Make `SimpleValue` deserializable within other types

Fixes https://github.com/Nadrieril/dhall-rust/issues/184

2 files changed, 112 insertions, 18 deletions

diff --git a/serde_dhall/src/deserialize.rs b/serde_dhall/src/deserialize.rs
index 6d3aab8..1206033 100644
--- a/serde_dhall/src/deserialize.rs
+++ b/serde_dhall/src/deserialize.rs
@@ -1,12 +1,17 @@
+use std::borrow::Cow;
+use std::collections::BTreeMap;
+use std::convert::TryInto as _;
+use std::fmt;
+
 use serde::de::value::{
     MapAccessDeserializer, MapDeserializer, SeqDeserializer,
 };
-use std::borrow::Cow;
+use serde::de::VariantAccess as _;
 
 use dhall::syntax::NumKind;
 
 use crate::value::SimpleValue;
-use crate::{Error, ErrorKind, Result, Value};
+use crate::{Error, ErrorKind, Value};
 
 pub trait Sealed {}
 
@@ -38,7 +43,7 @@ pub trait Sealed {}
 /// [serde]: https://serde.rs
 pub trait FromDhall: Sealed + Sized {
     #[doc(hidden)]
-    fn from_dhall(v: &Value) -> Result<Self>;
+    fn from_dhall(v: &Value) -> crate::Result<Self>;
 }
 
 impl<T> Sealed for T where T: serde::de::DeserializeOwned {}
@@ -83,7 +88,7 @@ struct Deserializer<'a>(Cow<'a, SimpleValue>);
 /// ```
 ///
 /// [`SimpleValue`]: enum.SimpleValue.html
-pub fn from_simple_value<T>(v: SimpleValue) -> Result<T>
+pub fn from_simple_value<T>(v: SimpleValue) -> crate::Result<T>
 where
     T: serde::de::DeserializeOwned,
 {
@@ -94,7 +99,7 @@ impl<T> FromDhall for T
 where
     T: serde::de::DeserializeOwned,
 {
-    fn from_dhall(v: &Value) -> Result<Self> {
+    fn from_dhall(v: &Value) -> crate::Result<Self> {
         let sval = v.to_simple_value().ok_or_else(|| {
             Error(ErrorKind::Deserialize(format!(
                 "this cannot be deserialized into the serde data model: {}",
@@ -115,7 +120,7 @@ impl<'de: 'a, 'a> serde::de::IntoDeserializer<'de, Error> for Deserializer<'a> {
 impl<'de: 'a, 'a> serde::Deserializer<'de> for Deserializer<'a> {
     type Error = Error;
 
-    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value>
+    fn deserialize_any<V>(self, visitor: V) -> crate::Result<V::Value>
     where
         V: serde::de::Visitor<'de>,
     {
@@ -167,7 +172,11 @@ impl<'de: 'a, 'a> serde::Deserializer<'de> for Deserializer<'a> {
         }
     }
 
-    fn deserialize_tuple<V>(self, _: usize, visitor: V) -> Result<V::Value>
+    fn deserialize_tuple<V>(
+        self,
+        _: usize,
+        visitor: V,
+    ) -> crate::Result<V::Value>
     where
         V: serde::de::Visitor<'de>,
     {
@@ -186,3 +195,99 @@ impl<'de: 'a, 'a> serde::Deserializer<'de> for Deserializer<'a> {
         tuple_struct map struct enum identifier ignored_any
     }
 }
+
+struct SimpleValueVisitor;
+
+impl<'de> serde::de::Visitor<'de> for SimpleValueVisitor {
+    type Value = SimpleValue;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("any valid Dhall value")
+    }
+
+    fn visit_bool<E>(self, value: bool) -> Result<SimpleValue, E> {
+        Ok(SimpleValue::Num(NumKind::Bool(value)))
+    }
+
+    fn visit_i64<E>(self, value: i64) -> Result<SimpleValue, E> {
+        // TODO: use `i64` instead of `isize` in `NumKind`.
+        Ok(SimpleValue::Num(NumKind::Integer(
+            value.try_into().unwrap(),
+        )))
+    }
+
+    fn visit_u64<E>(self, value: u64) -> Result<SimpleValue, E> {
+        // TODO: use `u64` instead of `usize` in `NumKind`.
+        Ok(SimpleValue::Num(NumKind::Natural(
+            value.try_into().unwrap(),
+        )))
+    }
+
+    fn visit_f64<E>(self, value: f64) -> Result<SimpleValue, E> {
+        Ok(SimpleValue::Num(NumKind::Double(value.into())))
+    }
+
+    fn visit_str<E>(self, value: &str) -> Result<SimpleValue, E> {
+        Ok(SimpleValue::Text(String::from(value)))
+    }
+
+    fn visit_string<E>(self, value: String) -> Result<SimpleValue, E> {
+        Ok(SimpleValue::Text(value))
+    }
+
+    fn visit_none<E>(self) -> Result<SimpleValue, E> {
+        Ok(SimpleValue::Optional(None))
+    }
+
+    fn visit_some<D>(self, val: D) -> Result<SimpleValue, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let val = val.deserialize_any(SimpleValueVisitor)?;
+        Ok(SimpleValue::Optional(Some(Box::new(val))))
+    }
+
+    fn visit_enum<V>(self, visitor: V) -> Result<SimpleValue, V::Error>
+    where
+        V: serde::de::EnumAccess<'de>,
+    {
+        let (name, variant): (String, _) = visitor.variant()?;
+        // Serde does not allow me to check what kind of variant it is. This will work for dhall
+        // values, because there are only two possible kinds of cvariants, but doesn't work in
+        // general. Given that the `serde_value` crate ignores enums, I assume this is not fixable
+        // :(.
+        let val = variant.newtype_variant().ok();
+        Ok(SimpleValue::Union(name, val))
+    }
+
+    fn visit_seq<V>(self, mut visitor: V) -> Result<SimpleValue, V::Error>
+    where
+        V: serde::de::SeqAccess<'de>,
+    {
+        let mut vec = Vec::new();
+        while let Some(elem) = visitor.next_element()? {
+            vec.push(elem);
+        }
+        Ok(SimpleValue::List(vec))
+    }
+
+    fn visit_map<V>(self, mut visitor: V) -> Result<SimpleValue, V::Error>
+    where
+        V: serde::de::MapAccess<'de>,
+    {
+        let mut record = BTreeMap::default();
+        while let Some((key, value)) = visitor.next_entry()? {
+            record.insert(key, value);
+        }
+        Ok(SimpleValue::Record(record))
+    }
+}
+
+impl<'de> serde::de::Deserialize<'de> for SimpleValue {
+    fn deserialize<D>(deserializer: D) -> Result<SimpleValue, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        deserializer.deserialize_any(SimpleValueVisitor)
+    }
+}
diff --git a/serde_dhall/src/value.rs b/serde_dhall/src/value.rs
index 79ba75a..d7116d9 100644
--- a/serde_dhall/src/value.rs
+++ b/serde_dhall/src/value.rs
@@ -377,7 +377,6 @@ impl SimpleType {
 }
 
 impl Sealed for Value {}
-impl Sealed for SimpleValue {}
 impl Sealed for SimpleType {}
 
 impl FromDhall for Value {
@@ -385,16 +384,6 @@ impl FromDhall for Value {
         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(|| {