Enforce `NextProcess` return type in chain functions and update examples

2 files changed, 44 insertions, 8 deletions

diff --git a/mingling_macros/src/chain.rs b/mingling_macros/src/chain.rs
index a91949d..e7b2db2 100644
--- a/mingling_macros/src/chain.rs
+++ b/mingling_macros/src/chain.rs
@@ -1,7 +1,9 @@
 use proc_macro::TokenStream;
 use quote::{ToTokens, quote};
 use syn::spanned::Spanned;
-use syn::{FnArg, Ident, ItemFn, Pat, PatType, Signature, Type, TypePath, parse_macro_input};
+use syn::{
+    FnArg, Ident, ItemFn, Pat, PatType, ReturnType, Signature, Type, TypePath, parse_macro_input,
+};
 
 /// Extracts the previous type and parameter name from function arguments
 fn extract_previous_info(sig: &Signature) -> syn::Result<(Pat, TypePath)> {
@@ -67,6 +69,40 @@ pub fn chain_attr(attr: TokenStream, item: TokenStream) -> TokenStream {
         }
     }
 
+    // Check that return type is NextProcess
+    let return_type = &input_fn.sig.output;
+    match return_type {
+        ReturnType::Type(_, ty) => {
+            // Check if the return type is NextProcess
+            match &**ty {
+                Type::Path(type_path) => {
+                    let last_segment = type_path.path.segments.last().unwrap();
+                    if last_segment.ident.to_string() != "NextProcess" {
+                        return syn::Error::new(
+                            ty.span(),
+                            "Chain function must return `NextProcess`",
+                        )
+                        .to_compile_error()
+                        .into();
+                    }
+                }
+                _ => {
+                    return syn::Error::new(ty.span(), "Chain function must return `NextProcess`")
+                        .to_compile_error()
+                        .into();
+                }
+            }
+        }
+        ReturnType::Default => {
+            return syn::Error::new(
+                input_fn.sig.span(),
+                "Chain function must specify a return type (must be `NextProcess`)",
+            )
+            .to_compile_error()
+            .into();
+        }
+    }
+
     // Extract the previous type and parameter name from function arguments
     let (prev_param, previous_type) = match extract_previous_info(&input_fn.sig) {
         Ok(info) => info,
diff --git a/mingling_macros/src/lib.rs b/mingling_macros/src/lib.rs
index 70a5c3e..818cda6 100644
--- a/mingling_macros/src/lib.rs
+++ b/mingling_macros/src/lib.rs
@@ -316,14 +316,14 @@ pub fn r_println(input: TokenStream) -> TokenStream {
 /// ```rust,ignore
 /// // Default program (ThisProgram):
 /// #[chain]
-/// fn my_step(prev: InputType) -> ChainProcess<ThisProgram> {
+/// fn my_step(prev: InputType) -> NextProcess {
 ///     // transform `prev`...
 ///     OutputType::new(result).to_render()
 /// }
 ///
 /// // Explicit program name:
 /// #[chain(MyProgram)]
-/// fn my_step(prev: InputType) -> ChainProcess<MyProgram> {
+/// fn my_step(prev: InputType) -> NextProcess {
 ///     // ...
 /// }
 /// ```
@@ -336,7 +336,7 @@ pub fn r_println(input: TokenStream) -> TokenStream {
 /// pack!(MyOutput = String);
 ///
 /// #[chain]
-/// fn greet(prev: HelloEntry) -> ChainProcess<ThisProgram> {
+/// fn greet(prev: HelloEntry) -> NextProcess {
 ///     let name = prev.first().cloned().unwrap_or_else(|| "World".to_string());
 ///     MyOutput::new(name).to_render()
 /// }
@@ -350,7 +350,7 @@ pub fn r_println(input: TokenStream) -> TokenStream {
 /// pack!(MyOutput = String);
 ///
 /// #[chain]
-/// async fn greet(prev: HelloEntry) -> ChainProcess<ThisProgram> {
+/// async fn greet(prev: HelloEntry) -> NextProcess {
 ///     let name = prev.first().cloned().unwrap_or_else(|| "World".to_string());
 ///     some_async_fn(&name).await;
 ///     MyOutput::new(name).to_render()
@@ -360,7 +360,7 @@ pub fn r_println(input: TokenStream) -> TokenStream {
 /// # Requirements
 ///
 /// - The function must have exactly **one** parameter (the previous type in the chain).
-/// - The function must return `ChainProcess<ProgramName>` (or `impl Into<ChainProcess<ProgramName>>`).
+/// - The function must return `NextProcess` (the type alias generated by `gen_program!`, which equals `ChainProcess<ProgramName>`).
 /// - With the `async` feature, async functions are supported; without it, async functions are rejected.
 #[proc_macro_attribute]
 pub fn chain(attr: TokenStream, item: TokenStream) -> TokenStream {
@@ -857,7 +857,7 @@ pub fn program_comp_gen(input: TokenStream) -> TokenStream {
     #[cfg(feature = "async")]
     let fn_exec_comp = quote! {
         #[::mingling::macros::chain(#name)]
-        pub async fn __exec_completion(prev: CompletionContext) -> ::mingling::ChainProcess<#name> {
+        pub async fn __exec_completion(prev: CompletionContext) -> NextProcess {
             let read_ctx = ::mingling::ShellContext::try_from(prev.inner);
             match read_ctx {
                 Ok(ctx) => {
@@ -872,7 +872,7 @@ pub fn program_comp_gen(input: TokenStream) -> TokenStream {
     #[cfg(not(feature = "async"))]
     let fn_exec_comp = quote! {
         #[::mingling::macros::chain(#name)]
-        pub fn __exec_completion(prev: CompletionContext) -> ::mingling::ChainProcess<#name> {
+        pub fn __exec_completion(prev: CompletionContext) -> NextProcess {
             let read_ctx = ::mingling::ShellContext::try_from(prev.inner);
             match read_ctx {
                 Ok(ctx) => {