3 files changed, 83 insertions, 8 deletions
diff --git a/dhall_generator/Cargo.toml b/dhall_generator/Cargo.toml
index ada6707..2a6ca67 100644
--- a/dhall_generator/Cargo.toml
+++ b/dhall_generator/Cargo.toml
@@ -12,4 +12,5 @@ doctest = false
 itertools = "0.8.0"
 quote = "0.6.11"
 proc-macro2 = "0.4.27"
+syn = "0.15.29"
 dhall_core = { path = "../dhall_core" }
diff --git a/dhall_generator/src/dhall_expr.rs b/dhall_generator/src/dhall_expr.rs
index 1ee4d1a..bc9da22 100644
--- a/dhall_generator/src/dhall_expr.rs
+++ b/dhall_generator/src/dhall_expr.rs
@@ -89,6 +89,10 @@ fn dhall_to_tokenstream(
             let m = map_to_tokenstream(m, ctx);
             quote! { dhall_core::Expr::RecordLit(#m) }
         }
+        UnionType(m) => {
+            let m = map_to_tokenstream(m, ctx);
+            quote! { dhall_core::Expr::UnionType(#m) }
+        }
         e => unimplemented!("{:?}", e),
     }
 }
diff --git a/dhall_generator/src/dhall_type.rs b/dhall_generator/src/dhall_type.rs
index 7cfd945..b305ca0 100644
--- a/dhall_generator/src/dhall_type.rs
+++ b/dhall_generator/src/dhall_type.rs
@@ -1,15 +1,85 @@
 extern crate proc_macro;
 // use dhall_core::*;
-// use proc_macro2::TokenStream;
-use quote::quote;
+use proc_macro::TokenStream;
+use quote::{quote, quote_spanned};
+use syn::spanned::Spanned;
+use syn::{parse_macro_input, parse_quote, DeriveInput};
 
-pub fn derive_dhall_type(_input: proc_macro::TokenStream) -> proc_macro::TokenStream {
-    (quote!{
-        impl DhallType for A {
+pub fn derive_dhall_type(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    // List of types that must impl DhallType
+    let mut constraints = vec![];
+
+    let dhall_type = match &input.data {
+        syn::Data::Struct(data) => match &data.fields {
+            syn::Fields::Named(fields) => {
+                let fields = fields.named.iter()
+                    .map(|f| {
+                        let name = f.ident.as_ref().unwrap().to_string();
+                        let ty = &f.ty;
+                        constraints.push(ty.clone());
+                        quote!( m.insert(dhall_core::Label::from(#name), <#ty as dhall::DhallType>::dhall_type()); )
+                    });
+                quote! { dhall_core::rc(dhall_core::Expr::RecordType({
+                    use std::collections::BTreeMap;
+                    let mut m = BTreeMap::new();
+                    #(#fields)*
+                    m
+                })) }
+            }
+            _ => quote!(dhall_generator::dhall_expr!(Bool)),
+        },
+        _ => quote!(dhall_generator::dhall_expr!(Bool)),
+    };
+
+    let mut generics = input.generics.clone();
+    generics.make_where_clause();
+    let (impl_generics, ty_generics, orig_where_clause) =
+        generics.split_for_impl();
+    let orig_where_clause = orig_where_clause.unwrap();
+
+    // Hygienic errors
+    let assertions = constraints.iter().enumerate().map(|(i, ty)| {
+        // Ensure that ty: DhallType, with an appropriate span
+        let assert_name =
+            syn::Ident::new(&format!("_AssertDhallType{}", i), ty.span());
+        let mut local_where_clause = orig_where_clause.clone();
+        local_where_clause
+            .predicates
+            .push(parse_quote!(#ty: dhall::DhallType));
+        let phantoms = generics.params.iter().map(|param| match param {
+            syn::GenericParam::Type(syn::TypeParam { ident, .. }) => {
+                quote!(#ident)
+            }
+            syn::GenericParam::Lifetime(syn::LifetimeDef {
+                lifetime, ..
+            }) => quote!(&#lifetime ()),
+            _ => unimplemented!(),
+        });
+        quote_spanned! {ty.span()=>
+            struct #assert_name #impl_generics #local_where_clause {
+                _phantom: std::marker::PhantomData<(#(#phantoms),*)>
+            };
+        }
+    });
+
+    // Ensure that all the fields have a DhallType impl
+    let mut where_clause = orig_where_clause.clone();
+    for ty in constraints.iter() {
+        where_clause
+            .predicates
+            .push(parse_quote!(#ty: dhall::DhallType));
+    }
+
+    let ident = &input.ident;
+    let tokens = quote! {
+        impl #impl_generics dhall::DhallType for #ident #ty_generics #where_clause {
             fn dhall_type() -> dhall_core::DhallExpr {
-                dhall_core::rc(dhall_core::Expr::BoolLit(false))
+                #(#assertions)*
+                #dhall_type
             }
         }
-    }).into()
+    };
+    TokenStream::from(tokens)
 }
-