use [`type_annotation`]
+    /// to provide a type manually.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// use serde::Deserialize;
+    /// use serde_dhall::StaticType;
+    ///
+    /// #[derive(Deserialize, StaticType)]
+    /// struct Point {
+    ///     x: u64,
+    ///     y: Option<u64>,
+    /// }
+    ///
+    /// // Some Dhall data
+    /// let data = "{ x = 1, y = Some (1 + 1) }";
+    ///
+    /// // Convert the Dhall string to a Point.
+    /// let point = serde_dhall::from_str(data)
+    ///     .static_type_annotation()
+    ///     .parse::<Point>()?;
+    /// assert_eq!(point.x, 1);
+    /// assert_eq!(point.y, Some(2));
+    ///
+    /// // Invalid data fails the type validation; deserialization would have succeeded otherwise.
+    /// let invalid_data = "{ x = 1 }";
+    /// assert!(
+    ///     serde_dhall::from_str(invalid_data)
+    ///         .static_type_annotation()
+    ///         .parse::<Point>()
+    ///         .is_err()
+    /// );
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// [`type_annotation`]: struct.Deserializer.html#method.type_annotation
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn static_type_annotation(self) -> Deserializer<'a, StaticAnnot> {
+        Deserializer {
+            annot: StaticAnnot,
+            source: self.source,
+            allow_imports: self.allow_imports,
+        }
+    }
+}
+
+impl<'a, A> Deserializer<'a, A> {
+    /// Sets whether to enable imports.
+    ///
+    /// By default, imports are enabled.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// use serde::Deserialize;
+    /// use serde_dhall::SimpleType;
+    ///
+    /// let data = "12 + ./other_file.dhall : Natural";
+    /// assert!(
+    ///     serde_dhall::from_str(data)
+    ///         .imports(false)
+    ///         .parse::<u64>()
+    ///         .is_err()
+    /// );
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// [`static_type_annotation`]: struct.Deserializer.html#method.static_type_annotation
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn imports(self, imports: bool) -> Self {
+        Deserializer {
+            allow_imports: imports,
+            ..self
+        }
+    }
+
+    // /// TODO
+    // pub fn remote_imports(&mut self, imports: bool) -> &mut Self {
+    //     self.allow_remote_imports = imports;
+    //     if imports {
+    //         self.allow_imports = true;
+    //     }
+    //     self
+    // }
+
+    fn _parse<T>(&self) -> dhall::error::Result<Value>
+    where
+        T: HasAnnot<A>,
+    {
+        let parsed = match &self.source {
+            Source::Str(s) => Parsed::parse_str(s)?,
+            Source::File(p) => Parsed::parse_file(p.as_ref())?,
+        };
+        let resolved = if self.allow_imports {
+            parsed.resolve()?
+        } else {
+            parsed.skip_resolve()?
+        };
+        let typed = match &T::get_annot(&self.annot) {
+            None => resolved.typecheck()?,
+            Some(ty) => resolved.typecheck_with(ty.to_value().as_hir())?,
+        };
+        Ok(Value::from_nir(typed.normalize().as_nir()))
+    }
+
+    /// Parses the chosen dhall value with the options provided.
+    ///
+    /// If you enabled static annotations, `T` is required to implement [`StaticType`].
+    ///
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// let data = serde_dhall::from_str("6 * 7").parse::<u64>()?;
+    /// assert_eq!(data, 42);
+    /// # Ok(())
+    /// # }
+    /// ```
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn parse<T>(&self) -> Result<T>
+    where
+        T: FromDhall + HasAnnot<A>,
+    {
+        let val = self
+            ._parse::<T>()
+            .map_err(ErrorKind::Dhall)
+            .map_err(Error)?;
+        T::from_dhall(&val)
+    }
+}
+
+/// Deserialize a value from a string of Dhall text.
+///
+/// This returns a [`Deserializer`] object. Call the [`parse`] method to get the deserialized
+/// value, or use other [`Deserializer`] methods to control the deserialization process.
+///
+/// # Example
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde::Deserialize;
+///
+/// // We use serde's derive feature
+/// #[derive(Deserialize)]
+/// struct Point {
+///     x: u64,
+///     y: u64,
+/// }
+///
+/// // Some Dhall data
+/// let data = "{ x = 1, y = 1 + 1 } : { x: Natural, y: Natural }";
+///
+/// // Parse the Dhall string as a Point.
+/// let point: Point = serde_dhall::from_str(data).parse()?;
+///
+/// assert_eq!(point.x, 1);
+/// assert_eq!(point.y, 2);
+/// # Ok(())
+/// # }
+/// ```
+///
+/// [`Deserializer`]: struct.Deserializer.html
+/// [`parse`]: struct.Deserializer.html#method.parse
+pub fn from_str(s: &str) -> Deserializer<'_, NoAnnot> {
+    Deserializer::from_str(s)
+}
+
+/// Deserialize a value from a Dhall file.
+///
+/// This returns a [`Deserializer`] object. Call the [`parse`] method to get the deserialized
+/// value, or use other [`Deserializer`] methods to control the deserialization process.
+///
+/// # Example
+///
+/// ```no_run
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde::Deserialize;
+///
+/// // We use serde's derive feature
+/// #[derive(Deserialize)]
+/// struct Point {
+///     x: u64,
+///     y: u64,
+/// }
+///
+/// // Parse the Dhall file as a Point.
+/// let point: Point = serde_dhall::from_file("foo.dhall").parse()?;
+/// # Ok(())
+/// # }
+/// ```
+///
+/// [`Deserializer`]: struct.Deserializer.html
+/// [`parse`]: struct.Deserializer.html#method.parse
+pub fn from_file<'a, P: AsRef<Path>>(path: P) -> Deserializer<'a, NoAnnot> {
+    Deserializer::from_file(path)
+}
+
+// pub fn from_url(url: &str) -> Deserializer<'_, NoAnnot> {
+//     Deserializer::from_url(url)
+// }
diff --git a/serde_dhall/src/serde.rs b/serde_dhall/src/serde.rs
deleted file mode 100644
index 4fd7815..0000000
--- a/serde_dhall/src/serde.rs
+++ /dev/null
@@ -1,144 +0,0 @@
-use std::borrow::Cow;
-
-use serde::de::value::{
-    MapAccessDeserializer, MapDeserializer, SeqDeserializer,
-};
-
-use dhall::syntax::{ExprKind, LitKind};
-use dhall::NormalizedExpr;
-
-use crate::de::{Deserialize, Error, Result};
-use crate::Value;
-
-impl<'a, T> crate::de::sealed::Sealed for T where T: serde::Deserialize<'a> {}
-
-impl<'a, T> Deserialize for T
-where
-    T: serde::Deserialize<'a>,
-{
-    fn from_dhall(v: &Value) -> Result<Self> {
-        T::deserialize(Deserializer(Cow::Owned(v.to_expr())))
-    }
-}
-
-struct Deserializer<'a>(Cow<'a, NormalizedExpr>);
-
-impl<'de: 'a, 'a> serde::de::IntoDeserializer<'de, Error> for Deserializer<'a> {
-    type Deserializer = Deserializer<'a>;
-    fn into_deserializer(self) -> Self::Deserializer {
-        self
-    }
-}
-
-impl<'de: 'a, 'a> serde::Deserializer<'de> for Deserializer<'a> {
-    type Error = Error;
-
-    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value>
-    where
-        V: serde::de::Visitor<'de>,
-    {
-        use std::convert::TryInto;
-        use ExprKind::*;
-        use LitKind::*;
-        let expr = self.0.as_ref();
-        let not_serde_compatible = || {
-            Err(Error::Deserialize(format!(
-                "this cannot be deserialized into the serde data model: {}",
-                expr
-            )))
-        };
-
-        match expr.kind() {
-            Lit(Bool(x)) => visitor.visit_bool(*x),
-            Lit(Natural(x)) => {
-                if let Ok(x64) = (*x).try_into() {
-                    visitor.visit_u64(x64)
-                } else if let Ok(x32) = (*x).try_into() {
-                    visitor.visit_u32(x32)
-                } else {
-                    unimplemented!()
-                }
-            }
-            Lit(Integer(x)) => {
-                if let Ok(x64) = (*x).try_into() {
-                    visitor.visit_i64(x64)
-                } else if let Ok(x32) = (*x).try_into() {
-                    visitor.visit_i32(x32)
-                } else {
-                    unimplemented!()
-                }
-            }
-            Lit(Double(x)) => visitor.visit_f64((*x).into()),
-            TextLit(x) => {
-                // Normal form ensures that the tail is empty.
-                assert!(x.tail().is_empty());
-                visitor.visit_str(x.head())
-            }
-            EmptyListLit(..) => {
-                visitor.visit_seq(SeqDeserializer::new(None::<()>.into_iter()))
-            }
-            NEListLit(xs) => visitor.visit_seq(SeqDeserializer::new(
-                xs.iter().map(|x| Deserializer(Cow::Borrowed(x))),
-            )),
-            SomeLit(x) => visitor.visit_some(Deserializer(Cow::Borrowed(x))),
-            App(f, x) => match f.kind() {
-                Builtin(dhall::syntax::Builtin::OptionalNone) => {
-                    visitor.visit_none()
-                }
-                Field(y, name) => match y.kind() {
-                    UnionType(..) => {
-                        let name: String = name.into();
-                        visitor.visit_enum(MapAccessDeserializer::new(
-                            MapDeserializer::new(
-                                Some((name, Deserializer(Cow::Borrowed(x))))
-                                    .into_iter(),
-                            ),
-                        ))
-                    }
-                    _ => not_serde_compatible(),
-                },
-                _ => not_serde_compatible(),
-            },
-            RecordLit(m) => visitor
-                .visit_map(MapDeserializer::new(m.iter().map(|(k, v)| {
-                    (k.as_ref(), Deserializer(Cow::Borrowed(v)))
-                }))),
-            Field(y, name) => match y.kind() {
-                UnionType(..) => {
-                    let name: String = name.into();
-                    visitor.visit_enum(MapAccessDeserializer::new(
-                        MapDeserializer::new(Some((name, ())).into_iter()),
-                    ))
-                }
-                _ => not_serde_compatible(),
-            },
-            Const(..) | Var(..) | Lam(..) | Pi(..) | Let(..) | Annot(..)
-            | Assert(..) | Builtin(..) | BinOp(..) | BoolIf(..)
-            | RecordType(..) | UnionType(..) | Merge(..) | ToMap(..)
-            | Projection(..) | ProjectionByExpr(..) | Completion(..)
-            | Import(..) => not_serde_compatible(),
-        }
-    }
-
-    fn deserialize_tuple<V>(self, _: usize, visitor: V) -> Result<V::Value>
-    where
-        V: serde::de::Visitor<'de>,
-    {
-        use ExprKind::*;
-        let expr = self.0.as_ref();
-
-        match expr.kind() {
-            // Blindly takes keys in sorted order.
-            RecordLit(m) => visitor.visit_seq(SeqDeserializer::new(
-                m.iter().map(|(_, v)| Deserializer(Cow::Borrowed(v))),
-            )),
-            _ => self.deserialize_any(visitor),
-        }
-    }
-
-    serde::forward_to_deserialize_any! {
-        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
-        bytes byte_buf option unit unit_struct newtype_struct seq
-        tuple_struct map struct enum identifier ignored_any
-    }
-}
diff --git a/serde_dhall/src/static_type.rs b/serde_dhall/src/static_type.rs
index 1eb9150..26c70cd 100644
--- a/serde_dhall/src/static_type.rs
+++ b/serde_dhall/src/static_type.rs
@@ -1,27 +1,92 @@
-use dhall::syntax::Builtin;
-
-use crate::Value;
+use crate::SimpleType;
 
 /// A Rust type that can be represented as a Dhall type.
 ///
-/// A typical example is `Option<bool>`,
-/// represented by the dhall expression `Optional Bool`.
+/// A typical example is `Option<bool>`, represented by the Dhall expression `Optional Bool`.
+///
+/// This trait can be automatically derived, and this is the recommended way of implementing it.
+///
+/// Some Rust types cannot implement this trait, because there isn't a single Dhall type that
+/// corresponds to them. For example, `HashMap<String, u64>` could correspond to multiple different
+/// Dhall types, e.g. `{ foo: Natural, bar: Natural }` and `{ baz: Natural }`.
+///
+/// # Example
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde_dhall::{SimpleType, StaticType};
+///
+/// #[derive(StaticType)]
+/// struct Foo {
+///     x: bool,
+///     y: Vec<u64>,
+/// }
+///
+/// let ty: SimpleType =
+///     serde_dhall::from_str("{ x: Bool, y: List Natural }").parse()?;
+///
+/// assert_eq!(Foo::static_type(), ty);
+/// # Ok(())
+/// # }
+/// ```
+///
+/// # Type correspondence
 ///
-/// This trait can and should be automatically derived.
+/// The following Dhall types correspond to the following Rust types:
 ///
-/// The representation needs to be independent of the value.
-/// For this reason, something like `HashMap<String, bool>` cannot implement
-/// [StaticType] because each different value would
-/// have a different Dhall record type.
+/// Dhall  | Rust
+/// -------|------
+/// `Bool`  | `bool`
+/// `Natural`  | `u64`, `u32`, ...
+/// `Integer`  | `i64`, `i32`, ...
+/// `Double`  | `f64`, `f32`, ...
+/// `Text`  | `String`
+/// `List T`  | `Vec<T>`
+/// `Optional T`  | `Option<T>`
+/// `{ x: T, y: U }`  | structs
+/// `{ _1: T, _2: U }`  | `(T, U)`, structs
+/// `{ x: T, y: T }`  | `HashMap<String, T>`, structs
+/// `< x: T \| y: U >`  | enums
+/// `T -> U`  | unsupported
+/// `Prelude.JSON.Type`  | unsupported
+/// `Prelude.Map.Type T U`  | unsupported
 pub trait StaticType {
-    fn static_type() -> Value;
+    /// Return the Dhall type that represents this type.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// use serde::Deserialize;
+    /// use serde_dhall::{SimpleType, StaticType};
+    ///
+    /// // Using `derive(StaticType)` here would give it the type `{ _1: List Natural }`.
+    /// #[derive(Deserialize)]
+    /// #[serde(transparent)]
+    /// struct Foo(Vec<u64>);
+    ///
+    /// impl StaticType for Foo {
+    ///     fn static_type() -> SimpleType {
+    ///         SimpleType::List(Box::new(SimpleType::Natural))
+    ///     }
+    /// }
+    ///
+    /// let foo = serde_dhall::from_str("[ 1, 2 ]")
+    ///     .static_type_annotation()
+    ///     .parse::<Foo>()?;
+    ///
+    /// assert_eq!(foo.0, vec![1, 2]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    fn static_type() -> SimpleType;
 }
 
 macro_rules! derive_builtin {
-    ($ty:ty, $builtin:ident) => {
-        impl StaticType for $ty {
-            fn static_type() -> Value {
-                Value::make_builtin_type(Builtin::$builtin)
+    ($rust_ty:ty, $dhall_ty:ident) => {
+        impl StaticType for $rust_ty {
+            fn static_type() -> SimpleType {
+                SimpleType::$dhall_ty
             }
         }
     };
@@ -43,13 +108,14 @@ where
     A: StaticType,
     B: StaticType,
 {
-    fn static_type() -> Value {
-        Value::make_record_type(
+    fn static_type() -> SimpleType {
+        SimpleType::Record(
             vec![
                 ("_1".to_owned(), A::static_type()),
                 ("_2".to_owned(), B::static_type()),
             ]
-            .into_iter(),
+            .into_iter()
+            .collect(),
         )
     }
 }
@@ -59,13 +125,14 @@ where
     T: StaticType,
     E: StaticType,
 {
-    fn static_type() -> Value {
-        Value::make_union_type(
+    fn static_type() -> SimpleType {
+        SimpleType::Union(
             vec![
                 ("Ok".to_owned(), Some(T::static_type())),
                 ("Err".to_owned(), Some(E::static_type())),
             ]
-            .into_iter(),
+            .into_iter()
+            .collect(),
         )
     }
 }
@@ -74,8 +141,8 @@ impl<T> StaticType for Option<T>
 where
     T: StaticType,
 {
-    fn static_type() -> Value {
-        Value::make_optional_type(T::static_type())
+    fn static_type() -> SimpleType {
+        SimpleType::Optional(Box::new(T::static_type()))
     }
 }
 
@@ -83,8 +150,8 @@ impl<T> StaticType for Vec<T>
 where
     T: StaticType,
 {
-    fn static_type() -> Value {
-        Value::make_list_type(T::static_type())
+    fn static_type() -> SimpleType {
+        SimpleType::List(Box::new(T::static_type()))
     }
 }
 
@@ -92,7 +159,7 @@ impl<'a, T> StaticType for &'a T
 where
     T: StaticType,
 {
-    fn static_type() -> Value {
+    fn static_type() -> SimpleType {
         T::static_type()
     }
 }
diff --git a/serde_dhall/src/value.rs b/serde_dhall/src/value.rs
new file mode 100644
index 0000000..d6631da
--- /dev/null
+++ b/serde_dhall/src/value.rs
@@ -0,0 +1,287 @@
+use std::collections::{BTreeMap, HashMap};
+
+use dhall::semantics::{Hir, HirKind, Nir, NirKind};
+use dhall::syntax::{Builtin, Expr, ExprKind, NumKind, Span};
+
+use crate::{Error, ErrorKind, FromDhall, Result, Sealed};
+
+#[doc(hidden)]
+/// An arbitrary Dhall value.
+#[derive(Debug, Clone)]
+pub struct Value {
+    /// Invariant: in normal form
+    hir: Hir,
+    /// Cached conversions because they are annoying to construct from Hir.
+    /// At most one of them will be `Some`.
+    as_simple_val: Option<SimpleValue>,
+    as_simple_ty: Option<SimpleType>,
+}
+
+/// A simple value of the kind that can be decoded with serde
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) enum SimpleValue {
+    Num(NumKind),
+    Text(String),
+    Optional(Option<Box<SimpleValue>>),
+    List(Vec<SimpleValue>),
+    Record(BTreeMap<String, SimpleValue>),
+    Union(String, Option<Box<SimpleValue>>),
+}
+
+/// The type of a value that can be decoded by `serde_dhall`, e.g. `{ x: Bool, y: List Natural }`.
+///
+/// A `SimpleType` is used when deserializing values to ensure they are of the expected type.
+/// Rather than letting `serde` handle potential type mismatches, this uses the type-checking
+/// capabilities of Dhall to catch errors early and cleanly indicate in the user's code where the
+/// mismatch happened.
+///
+/// You would typically not manipulate `SimpleType`s by hand but rather let Rust infer it for your
+/// datatype by deriving the [`StaticType`] trait, and using
+/// [`Deserializer::static_type_annotation`]. If you need to supply a `SimpleType` manually, you
+/// can either deserialize it like any other Dhall value, or construct it manually.
+///
+/// [`StaticType`]: trait.StaticType.html
+/// [`Deserializer::static_type_annotation`]: options/struct.Deserializer.html#method.static_type_annotation
+///
+/// # Examples
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde_dhall::{SimpleType, StaticType};
+///
+/// #[derive(StaticType)]
+/// struct Foo {
+///     x: bool,
+///     y: Vec<u64>,
+/// }
+///
+/// let ty: SimpleType =
+///     serde_dhall::from_str("{ x: Bool, y: List Natural }").parse()?;
+///
+/// assert_eq!(Foo::static_type(), ty);
+/// # Ok(())
+/// # }
+/// ```
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use std::collections::HashMap;
+/// use serde_dhall::SimpleType;
+///
+/// let ty: SimpleType =
+///     serde_dhall::from_str("{ x: Natural, y: Natural }").parse()?;
+///
+/// let mut map = HashMap::new();
+/// map.insert("x".to_string(), SimpleType::Natural);
+/// map.insert("y".to_string(), SimpleType::Natural);
+/// assert_eq!(ty, SimpleType::Record(map));
+/// # Ok(())
+/// # }
+/// ```
+///
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum SimpleType {
+    /// Corresponds to the Dhall type `Bool`
+    Bool,
+    /// Corresponds to the Dhall type `Natural`
+    Natural,
+    /// Corresponds to the Dhall type `Integer`
+    Integer,
+    /// Corresponds to the Dhall type `Double`
+    Double,
+    /// Corresponds to the Dhall type `Text`
+    Text,
+    /// Corresponds to the Dhall type `Optional T`
+    Optional(Box<SimpleType>),
+    /// Corresponds to the Dhall type `List T`
+    List(Box<SimpleType>),
+    /// Corresponds to the Dhall type `{ x : T, y : U }`
+    Record(HashMap<String, SimpleType>),
+    /// Corresponds to the Dhall type `< x : T | y : U >`
+    Union(HashMap<String, Option<SimpleType>>),
+}
+
+impl Value {
+    pub(crate) fn from_nir(x: &Nir) -> Self {
+        Value {
+            hir: x.to_hir_noenv(),
+            as_simple_val: SimpleValue::from_nir(x),
+            as_simple_ty: SimpleType::from_nir(x),
+        }
+    }
+
+    pub(crate) fn as_hir(&self) -> &Hir {
+        &self.hir
+    }
+
+    /// Converts a Value into a SimpleValue.
+    pub(crate) fn to_simple_value(&self) -> Option<SimpleValue> {
+        self.as_simple_val.clone()
+    }
+
+    /// Converts a Value into a SimpleType.
+    pub(crate) fn to_simple_type(&self) -> Option<SimpleType> {
+        self.as_simple_ty.clone()
+    }
+
+    /// Converts a value back to the corresponding AST expression.
+    pub(crate) fn to_expr(&self) -> Expr {
+        self.hir.to_expr(Default::default())
+    }
+}
+
+impl SimpleValue {
+    pub(crate) fn from_nir(nir: &Nir) -> Option<Self> {
+        Some(match nir.kind() {
+            NirKind::Num(lit) => SimpleValue::Num(lit.clone()),
+            NirKind::TextLit(x) => SimpleValue::Text(
+                x.as_text()
+                    .expect("Normal form should ensure the text is a string"),
+            ),
+            NirKind::EmptyOptionalLit(_) => SimpleValue::Optional(None),
+            NirKind::NEOptionalLit(x) => {
+                SimpleValue::Optional(Some(Box::new(Self::from_nir(x)?)))
+            }
+            NirKind::EmptyListLit(_) => SimpleValue::List(vec![]),
+            NirKind::NEListLit(xs) => SimpleValue::List(
+                xs.iter().map(Self::from_nir).collect::<Option<_>>()?,
+            ),
+            NirKind::RecordLit(kvs) => SimpleValue::Record(
+                kvs.iter()
+                    .map(|(k, v)| Some((k.into(), Self::from_nir(v)?)))
+                    .collect::<Option<_>>()?,
+            ),
+            NirKind::UnionLit(field, x, _) => SimpleValue::Union(
+                field.into(),
+                Some(Box::new(Self::from_nir(x)?)),
+            ),
+            NirKind::UnionConstructor(field, ty)
+                if ty.get(field).map(|f| f.is_some()) == Some(false) =>
+            {
+                SimpleValue::Union(field.into(), None)
+            }
+            _ => return None,
+        })
+    }
+}
+
+impl SimpleType {
+    pub(crate) fn from_nir(nir: &Nir) -> Option<Self> {
+        Some(match nir.kind() {
+            NirKind::BuiltinType(b) => match b {
+                Builtin::Bool => SimpleType::Bool,
+                Builtin::Natural => SimpleType::Natural,
+                Builtin::Integer => SimpleType::Integer,
+                Builtin::Double => SimpleType::Double,
+                Builtin::Text => SimpleType::Text,
+                _ => unreachable!(),
+            },
+            NirKind::OptionalType(t) => {
+                SimpleType::Optional(Box::new(Self::from_nir(t)?))
+            }
+            NirKind::ListType(t) => {
+                SimpleType::List(Box::new(Self::from_nir(t)?))
+            }
+            NirKind::RecordType(kts) => SimpleType::Record(
+                kts.iter()
+                    .map(|(k, v)| Some((k.into(), Self::from_nir(v)?)))
+                    .collect::<Option<_>>()?,
+            ),
+            NirKind::UnionType(kts) => SimpleType::Union(
+                kts.iter()
+                    .map(|(k, v)| {
+                        Some((
+                            k.into(),
+                            v.as_ref()
+                                .map(|v| Ok(Self::from_nir(v)?))
+                                .transpose()?,
+                        ))
+                    })
+                    .collect::<Option<_>>()?,
+            ),
+            _ => return None,
+        })
+    }
+
+    pub(crate) fn to_value(&self) -> Value {
+        Value {
+            hir: self.to_hir(),
+            as_simple_val: None,
+            as_simple_ty: Some(self.clone()),
+        }
+    }
+    pub(crate) fn to_hir(&self) -> Hir {
+        let hir = |k| Hir::new(HirKind::Expr(k), Span::Artificial);
+        hir(match self {
+            SimpleType::Bool => ExprKind::Builtin(Builtin::Bool),
+            SimpleType::Natural => ExprKind::Builtin(Builtin::Natural),
+            SimpleType::Integer => ExprKind::Builtin(Builtin::Integer),
+            SimpleType::Double => ExprKind::Builtin(Builtin::Double),
+            SimpleType::Text => ExprKind::Builtin(Builtin::Text),
+            SimpleType::Optional(t) => ExprKind::App(
+                hir(ExprKind::Builtin(Builtin::Optional)),
+                t.to_hir(),
+            ),
+            SimpleType::List(t) => {
+                ExprKind::App(hir(ExprKind::Builtin(Builtin::List)), t.to_hir())
+            }
+            SimpleType::Record(kts) => ExprKind::RecordType(
+                kts.iter()
+                    .map(|(k, t)| (k.as_str().into(), t.to_hir()))
+                    .collect(),
+            ),
+            SimpleType::Union(kts) => ExprKind::UnionType(
+                kts.iter()
+                    .map(|(k, t)| {
+                        (k.as_str().into(), t.as_ref().map(|t| t.to_hir()))
+                    })
+                    .collect(),
+            ),
+        })
+    }
+}
+
+impl Sealed for Value {}
+impl Sealed for SimpleValue {}
+impl Sealed for SimpleType {}
+
+impl FromDhall for Value {
+    fn from_dhall(v: &Value) -> Result<Self> {
+        Ok(v.clone())
+    }
+}
+impl FromDhall for SimpleValue {
+    fn from_dhall(v: &Value) -> Result<Self> {
+        v.to_simple_value().ok_or_else(|| {
+            Error(ErrorKind::Deserialize(format!(
+                "this cannot be deserialized into a simple type: {}",
+                v
+            )))
+        })
+    }
+}
+impl FromDhall for SimpleType {
+    fn from_dhall(v: &Value) -> Result<Self> {
+        v.to_simple_type().ok_or_else(|| {
+            Error(ErrorKind::Deserialize(format!(
+                "this cannot be deserialized into a simple type: {}",
+                v
+            )))
+        })
+    }
+}
+
+impl Eq for Value {}
+impl PartialEq for Value {
+    fn eq(&self, other: &Self) -> bool {
+        self.hir == other.hir
+    }
+}
+impl std::fmt::Display for Value {
+    fn fmt(
+        &self,
+        f: &mut std::fmt::Formatter,
+    ) -> std::result::Result<(), std::fmt::Error> {
+        self.to_expr().fmt(f)
+    }
+}