Merge pull request #189 from Nadrieril/serialize

18 files changed, 1278 insertions, 224 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5329eca..0fe7814 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 #### [Unreleased]
 
+- Implement serialization (https://github.com/Nadrieril/dhall-rust/issues/164)
 - BREAKING CHANGE: use u64/i64 instead of usize/isize in `NumKind`
 
 #### [0.7.5] - 2020-10-28
diff --git a/dhall/src/syntax/text/printer.rs b/dhall/src/syntax/text/printer.rs
index 0c2eb2e..4b7b8b8 100644
--- a/dhall/src/syntax/text/printer.rs
+++ b/dhall/src/syntax/text/printer.rs
@@ -156,7 +156,9 @@ fn fmt_label(label: &Label, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
     };
     if s.is_empty() {
         write!(f, "``")
-    } else if !is_reserved && s.chars().all(|c| c.is_ascii_alphanumeric()) {
+    } else if !is_reserved
+        && s.chars().all(|c| c.is_ascii_alphanumeric() || c == '_')
+    {
         write!(f, "{}", s)
     } else {
         write!(f, "`{}`", s)
diff --git a/dhall/tests/parser/success/text/templateB.txt b/dhall/tests/parser/success/text/templateB.txt
index 74303d3..939aa18 100644
--- a/dhall/tests/parser/success/text/templateB.txt
+++ b/dhall/tests/parser/success/text/templateB.txt
@@ -1 +1 @@
-λ(record : { `in_ca` : Bool, name : Text, `taxed_value` : Double, value : Double }) → "Hello ${ record.name }\nYou have just won ${ Double/show record.value } dollars!\n${ if record.`in_ca` then "Well, ${ Double/show record.`taxed_value` } dollars, after taxes" else "" }\n"
+λ(record : { in_ca : Bool, name : Text, taxed_value : Double, value : Double }) → "Hello ${ record.name }\nYou have just won ${ Double/show record.value } dollars!\n${ if record.in_ca then "Well, ${ Double/show record.taxed_value } dollars, after taxes" else "" }\n"
diff --git a/dhall/tests/parser/success/unit/LambdaUnderscoreB.txt b/dhall/tests/parser/success/unit/LambdaUnderscoreB.txt
index 10000d0..32719f0 100644
--- a/dhall/tests/parser/success/unit/LambdaUnderscoreB.txt
+++ b/dhall/tests/parser/success/unit/LambdaUnderscoreB.txt
@@ -1 +1 @@
-λ(`_` : T) → x
+λ(_ : T) → x
diff --git a/dhall/tests/parser/success/unit/VariableUnderscoreB.txt b/dhall/tests/parser/success/unit/VariableUnderscoreB.txt
index 5824cab..31354ec 100644
--- a/dhall/tests/parser/success/unit/VariableUnderscoreB.txt
+++ b/dhall/tests/parser/success/unit/VariableUnderscoreB.txt
@@ -1 +1 @@
-`_`
+_
diff --git a/dhall/tests/type-inference/failure/unit/AssertAlphaTrap.txt b/dhall/tests/type-inference/failure/unit/AssertAlphaTrap.txt
index 0798b19..7241266 100644
--- a/dhall/tests/type-inference/failure/unit/AssertAlphaTrap.txt
+++ b/dhall/tests/type-inference/failure/unit/AssertAlphaTrap.txt
@@ -1,4 +1,4 @@
-Type error: error: unbound variable ``_``
+Type error: error: unbound variable `_`
  --> <current file>:1:47
   |
 1 | assert : (\(_: Bool) -> _) === (\(x: Bool) -> _)
diff --git a/serde_dhall/src/deserialize.rs b/serde_dhall/src/deserialize.rs
index 12b4703..0e518ea 100644
--- a/serde_dhall/src/deserialize.rs
+++ b/serde_dhall/src/deserialize.rs
@@ -47,8 +47,6 @@ pub trait FromDhall: Sealed + Sized {
 
 impl<T> Sealed for T where T: serde::de::DeserializeOwned {}
 
-struct Deserializer<'a>(Cow<'a, SimpleValue>);
-
 /// Deserialize a Rust value from a Dhall [`SimpleValue`].
 ///
 /// # Example
@@ -109,6 +107,8 @@ where
     }
 }
 
+struct Deserializer<'a>(Cow<'a, SimpleValue>);
+
 impl<'de: 'a, 'a> serde::de::IntoDeserializer<'de, Error> for Deserializer<'a> {
     type Deserializer = Deserializer<'a>;
     fn into_deserializer(self) -> Self::Deserializer {
@@ -171,9 +171,19 @@ impl<'de: 'a, 'a> serde::Deserializer<'de> for Deserializer<'a> {
         }
     }
 
+    fn deserialize_unit<V>(self, visitor: V) -> crate::Result<V::Value>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        match self.0.as_ref() {
+            SimpleValue::Record(m) if m.is_empty() => visitor.visit_unit(),
+            _ => 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
+        bytes byte_buf option 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
index 896e8b9..3309fe4 100644
--- a/serde_dhall/src/error.rs
+++ b/serde_dhall/src/error.rs
@@ -11,6 +11,7 @@ pub struct Error(pub(crate) ErrorKind);
 pub(crate) enum ErrorKind {
     Dhall(DhallError),
     Deserialize(String),
+    Serialize(String),
 }
 
 impl From<ErrorKind> for Error {
@@ -24,6 +25,7 @@ impl std::fmt::Display for Error {
         match &self.0 {
             ErrorKind::Dhall(err) => write!(f, "{}", err),
             ErrorKind::Deserialize(err) => write!(f, "{}", err),
+            ErrorKind::Serialize(err) => write!(f, "{}", err),
         }
     }
 }
@@ -38,3 +40,12 @@ impl serde::de::Error for Error {
         ErrorKind::Deserialize(msg.to_string()).into()
     }
 }
+
+impl serde::ser::Error for Error {
+    fn custom<T>(msg: T) -> Self
+    where
+        T: std::fmt::Display,
+    {
+        ErrorKind::Serialize(msg.to_string()).into()
+    }
+}
diff --git a/serde_dhall/src/lib.rs b/serde_dhall/src/lib.rs
index 8291f74..387278b 100644
--- a/serde_dhall/src/lib.rs
+++ b/serde_dhall/src/lib.rs
@@ -12,13 +12,12 @@
 //! YAML. It uses the [Serde][serde] serialization library to provide drop-in support for Dhall
 //! for any datatype that supports serde (and that's a lot of them !).
 //!
-//! This library is limited to deserializing (reading) Dhall values; serializing (writing)
-//! values to Dhall is not supported.
-//!
 //! # Basic usage
 //!
-//! 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.
+//! ## Deserialization (reading)
+//!
+//! The entrypoint for deserialization 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`:
 //!
@@ -65,16 +64,69 @@
 //! # }
 //! ```
 //!
+//! ## Serialization (writing)
+//!
+//! The entrypoint for serialization is the [`serialize`](fn.serialize.html) function. It takes a
+//! serde-compatible type value and serializes it to a string containing a Dhall expression.
+//!
+//! This could mean a common Rust type like `HashMap`:
+//!
+//! ```rust
+//! # fn main() -> serde_dhall::Result<()> {
+//! use std::collections::HashMap;
+//!
+//! let mut map = HashMap::new();
+//! map.insert("x".to_string(), 1u64);
+//! map.insert("y".to_string(), 2u64);
+//!
+//! let string = serde_dhall::serialize(&map).to_string()?;
+//! assert_eq!(
+//!     string,
+//!     "{ x = 1, y = 2 }".to_string(),
+//! );
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! or a custom datatype, using serde's `derive` mechanism:
+//!
+//! ```rust
+//! # fn main() -> serde_dhall::Result<()> {
+//! use serde::Serialize;
+//!
+//! #[derive(Serialize)]
+//! struct Point {
+//!     x: u64,
+//!     y: u64,
+//! }
+//!
+//! let data = Point { x: 1, y: 2 };
+//! let string = serde_dhall::serialize(&data).to_string()?;
+//! assert_eq!(
+//!     string,
+//!     "{ x = 1, y = 2 }".to_string(),
+//! );
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! Beware that in order to serialize empty options, empty lists or enums correctly, you will need
+//! to provide a type annotation!
+//!
 //! # 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(…).parse()`](fn.from_str.html).
+//! If you used to produce JSON or YAML, you only need to replace [`serde_json::to_string`] or
+//! [`serde_yaml::to_string`] with [`serde_dhall::serialize(…).to_string()`](fn.serialize.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
+//! [`serde_json::to_string`]: https://docs.serde.rs/serde_json/fn.to_string.html
+//! [`serde_yaml::to_string`]: https://docs.serde.rs/serde_yaml/fn.to_string.html
 //!
 //!
-//! # Additional Dhall typechecking
+//! # Additional type annotations
 //!
 //! When deserializing, normal type checking is done to ensure that the returned value is a valid
 //! Dhall value. However types are
@@ -82,11 +134,16 @@
 //! 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 in this way: you can provide the type manually or
-//! you can let Rust infer it for you.
+//! It is also possible to provide a type annotation when serializing. This is useful in particular
+//! for types like `HashMap` or [`SimpleValue`] that do not have a fixed type as Dhall values.
+//!
+//! Moreover, some values (namely empty options, empty lists, and enums) _require_ a type annotation
+//! in order to be converted to Dhall, because the resulting Dhall value will contain the type
+//! explicitly.
 //!
-//! To let Rust infer the appropriate Dhall type, use the [StaticType](trait.StaticType.html)
-//! trait.
+//! There are two ways to provide a type in this way: you can provide it manually or you can let
+//! Rust infer it for you. To let Rust infer the appropriate Dhall type, use the
+//! [StaticType](trait.StaticType.html) trait.
 //!
 //! ```rust
 //! # fn main() -> serde_dhall::Result<()> {
@@ -121,6 +178,28 @@
 //! # }
 //! ```
 //!
+//! ```
+//! # fn main() -> serde_dhall::Result<()> {
+//! use serde::Serialize;
+//! use serde_dhall::{serialize, StaticType};
+//!
+//! #[derive(Serialize, StaticType)]
+//! enum MyOption {
+//!     MyNone,
+//!     MySome(u64),
+//! }
+//!
+//! let data = MyOption::MySome(0);
+//! let string = serialize(&data)
+//!     .static_type_annotation()
+//!     .to_string()?;
+//! // The resulting Dhall string depends on the type annotation; it could not have been
+//! // printed without it.
+//! assert_eq!(string, "< MyNone | MySome: Natural >.MySome 0".to_string());
+//! # 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.
 //!
@@ -151,12 +230,27 @@
 //! # }
 //! ```
 //!
+//! ```
+//! # fn main() -> serde_dhall::Result<()> {
+//! use serde_dhall::{serialize, from_str, SimpleValue};
+//!
+//! let ty = from_str("< A | B: Bool >").parse()?;
+//! let data = SimpleValue::Union("A".to_string(), None);
+//! let string = serialize(&data)
+//!     .type_annotation(&ty)
+//!     .to_string()?;
+//! assert_eq!(string, "< A | B: Bool >.A".to_string());
+//! # Ok(())
+//! # }
+//! ```
+//!
 //! # Controlling deserialization
 //!
 //! If you need more control over the process of reading Dhall values, e.g. disabling
 //! imports, see the [`Deserializer`] methods.
 //!
 //! [`Deserializer`]: struct.Deserializer.html
+//! [`SimpleValue`]: enum.SimpleValue.html
 //! [dhall]: https://dhall-lang.org/
 //! [serde]: https://docs.serde.rs/serde/
 //! [serde::Deserialize]: https://docs.serde.rs/serde/trait.Deserialize.html
@@ -169,6 +263,7 @@ mod test_readme {
 mod deserialize;
 mod error;
 mod options;
+mod serialize;
 mod static_type;
 /// Dhall values
 mod value;
@@ -176,10 +271,11 @@ mod value;
 #[doc(hidden)]
 pub use dhall_proc_macros::StaticType;
 
-pub(crate) use deserialize::Sealed;
 pub use deserialize::{from_simple_value, FromDhall};
 pub(crate) use error::ErrorKind;
 pub use error::{Error, Result};
-pub use options::{from_file, from_str, Deserializer};
+pub use options::de::{from_file, from_str, Deserializer};
+pub use options::ser::{serialize, Serializer};
+pub use serialize::ToDhall;
 pub use static_type::StaticType;
 pub use value::{NumKind, SimpleType, SimpleValue, Value};
diff --git a/serde_dhall/src/options.rs b/serde_dhall/src/options/de.rs
index 7b27114..e4f3456 100644
--- a/serde_dhall/src/options.rs
+++ b/serde_dhall/src/options/de.rs
@@ -2,8 +2,9 @@ use std::path::{Path, PathBuf};
 
 use dhall::Parsed;
 
+use crate::options::{HasAnnot, ManualAnnot, NoAnnot, StaticAnnot, TypeAnnot};
 use crate::SimpleType;
-use crate::{Error, ErrorKind, FromDhall, Result, StaticType, Value};
+use crate::{Error, ErrorKind, FromDhall, Result, Value};
 
 #[derive(Debug, Clone)]
 enum Source<'a> {
@@ -12,32 +13,6 @@ enum Source<'a> {
     // 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
@@ -252,6 +227,7 @@ impl<'a, A> Deserializer<'a, A> {
 
     fn _parse<T>(&self) -> dhall::error::Result<Value>
     where
+        A: TypeAnnot,
         T: HasAnnot<A>,
     {
         let parsed = match &self.source {
@@ -263,7 +239,7 @@ impl<'a, A> Deserializer<'a, A> {
         } else {
             parsed.skip_resolve()?
         };
-        let typed = match &T::get_annot(&self.annot) {
+        let typed = match &T::get_annot(self.annot) {
             None => resolved.typecheck()?,
             Some(ty) => resolved.typecheck_with(ty.to_value().as_hir())?,
         };
@@ -287,6 +263,7 @@ impl<'a, A> Deserializer<'a, A> {
     /// [`StaticType`]: trait.StaticType.html
     pub fn parse<T>(&self) -> Result<T>
     where
+        A: TypeAnnot,
         T: FromDhall + HasAnnot<A>,
     {
         let val = self
diff --git a/serde_dhall/src/options/mod.rs b/serde_dhall/src/options/mod.rs
new file mode 100644
index 0000000..9241c45
--- /dev/null
+++ b/serde_dhall/src/options/mod.rs
@@ -0,0 +1,36 @@
+use crate::{SimpleType, StaticType};
+
+pub(crate) mod de;
+pub(crate) mod ser;
+
+#[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 TypeAnnot: Copy {}
+pub trait HasAnnot<A: TypeAnnot> {
+    fn get_annot(a: A) -> Option<SimpleType>;
+}
+
+impl TypeAnnot for NoAnnot {}
+impl TypeAnnot for ManualAnnot<'_> {}
+impl TypeAnnot for StaticAnnot {}
+
+impl<T> HasAnnot<NoAnnot> for T {
+    fn get_annot(_: NoAnnot) -> Option<SimpleType> {
+        None
+    }
+}
+impl<T> HasAnnot<ManualAnnot<'_>> for T {
+    fn get_annot(a: ManualAnnot<'_>) -> Option<SimpleType> {
+        Some(a.0.clone())
+    }
+}
+impl<T: StaticType> HasAnnot<StaticAnnot> for T {
+    fn get_annot(_: StaticAnnot) -> Option<SimpleType> {
+        Some(T::static_type())
+    }
+}
diff --git a/serde_dhall/src/options/ser.rs b/serde_dhall/src/options/ser.rs
new file mode 100644
index 0000000..d74beb0
--- /dev/null
+++ b/serde_dhall/src/options/ser.rs
@@ -0,0 +1,234 @@
+use crate::options::{HasAnnot, ManualAnnot, NoAnnot, StaticAnnot, TypeAnnot};
+use crate::{Result, SimpleType, ToDhall};
+
+/// Controls how a Dhall value is written.
+///
+/// This builder exposes the ability to configure how a value is serialized, and to set type
+/// annotations.
+///
+/// When using [`Serializer`], you'll create it with [`serialize`], then chain calls to methods to
+/// set each option, then call [`to_string`]. This will give you a [`Result<String>`] containing
+/// the input serialized to Dhall.
+///
+/// Note that if you do not provide a type annotation, some values may not be convertible to Dhall,
+/// like empty lists or enums.
+///
+/// [`Serializer`]: struct.Serializer.html
+/// [`serialize`]: fn.serialize.html
+/// [`to_string`]: struct.Serializer.html#method.to_string
+/// [`Result<String>`]: type.Result.html
+///
+/// # Examples
+///
+/// Serializing without a type annotation:
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde_dhall::serialize;
+///
+/// let string = serialize(&1i64).to_string()?;
+/// assert_eq!(string, "+1".to_string());
+/// # Ok(())
+/// # }
+/// ```
+///
+/// Serializing with an automatic type annotation:
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde_dhall::serialize;
+///
+/// let data: Option<u64> = None;
+/// let string = serialize(&data).static_type_annotation().to_string()?;
+/// assert_eq!(string, "None Natural".to_string());
+/// # Ok(())
+/// # }
+/// ```
+#[derive(Debug, Clone)]
+pub struct Serializer<'a, T, A> {
+    data: &'a T,
+    annot: A,
+}
+
+impl<'a, T> Serializer<'a, T, NoAnnot> {
+    /// Provides a type to the serialization process. The provided value will be checked against
+    /// that type, and the type will be used when Dhall needs it, like for empty lists or for
+    /// unions.
+    ///
+    /// 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 serde_dhall::{serialize, from_str, SimpleType, SimpleValue};
+    ///
+    /// let ty = from_str("< A | B: Bool >").parse()?;
+    /// let data = SimpleValue::Union("A".to_string(), None);
+    /// let string = serialize(&data)
+    ///     .type_annotation(&ty)
+    ///     .to_string()?;
+    /// assert_eq!(string, "< A | B: Bool >.A".to_string());
+    ///
+    /// // Invalid data fails the type validation; serialization would have succeeded otherwise.
+    /// let ty = SimpleType::Integer;
+    /// assert!(
+    ///     serialize(&Some(0u64))
+    ///         .type_annotation(&ty)
+    ///         .to_string()
+    ///         .is_err()
+    /// );
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// [`static_type_annotation`]: struct.Serializer.html#method.static_type_annotation
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn type_annotation<'ty>(
+        self,
+        ty: &'ty SimpleType,
+    ) -> Serializer<'a, T, ManualAnnot<'ty>> {
+        Serializer {
+            annot: ManualAnnot(ty),
+            data: self.data,
+        }
+    }
+
+    /// Uses the type of `T` in the serialization process. This will be used when Dhall needs it,
+    /// like for empty lists or for unions.
+    ///
+    /// `T` must implement the [`StaticType`] trait. If it doesn't, you can use [`type_annotation`]
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// use serde::Serialize;
+    /// use serde_dhall::{serialize, StaticType};
+    ///
+    /// #[derive(Serialize, StaticType)]
+    /// enum MyOption {
+    ///     MyNone,
+    ///     MySome(u64),
+    /// }
+    ///
+    /// let data = MyOption::MySome(0);
+    /// let string = serialize(&data)
+    ///     .static_type_annotation()
+    ///     .to_string()?;
+    /// // The resulting Dhall string depends on the type annotation; it could not have been
+    /// // printed without it.
+    /// assert_eq!(string, "< MyNone | MySome: Natural >.MySome 0".to_string());
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// [`type_annotation`]: struct.Serializer.html#method.type_annotation
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn static_type_annotation(self) -> Serializer<'a, T, StaticAnnot> {
+        Serializer {
+            annot: StaticAnnot,
+            data: self.data,
+        }
+    }
+}
+
+impl<'a, T, A> Serializer<'a, T, A>
+where
+    A: TypeAnnot,
+{
+    /// Prints the chosen value with the options provided.
+    ///
+    /// If you enabled static annotations, `T` is required to implement [`StaticType`].
+    ///
+    /// Note that if you do not provide a type annotation, some values may not be convertible to
+    /// Dhall, like empty lists or enums.
+    ///
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # fn main() -> serde_dhall::Result<()> {
+    /// use serde_dhall::serialize;
+    ///
+    /// let string = serialize(&1i64).static_type_annotation().to_string()?;
+    /// assert_eq!(string, "+1".to_string());
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// [`StaticType`]: trait.StaticType.html
+    pub fn to_string(&self) -> Result<String>
+    where
+        T: ToDhall + HasAnnot<A>,
+    {
+        let val = self.data.to_dhall(T::get_annot(self.annot).as_ref())?;
+        Ok(val.to_string())
+    }
+}
+
+/// Serialize a value to a string of Dhall text.
+///
+/// This returns a [`Serializer`] object. Call the [`to_string`] method to get the serialized
+/// value, or use other [`Serializer`] methods to control the serialization process.
+///
+/// In order to process certain values (like unions or empty lists) correctly, it is necessary to
+/// add a type annotation (with [`static_type_annotation`] or [`type_annotation`]).
+///
+/// # Examples
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde::Serialize;
+/// use serde_dhall::{serialize, StaticType};
+///
+/// #[derive(Serialize)]
+/// struct Point {
+///     x: u64,
+///     y: u64,
+/// }
+///
+///
+/// let data = Point { x: 0, y: 0 };
+/// let string = serialize(&data).to_string()?;
+/// assert_eq!(string, "{ x = 0, y = 0 }");
+/// # Ok(())
+/// # }
+/// ```
+///
+/// ```rust
+/// # fn main() -> serde_dhall::Result<()> {
+/// use serde::Serialize;
+/// use serde_dhall::{serialize, StaticType};
+///
+/// #[derive(Serialize, StaticType)]
+/// enum MyOption {
+///     MyNone,
+///     MySome(u64),
+/// }
+///
+/// let data = MyOption::MySome(0);
+/// let string = serialize(&data)
+///     .static_type_annotation()
+///     .to_string()?;
+/// // The resulting Dhall string depends on the type annotation; it could not have been
+/// // printed without it.
+/// assert_eq!(string, "< MyNone | MySome: Natural >.MySome 0".to_string());
+/// # Ok(())
+/// # }
+/// ```
+///
+/// [`Serializer`]: struct.Serializer.html
+/// [`type_annotation`]: struct.Serializer.html#method.type_annotation
+/// [`static_type_annotation`]: struct.Serializer.html#method.static_type_annotation
+/// [`to_string`]: struct.Serializer.html#method.to_string
+pub fn serialize<T>(data: &T) -> Serializer<'_, T, NoAnnot>
+where
+    T: ToDhall,
+{
+    Serializer {
+        data,
+        annot: NoAnnot,
+    }
+}
diff --git a/serde_dhall/src/serialize.rs b/serde_dhall/src/serialize.rs
new file mode 100644
index 0000000..286c04d
--- /dev/null
+++ b/serde_dhall/src/serialize.rs
@@ -0,0 +1,396 @@
+use serde::ser;
+use std::collections::BTreeMap;
+
+use dhall::syntax::NumKind;
+
+use crate::value::SimpleValue;
+use crate::{Error, ErrorKind, Result, SimpleType, Value};
+use SimpleValue::*;
+
+pub trait Sealed {}
+
+/// A data structure that can be serialized from a Dhall expression.
+///
+/// This is automatically implemented for any type that [serde] can serialize.
+/// 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::Serialize;
+///
+/// // Use serde's derive
+/// #[derive(Serialize)]
+/// struct Point {
+///     x: u64,
+///     y: u64,
+/// }
+///
+/// // Convert a Point to a Dhall string.
+/// let point = Point { x: 0, y: 0 };
+/// let point_str = serde_dhall::serialize(&point).to_string()?;
+/// assert_eq!(point_str, "{ x = 0, y = 0 }".to_string());
+/// # Ok(())
+/// # }
+/// ```
+///
+/// [serde]: https://serde.rs
+pub trait ToDhall: Sealed {
+    #[doc(hidden)]
+    fn to_dhall(&self, ty: Option<&SimpleType>) -> Result<Value>;
+}
+
+impl<T> Sealed for T where T: ser::Serialize {}
+
+impl<T> ToDhall for T
+where
+    T: ser::Serialize,
+{
+    fn to_dhall(&self, ty: Option<&SimpleType>) -> Result<Value> {
+        let sval: SimpleValue = self.serialize(Serializer)?;
+        sval.into_value(ty)
+    }
+}
+
+#[derive(Default, Clone, Copy)]
+struct Serializer;
+
+impl ser::Serializer for Serializer {
+    type Ok = SimpleValue;
+    type Error = Error;
+
+    type SerializeSeq = SeqSerializer;
+    type SerializeTuple = TupleSerializer;
+    type SerializeTupleStruct = ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeTupleVariant = ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeMap = MapSerializer;
+    type SerializeStruct = StructSerializer;
+    type SerializeStructVariant = ser::Impossible<Self::Ok, Self::Error>;
+
+    fn serialize_bool(self, v: bool) -> Result<Self::Ok> {
+        Ok(Num(NumKind::Bool(v)))
+    }
+
+    fn serialize_i8(self, v: i8) -> Result<Self::Ok> {
+        self.serialize_i64(i64::from(v))
+    }
+    fn serialize_i16(self, v: i16) -> Result<Self::Ok> {
+        self.serialize_i64(i64::from(v))
+    }
+    fn serialize_i32(self, v: i32) -> Result<Self::Ok> {
+        self.serialize_i64(i64::from(v))
+    }
+    fn serialize_i64(self, v: i64) -> Result<Self::Ok> {
+        Ok(Num(NumKind::Integer(v)))
+    }
+
+    fn serialize_u8(self, v: u8) -> Result<Self::Ok> {
+        self.serialize_u64(u64::from(v))
+    }
+    fn serialize_u16(self, v: u16) -> Result<Self::Ok> {
+        self.serialize_u64(u64::from(v))
+    }
+    fn serialize_u32(self, v: u32) -> Result<Self::Ok> {
+        self.serialize_u64(u64::from(v))
+    }
+    fn serialize_u64(self, v: u64) -> Result<Self::Ok> {
+        Ok(Num(NumKind::Natural(v)))
+    }
+
+    fn serialize_f32(self, v: f32) -> Result<Self::Ok> {
+        self.serialize_f64(f64::from(v))
+    }
+    fn serialize_f64(self, v: f64) -> Result<Self::Ok> {
+        Ok(Num(NumKind::Double(v.into())))
+    }
+
+    fn serialize_char(self, v: char) -> Result<Self::Ok> {
+        self.serialize_str(&v.to_string())
+    }
+    fn serialize_str(self, v: &str) -> Result<Self::Ok> {
+        Ok(Text(v.to_owned()))
+    }
+
+    fn serialize_bytes(self, _v: &[u8]) -> Result<Self::Ok> {
+        Err(ErrorKind::Serialize(
+            "Unsupported data for serialization: byte array".to_owned(),
+        )
+        .into())
+    }
+
+    fn serialize_none(self) -> Result<Self::Ok> {
+        Ok(Optional(None))
+    }
+    fn serialize_some<T>(self, v: &T) -> Result<Self::Ok>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        Ok(Optional(Some(Box::new(v.serialize(self)?))))
+    }
+
+    fn serialize_unit(self) -> Result<Self::Ok> {
+        Ok(Record(Default::default()))
+    }
+
+    fn serialize_unit_struct(self, _name: &'static str) -> Result<Self::Ok> {
+        self.serialize_unit()
+    }
+    fn serialize_newtype_struct<T>(
+        self,
+        _name: &'static str,
+        _value: &T,
+    ) -> Result<Self::Ok>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        Err(ErrorKind::Serialize(
+            "Unsupported data for serialization: newtype struct".to_owned(),
+        )
+        .into())
+    }
+    fn serialize_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStruct> {
+        Ok(StructSerializer::default())
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        variant: &'static str,
+    ) -> Result<Self::Ok> {
+        Ok(Union(variant.to_owned(), None))
+    }
+    fn serialize_newtype_variant<T>(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        variant: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        let value = value.serialize(self)?;
+        Ok(Union(variant.to_owned(), Some(Box::new(value))))
+    }
+    fn serialize_tuple_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleVariant> {
+        Err(ErrorKind::Serialize(
+            "Unsupported data for serialization: tuple variant".to_owned(),
+        )
+        .into())
+    }
+    fn serialize_struct_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStructVariant> {
+        Err(ErrorKind::Serialize(
+            "Unsupported data for serialization: struct variant".to_owned(),
+        )
+        .into())
+    }
+
+    fn serialize_tuple(self, _len: usize) -> Result<Self::SerializeTuple> {
+        Ok(TupleSerializer::default())
+    }
+    fn serialize_tuple_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleStruct> {
+        Err(ErrorKind::Serialize(
+            "Unsupported data for serialization: tuple struct".to_owned(),
+        )
+        .into())
+    }
+
+    fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq> {
+        Ok(SeqSerializer::default())
+    }
+
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap> {
+        Ok(MapSerializer::default())
+    }
+}
+
+#[derive(Default)]
+struct SeqSerializer(Vec<SimpleValue>);
+
+impl ser::SerializeSeq for SeqSerializer {
+    type Ok = SimpleValue;
+    type Error = Error;
+
+    fn serialize_element<T>(&mut self, value: &T) -> Result<()>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        self.0.push(value.serialize(Serializer)?);
+        Ok(())
+    }
+
+    fn end(self) -> Result<Self::Ok> {
+        Ok(List(self.0))
+    }
+}
+
+#[derive(Default)]
+struct TupleSerializer(Vec<SimpleValue>);
+
+impl ser::SerializeTuple for TupleSerializer {
+    type Ok = SimpleValue;
+    type Error = Error;
+
+    fn serialize_element<T>(&mut self, value: &T) -> Result<()>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        self.0.push(value.serialize(Serializer)?);
+        Ok(())
+    }
+
+    fn end(self) -> Result<Self::Ok> {
+        Ok(Record(
+            self.0
+                .into_iter()
+                .enumerate()
+                .map(|(i, x)| (format!("_{}", i + 1), x))
+                .collect(),
+        ))
+    }
+}
+
+#[derive(Default)]
+struct MapSerializer {
+    map: BTreeMap<String, SimpleValue>,
+    key: Option<String>,
+    val: Option<SimpleValue>,
+}
+
+impl ser::SerializeMap for MapSerializer {
+    type Ok = SimpleValue;
+    type Error = Error;
+
+    fn serialize_key<T>(&mut self, key: &T) -> Result<()>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        let key = match key.serialize(Serializer)? {
+            Text(key) => key,
+            _ => return Err(<Error as ser::Error>::custom("not a string")),
+        };
+        if let Some(val) = self.val.take() {
+            self.map.insert(key, val);
+        } else {
+            self.key = Some(key);
+        }
+        Ok(())
+    }
+
+    fn serialize_value<T>(&mut self, val: &T) -> Result<()>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        let val: SimpleValue = val.serialize(Serializer)?;
+        if let Some(key) = self.key.take() {
+            self.map.insert(key, val);
+        } else {
+            self.val = Some(val);
+        }
+        Ok(())
+    }
+
+    fn end(self) -> Result<Self::Ok> {
+        Ok(Record(self.map))
+    }
+}
+
+#[derive(Default)]
+struct StructSerializer(BTreeMap<String, SimpleValue>);
+
+impl ser::SerializeStruct for StructSerializer {
+    type Ok = SimpleValue;
+    type Error = Error;
+
+    fn serialize_field<T>(&mut self, key: &'static str, val: &T) -> Result<()>
+    where
+        T: ?Sized + ser::Serialize,
+    {
+        let val: SimpleValue = val.serialize(Serializer)?;
+        self.0.insert(key.into(), val);
+        Ok(())
+    }
+
+    fn end(self) -> Result<Self::Ok> {
+        Ok(Record(self.0))
+    }
+}
+
+impl serde::ser::Serialize for SimpleValue {
+    fn serialize<S>(
+        &self,
+        serializer: S,
+    ) -> std::result::Result<S::Ok, S::Error>
+    where
+        S: serde::ser::Serializer,
+    {
+        use serde::ser::{SerializeMap, SerializeSeq};
+        use NumKind::*;
+        use SimpleValue::*;
+
+        match self {
+            Num(Bool(x)) => serializer.serialize_bool(*x),
+            Num(Natural(x)) => serializer.serialize_u64(*x),
+            Num(Integer(x)) => serializer.serialize_i64(*x),
+            Num(Double(x)) => serializer.serialize_f64((*x).into()),
+            Text(x) => serializer.serialize_str(x),
+            List(xs) => {
+                let mut seq = serializer.serialize_seq(Some(xs.len()))?;
+                for x in xs {
+                    seq.serialize_element(x)?;
+                }
+                seq.end()
+            }
+            Optional(None) => serializer.serialize_none(),
+            Optional(Some(x)) => serializer.serialize_some(x),
+            Record(m) => {
+                let mut map = serializer.serialize_map(Some(m.len()))?;
+                for (k, v) in m {
+                    map.serialize_entry(k, v)?;
+                }
+                map.end()
+            }
+            // serde's enum support is yet again really limited. We can't avoid a memleak here :(.
+            Union(field_name, None) => {
+                let field_name: Box<str> = field_name.clone().into();
+                serializer.serialize_unit_variant(
+                    "SimpleValue",
+                    0,
+                    Box::leak(field_name),
+                )
+            }
+            Union(field_name, Some(x)) => {
+                let field_name: Box<str> = field_name.clone().into();
+                serializer.serialize_newtype_variant(
+                    "SimpleValue",
+                    0,
+                    Box::leak(field_name),
+                    x,
+                )
+            }
+        }
+    }
+}
diff --git a/serde_dhall/src/static_type.rs b/serde_dhall/src/static_type.rs
index 3c5da18..586cd4d 100644
--- a/serde_dhall/src/static_type.rs
+++ b/serde_dhall/src/static_type.rs
@@ -10,6 +10,10 @@ use crate::SimpleType;
 /// corresponds to them. For example, `HashMap<String, u64>` could correspond to multiple different
 /// Dhall types, e.g. `{ foo: Natural, bar: Natural }` and `{ baz: Natural }`.
 ///
+/// See also [the table of type correspondances].
+///
+/// [the table of type correspondances]: enum.SimpleType.html#type-correspondence
+///
 /// # Example
 ///
 /// ```rust
@@ -29,28 +33,6 @@ use crate::SimpleType;
 /// # Ok(())
 /// # }
 /// ```
-///
-/// # 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
-/// `Prelude.Map.Type Text T`  | `HashMap<String, T>`, structs
-/// `T -> U`  | unsupported
-/// `Prelude.JSON.Type`  | unsupported
-/// `Prelude.Map.Type T U`  | unsupported
 pub trait StaticType {
     /// Return the Dhall type that represents this type.
     ///
@@ -103,6 +85,26 @@ derive_builtin!(i32, Integer);
 derive_builtin!(f64, Double);
 derive_builtin!(f32, Double);
 derive_builtin!(String, Text);
+derive_builtin!(&str, Text);
+
+impl StaticType for () {
+    fn static_type() -> SimpleType {
+        SimpleType::Record(vec![].into_iter().collect())
+    }
+}
+
+impl<A> StaticType for (A,)
+where
+    A: StaticType,
+{
+    fn static_type() -> SimpleType {
+        SimpleType::Record(
+            vec![("_1".to_owned(), A::static_type())]
+                .into_iter()
+                .collect(),
+        )
+    }
+}
 
 impl<A, B> StaticType for (A, B)
 where
@@ -121,6 +123,46 @@ where
     }
 }
 
+impl<A, B, C> StaticType for (A, B, C)
+where
+    A: StaticType,
+    B: StaticType,
+    C: StaticType,
+{
+    fn static_type() -> SimpleType {
+        SimpleType::Record(
+            vec![
+                ("_1".to_owned(), A::static_type()),
+                ("_2".to_owned(), B::static_type()),
+                ("_3".to_owned(), C::static_type()),
+            ]
+            .into_iter()
+            .collect(),
+        )
+    }
+}
+
+impl<A, B, C, D> StaticType for (A, B, C, D)
+where
+    A: StaticType,
+    B: StaticType,
+    C: StaticType,
+    D: StaticType,
+{
+    fn static_type() -> SimpleType {
+        SimpleType::Record(
+            vec![
+                ("_1".to_owned(), A::static_type()),
+                ("_2".to_owned(), B::static_type()),
+                ("_3".to_owned(), C::static_type()),
+                ("_4".to_owned(), D::static_type()),
+            ]
+            .into_iter()
+            .collect(),
+        )
+    }
+}
+
 impl<T, E> StaticType for std::result::Result<T, E>
 where
     T: StaticType,
diff --git a/serde_dhall/src/value.rs b/serde_dhall/src/value.rs
index d7116d9..b8ad199 100644
--- a/serde_dhall/src/value.rs
+++ b/serde_dhall/src/value.rs
@@ -6,7 +6,7 @@ use dhall::semantics::{Hir, HirKind, Nir, NirKind};
 pub use dhall::syntax::NumKind;
 use dhall::syntax::{Expr, ExprKind, Span};
 
-use crate::{Error, ErrorKind, FromDhall, Result, Sealed};
+use crate::{Error, ErrorKind, FromDhall, Result, ToDhall};
 
 #[doc(hidden)]
 /// An arbitrary Dhall value.
@@ -124,6 +124,28 @@ pub enum SimpleValue {
 /// [`StaticType`]: trait.StaticType.html
 /// [`Deserializer::static_type_annotation`]: options/struct.Deserializer.html#method.static_type_annotation
 ///
+/// # 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
+/// `Prelude.Map.Type Text T`  | `HashMap<String, T>`, structs
+/// `T -> U`  | unsupported
+/// `Prelude.JSON.Type`  | unsupported
+/// `Prelude.Map.Type T U`  | unsupported
+///
 /// # Examples
 ///
 /// ```rust
@@ -159,7 +181,6 @@ pub enum SimpleValue {
 /// # Ok(())
 /// # }
 /// ```
-///
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum SimpleType {
     /// Corresponds to the Dhall type `Bool`
@@ -292,6 +313,110 @@ impl SimpleValue {
             _ => return None,
         })
     }
+
+    // Converts this to `Hir`, using the optional type annotation. Without the type, things like
+    // empty lists and unions will fail to convert.
+    fn to_hir(&self, ty: Option<&SimpleType>) -> Result<Hir> {
+        use SimpleType as T;
+        use SimpleValue as V;
+        let hir = |k| Hir::new(HirKind::Expr(k), Span::Artificial);
+        let type_error = || {
+            Error(ErrorKind::Serialize(format!(
+                "expected a value of type {}, found {:?}",
+                ty.unwrap().to_hir().to_expr(Default::default()),
+                self
+            )))
+        };
+        let type_missing = || {
+            Error(ErrorKind::Serialize(format!(
+                "cannot serialize value without a type annotation: {:?}",
+                self
+            )))
+        };
+        let kind = match (self, ty) {
+            (V::Num(num @ NumKind::Bool(_)), Some(T::Bool))
+            | (V::Num(num @ NumKind::Natural(_)), Some(T::Natural))
+            | (V::Num(num @ NumKind::Integer(_)), Some(T::Integer))
+            | (V::Num(num @ NumKind::Double(_)), Some(T::Double))
+            | (V::Num(num), None) => ExprKind::Num(num.clone()),
+            (V::Text(v), Some(T::Text)) | (V::Text(v), None) => {
+                ExprKind::TextLit(v.clone().into())
+            }
+
+            (V::Optional(None), None) => return Err(type_missing()),
+            (V::Optional(None), Some(T::Optional(t))) => {
+                ExprKind::Op(OpKind::App(
+                    hir(ExprKind::Builtin(Builtin::OptionalNone)),
+                    t.to_hir(),
+                ))
+            }
+            (V::Optional(Some(v)), None) => ExprKind::SomeLit(v.to_hir(None)?),
+            (V::Optional(Some(v)), Some(T::Optional(t))) => {
+                ExprKind::SomeLit(v.to_hir(Some(t))?)
+            }
+
+            (V::List(v), None) if v.is_empty() => return Err(type_missing()),
+            (V::List(v), Some(T::List(t))) if v.is_empty() => {
+                ExprKind::EmptyListLit(hir(ExprKind::Op(OpKind::App(
+                    hir(ExprKind::Builtin(Builtin::List)),
+                    t.to_hir(),
+                ))))
+            }
+            (V::List(v), None) => ExprKind::NEListLit(
+                v.iter().map(|v| v.to_hir(None)).collect::<Result<_>>()?,
+            ),
+            (V::List(v), Some(T::List(t))) => ExprKind::NEListLit(
+                v.iter().map(|v| v.to_hir(Some(t))).collect::<Result<_>>()?,
+            ),
+
+            (V::Record(v), None) => ExprKind::RecordLit(
+                v.iter()
+                    .map(|(k, v)| Ok((k.clone().into(), v.to_hir(None)?)))
+                    .collect::<Result<_>>()?,
+            ),
+            (V::Record(v), Some(T::Record(t))) => ExprKind::RecordLit(
+                v.iter()
+                    .map(|(k, v)| match t.get(k) {
+                        Some(t) => Ok((k.clone().into(), v.to_hir(Some(t))?)),
+                        None => Err(type_error()),
+                    })
+                    .collect::<Result<_>>()?,
+            ),
+
+            (V::Union(..), None) => return Err(type_missing()),
+            (V::Union(variant, Some(v)), Some(T::Union(t))) => {
+                match t.get(variant) {
+                    Some(Some(variant_t)) => ExprKind::Op(OpKind::App(
+                        hir(ExprKind::Op(OpKind::Field(
+                            ty.unwrap().to_hir(),
+                            variant.clone().into(),
+                        ))),
+                        v.to_hir(Some(variant_t))?,
+                    )),
+                    _ => return Err(type_error()),
+                }
+            }
+            (V::Union(variant, None), Some(T::Union(t))) => {
+                match t.get(variant) {
+                    Some(None) => ExprKind::Op(OpKind::Field(
+                        ty.unwrap().to_hir(),
+                        variant.clone().into(),
+                    )),
+                    _ => return Err(type_error()),
+                }
+            }
+
+            (_, Some(_)) => return Err(type_error()),
+        };
+        Ok(hir(kind))
+    }
+    pub(crate) fn into_value(self, ty: Option<&SimpleType>) -> Result<Value> {
+        Ok(Value {
+            hir: self.to_hir(ty)?,
+            as_simple_val: Some(self),
+            as_simple_ty: None,
+        })
+    }
 }
 
 impl SimpleType {
@@ -376,8 +501,9 @@ impl SimpleType {
     }
 }
 
-impl Sealed for Value {}
-impl Sealed for SimpleType {}
+impl crate::deserialize::Sealed for Value {}
+impl crate::deserialize::Sealed for SimpleType {}
+impl crate::serialize::Sealed for Value {}
 
 impl FromDhall for Value {
     fn from_dhall(v: &Value) -> Result<Self> {
@@ -394,6 +520,11 @@ impl FromDhall for SimpleType {
         })
     }
 }
+impl ToDhall for Value {
+    fn to_dhall(&self, _ty: Option<&SimpleType>) -> Result<Value> {
+        Ok(self.clone())
+    }
+}
 
 impl Eq for Value {}
 impl PartialEq for Value {
diff --git a/serde_dhall/tests/de.rs b/serde_dhall/tests/de.rs
deleted file mode 100644
index 3abaad2..0000000
--- a/serde_dhall/tests/de.rs
+++ /dev/null
@@ -1,150 +0,0 @@
-use serde::Deserialize;
-use serde_dhall::{from_str, FromDhall, NumKind, SimpleValue, StaticType};
-
-#[test]
-fn test_de_typed() {
-    fn parse<T: FromDhall + StaticType>(s: &str) -> T {
-        from_str(s).static_type_annotation().parse().unwrap()
-    }
-
-    assert_eq!(parse::<bool>("True"), true);
-
-    assert_eq!(parse::<u64>("1"), 1);
-    assert_eq!(parse::<u32>("1"), 1);
-    assert_eq!(parse::<usize>("1"), 1);
-
-    assert_eq!(parse::<i64>("+1"), 1);
-    assert_eq!(parse::<i32>("+1"), 1);
-    assert_eq!(parse::<isize>("+1"), 1);
-
-    assert_eq!(parse::<f64>("1.0"), 1.0);
-    assert_eq!(parse::<f32>("1.0"), 1.0);
-
-    assert_eq!(parse::<String>(r#""foo""#), "foo".to_owned());
-    assert_eq!(parse::<Vec<u64>>("[] : List Natural"), <Vec<u64>>::new());
-    assert_eq!(parse::<Vec<u64>>("[1, 2]"), vec![1, 2]);
-    assert_eq!(parse::<Option<u64>>("None Natural"), None);
-    assert_eq!(parse::<Option<u64>>("Some 1"), Some(1));
-
-    assert_eq!(
-        parse::<(u64, String)>(r#"{ _1 = 1, _2 = "foo" }"#),
-        (1, "foo".to_owned())
-    );
-
-    #[derive(Debug, PartialEq, Eq, Deserialize, StaticType)]
-    struct Foo {
-        x: u64,
-        y: i64,
-    }
-    assert_eq!(parse::<Foo>("{ x = 1, y = -2 }"), Foo { x: 1, y: -2 });
-
-    #[derive(Debug, PartialEq, Eq, Deserialize, StaticType)]
-    enum Bar {
-        X(u64),
-        Y(i64),
-    }
-    assert_eq!(parse::<Bar>("< X: Natural | Y: Integer >.X 1"), Bar::X(1));
-
-    #[derive(Debug, PartialEq, Eq, Deserialize, StaticType)]
-    enum Baz {
-        X,
-        Y(i64),
-    }
-    assert_eq!(parse::<Baz>("< X | Y: Integer >.X"), Baz::X);
-
-    assert!(from_str("< X | Y: Integer >.Y")
-        .static_type_annotation()
-        .parse::<Baz>()
-        .is_err());
-}
-
-#[test]
-fn test_de_untyped() {
-    use std::collections::BTreeMap;
-    use std::collections::HashMap;
-
-    fn parse<T: FromDhall>(s: &str) -> T {
-        from_str(s).parse().unwrap()
-    }
-
-    // Test tuples on record of wrong type
-    assert_eq!(
-        parse::<(u64, String, i64)>(r#"{ y = "foo", x = 1, z = +42 }"#),
-        (1, "foo".to_owned(), 42)
-    );
-
-    let mut expected_map = HashMap::new();
-    expected_map.insert("x".to_string(), 1);
-    expected_map.insert("y".to_string(), 2);
-    assert_eq!(
-        parse::<HashMap<String, u64>>("{ x = 1, y = 2 }"),
-        expected_map
-    );
-    assert_eq!(
-        parse::<HashMap<String, u64>>("toMap { x = 1, y = 2 }"),
-        expected_map
-    );
-
-    let mut expected_map = HashMap::new();
-    expected_map.insert("if".to_string(), 1);
-    expected_map.insert("FOO_BAR".to_string(), 2);
-    expected_map.insert("baz-kux".to_string(), 3);
-    assert_eq!(
-        parse::<HashMap<String, u64>>("{ `if` = 1, FOO_BAR = 2, baz-kux = 3 }"),
-        expected_map
-    );
-
-    let mut expected_map = BTreeMap::new();
-    expected_map.insert("x".to_string(), 1);
-    expected_map.insert("y".to_string(), 2);
-    assert_eq!(
-        parse::<BTreeMap<String, u64>>("{ x = 1, y = 2 }"),
-        expected_map
-    );
-
-    #[derive(Debug, PartialEq, Eq, Deserialize)]
-    struct Foo {
-        x: u64,
-        y: Option<u64>,
-    }
-    // Omit optional field
-    assert_eq!(parse::<Foo>("{ x = 1 }"), Foo { x: 1, y: None });
-
-    // https://github.com/Nadrieril/dhall-rust/issues/155
-    assert!(from_str("List/length [True, 42]").parse::<bool>().is_err());
-}
-
-#[test]
-fn test_de_simplevalue() {
-    let bool_true = SimpleValue::Num(NumKind::Bool(true));
-    // https://github.com/Nadrieril/dhall-rust/issues/184
-    assert_eq!(
-        from_str("[ True ]").parse::<Vec<SimpleValue>>().unwrap(),
-        vec![bool_true.clone()]
-    );
-
-    assert_eq!(
-        from_str("< Foo >.Foo").parse::<SimpleValue>().unwrap(),
-        SimpleValue::Union("Foo".into(), None)
-    );
-    assert_eq!(
-        from_str("< Foo: Bool >.Foo True")
-            .parse::<SimpleValue>()
-            .unwrap(),
-        SimpleValue::Union("Foo".into(), Some(Box::new(bool_true.clone())))
-    );
-
-    #[derive(Debug, PartialEq, Eq, Deserialize)]
-    struct Foo {
-        foo: SimpleValue,
-    }
-    assert_eq!(
-        from_str("{ foo = True }").parse::<Foo>().unwrap(),
-        Foo {
-            foo: bool_true.clone()
-        },
-    );
-}
-
-// TODO: test various builder configurations
-// In particular test cloning and reusing builder
diff --git a/serde_dhall/tests/serde.rs b/serde_dhall/tests/serde.rs
new file mode 100644
index 0000000..4c184e7
--- /dev/null
+++ b/serde_dhall/tests/serde.rs
@@ -0,0 +1,210 @@
+mod serde {
+    use serde::{Deserialize, Serialize};
+    use serde_dhall::{from_str, serialize, FromDhall, StaticType, ToDhall};
+
+    fn assert_de<T>(s: &str, x: T)
+    where
+        T: FromDhall + StaticType + PartialEq + std::fmt::Debug,
+    {
+        assert_eq!(
+            from_str(s)
+                .static_type_annotation()
+                .parse::<T>()
+                .map_err(|e| e.to_string()),
+            Ok(x)
+        );
+    }
+    fn assert_ser<T>(s: &str, x: T)
+    where
+        T: ToDhall + StaticType + PartialEq + std::fmt::Debug,
+    {
+        assert_eq!(
+            serialize(&x)
+                .static_type_annotation()
+                .to_string()
+                .map_err(|e| e.to_string()),
+            Ok(s.to_string())
+        );
+    }
+    fn assert_serde<T>(s: &str, x: T)
+    where
+        T: ToDhall
+            + FromDhall
+            + StaticType
+            + PartialEq
+            + std::fmt::Debug
+            + Clone,
+    {
+        assert_de(s, x.clone());
+        assert_ser(s, x);
+    }
+
+    #[test]
+    fn numbers() {
+        assert_serde("True", true);
+
+        assert_serde("1", 1u64);
+        assert_serde("1", 1u32);
+        assert_serde("1", 1usize);
+
+        assert_serde("+1", 1i64);
+        assert_serde("+1", 1i32);
+        assert_serde("+1", 1isize);
+
+        assert_serde("1.0", 1.0f64);
+        assert_serde("1.0", 1.0f32);
+    }
+
+    #[test]
+    fn text() {
+        assert_serde(r#""foo""#, "foo".to_owned());
+        assert_ser(r#""foo""#, "foo");
+    }
+
+    #[test]
+    fn list() {
+        assert_serde("[] : List Natural", <Vec<u64>>::new());
+        assert_serde("[] : List Text", <Vec<String>>::new());
+        assert_serde(
+            r#"["foo", "bar"]"#,
+            vec!["foo".to_owned(), "bar".to_owned()],
+        );
+        assert_ser(r#"["foo", "bar"]"#, vec!["foo", "bar"]);
+        assert_serde::<Vec<u64>>("[1, 2]", vec![1, 2]);
+    }
+
+    #[test]
+    fn optional() {
+        assert_serde("None Natural", None::<u64>);
+        assert_serde("None Text", None::<String>);
+        assert_serde("Some 1", Some(1u64));
+        assert_eq!(
+            serialize(&None::<u64>).to_string().map_err(|e| e.to_string()),
+            Err("cannot serialize value without a type annotation: Optional(None)".to_string())
+        );
+    }
+
+    #[test]
+    fn tuple() {
+        assert_serde::<()>(r#"{=}"#, ());
+        assert_serde::<(u64, String)>(
+            r#"{ _1 = 1, _2 = "foo" }"#,
+            (1, "foo".to_owned()),
+        );
+        assert_serde::<(u64, u64, u64, u64)>(
+            r#"{ _1 = 1, _2 = 2, _3 = 3, _4 = 4 }"#,
+            (1, 2, 3, 4),
+        );
+    }
+
+    #[test]
+    fn structs() {
+        // #[derive(
+        //     Debug, Clone, PartialEq, Eq, Deserialize, Serialize, StaticType,
+        // )]
+        // struct Foo;
+        // assert_serde::<Foo>("{=}", Foo);
+
+        // #[derive(
+        //     Debug, Clone, PartialEq, Eq, Deserialize, Serialize, StaticType,
+        // )]
+        // struct Bar(u64);
+        // assert_serde::<Bar>("{ _1 = 1 }", Bar (1));
+
+        #[derive(
+            Debug, Clone, PartialEq, Eq, Deserialize, Serialize, StaticType,
+        )]
+        struct Baz {
+            x: u64,
+            y: i64,
+        }
+        assert_serde::<Baz>("{ x = 1, y = -2 }", Baz { x: 1, y: -2 });
+    }
+
+    #[test]
+    fn enums() {
+        #[derive(
+            Debug, Clone, PartialEq, Eq, Deserialize, Serialize, StaticType,
+        )]
+        enum Foo {
+            X(u64),
+            Y(i64),
+        }
+        assert_serde::<Foo>("< X: Natural | Y: Integer >.X 1", Foo::X(1));
+
+        #[derive(
+            Debug, Clone, PartialEq, Eq, Deserialize, Serialize, StaticType,
+        )]
+        enum Bar {
+            X,
+            Y(i64),
+        }
+        assert_serde::<Bar>("< X | Y: Integer >.X", Bar::X);
+
+        assert!(from_str("< X | Y: Integer >.Y")
+            .static_type_annotation()
+            .parse::<Bar>()
+            .is_err());
+    }
+
+    #[test]
+    fn test_de_untyped() {
+        use std::collections::BTreeMap;
+        use std::collections::HashMap;
+
+        fn parse<T: FromDhall>(s: &str) -> T {
+            from_str(s).parse().unwrap()
+        }
+
+        // Test tuples on record of wrong type. Fields are just taken in alphabetic order.
+        assert_eq!(
+            parse::<(u64, String, i64)>(r#"{ y = "foo", x = 1, z = +42 }"#),
+            (1, "foo".to_owned(), 42)
+        );
+
+        let mut expected_map = HashMap::new();
+        expected_map.insert("x".to_string(), 1);
+        expected_map.insert("y".to_string(), 2);
+        assert_eq!(
+            parse::<HashMap<String, u64>>("{ x = 1, y = 2 }"),
+            expected_map
+        );
+        assert_eq!(
+            parse::<HashMap<String, u64>>("toMap { x = 1, y = 2 }"),
+            expected_map
+        );
+
+        let mut expected_map = HashMap::new();
+        expected_map.insert("if".to_string(), 1);
+        expected_map.insert("FOO_BAR".to_string(), 2);
+        expected_map.insert("baz-kux".to_string(), 3);
+        assert_eq!(
+            parse::<HashMap<String, u64>>(
+                "{ `if` = 1, FOO_BAR = 2, baz-kux = 3 }"
+            ),
+            expected_map
+        );
+
+        let mut expected_map = BTreeMap::new();
+        expected_map.insert("x".to_string(), 1);
+        expected_map.insert("y".to_string(), 2);
+        assert_eq!(
+            parse::<BTreeMap<String, u64>>("{ x = 1, y = 2 }"),
+            expected_map
+        );
+
+        #[derive(Debug, PartialEq, Eq, Deserialize)]
+        struct Foo {
+            x: u64,
+            y: Option<u64>,
+        }
+        // Omit optional field
+        assert_eq!(parse::<Foo>("{ x = 1 }"), Foo { x: 1, y: None });
+
+        // https://github.com/Nadrieril/dhall-rust/issues/155
+        assert!(from_str("List/length [True, 42]").parse::<bool>().is_err());
+    }
+
+    // TODO: test various builder configurations
+    // In particular test cloning and reusing builder
+}
diff --git a/serde_dhall/tests/simple_value.rs b/serde_dhall/tests/simple_value.rs
new file mode 100644
index 0000000..3bd9d64
--- /dev/null
+++ b/serde_dhall/tests/simple_value.rs
@@ -0,0 +1,58 @@
+mod simple_value {
+    use serde::{Deserialize, Serialize};
+    use serde_dhall::{
+        from_str, serialize, FromDhall, NumKind, SimpleValue, ToDhall,
+    };
+
+    fn assert_de<T>(s: &str, x: T)
+    where
+        T: FromDhall + PartialEq + std::fmt::Debug,
+    {
+        assert_eq!(from_str(s).parse::<T>().map_err(|e| e.to_string()), Ok(x));
+    }
+    fn assert_ser<T>(s: &str, x: T)
+    where
+        T: ToDhall + PartialEq + std::fmt::Debug,
+    {
+        assert_eq!(
+            serialize(&x).to_string().map_err(|e| e.to_string()),
+            Ok(s.to_string())
+        );
+    }
+    fn assert_serde<T>(s: &str, x: T)
+    where
+        T: ToDhall + FromDhall + PartialEq + std::fmt::Debug + Clone,
+    {
+        assert_de(s, x.clone());
+        assert_ser(s, x);
+    }
+
+    #[test]
+    fn test_serde() {
+        let bool_true = SimpleValue::Num(NumKind::Bool(true));
+        // https://github.com/Nadrieril/dhall-rust/issues/184
+        assert_serde("[True]", vec![bool_true.clone()]);
+
+        assert_de("< Foo >.Foo", SimpleValue::Union("Foo".into(), None));
+        assert_de(
+            "< Foo: Bool >.Foo True",
+            SimpleValue::Union("Foo".into(), Some(Box::new(bool_true.clone()))),
+        );
+
+        assert_eq!(
+            serialize(&SimpleValue::Union("Foo".into(), None)).to_string().map_err(|e| e.to_string()),
+            Err("cannot serialize value without a type annotation: Union(\"Foo\", None)".to_string())
+        );
+
+        #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+        struct Foo {
+            foo: SimpleValue,
+        }
+        assert_serde(
+            "{ foo = True }",
+            Foo {
+                foo: bool_true.clone(),
+            },
+        );
+    }
+}