yeast-macros: add .map(p -> tpl) chain syntax for tree templates

After a {expr} or {..expr} placeholder, an optional chain of
.<builtin>() calls may follow. Currently the only builtin is:

  .map(param -> template)

which applies the template to each element of the iterable and
collects the resulting node IDs. A chain auto-splices into the
enclosing field/child position.

Example:
  path: {parts}.map(p -> (identifier #{p}))

The framework is extensible: additional builtins can be added by
matching on the method name in parse_chain_suffix.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: asgerf

shared/yeast-macros/src/lib.rs

@@ -44,8 +44,14 @@ pub fn query(input: TokenStream) -> TokenStream {
 /// {expr}                       - embed a Rust expression returning Id
 /// {..expr}                     - splice an iterable of Id (in child/field position)
 /// field: {..expr}              - splice into a named field
+/// {expr}.map(p -> tpl)         - apply tpl to each element; splice result
 /// ```
 ///
+/// Chain syntax after `{expr}` or `{..expr}`:
+/// - `.map(param -> template)` — produces one node per element of the iterable.
+///   The lambda parameter is bound in `template` (e.g. `{parts}.map(p -> (identifier #{p}))`).
+/// - Chains always splice (the result is iterable).
+///
 /// Can be called with an explicit context or using the implicit context
 /// from an enclosing `rule!`:
 ///

shared/yeast-macros/src/parse.rs

@@ -419,23 +419,35 @@ fn parse_direct_node_inner(tokens: &mut Tokens, ctx: &Ident) -> Result<TokenStre
         );
         field_counter += 1;
 
-        // Check for field: {..expr} — splice a Vec<Id> into the field
+        // Check for field: {..expr}.chain or field: {expr}.chain — splice a Vec<Id> into the field
         if peek_is_group(tokens, Delimiter::Brace) {
             let group_clone = tokens.clone().next().unwrap();
             if let TokenTree::Group(g) = &group_clone {
                 let mut inner_check = g.stream().into_iter();
                 let is_splice = matches!(inner_check.next(), Some(TokenTree::Punct(p)) if p.as_char() == '.')
                     && matches!(inner_check.next(), Some(TokenTree::Punct(p)) if p.as_char() == '.');
-                if is_splice {
+                // Determine if a chain (.map(..)) follows the `{}` group.
+                let mut after = tokens.clone();
+                after.next(); // skip the brace group
+                let has_chain = matches!(after.peek(), Some(TokenTree::Punct(p)) if p.as_char() == '.');
+
+                if is_splice || has_chain {
                     let group = expect_group(tokens, Delimiter::Brace)?;
-                    let mut inner = group.stream().into_iter().peekable();
-                    inner.next(); // consume first .
-                    inner.next(); // consume second .
-                    let expr: proc_macro2::TokenStream = inner.collect();
+                    let base: TokenStream = if is_splice {
+                        let mut inner = group.stream().into_iter().peekable();
+                        inner.next(); // consume first .
+                        inner.next(); // consume second .
+                        let expr: TokenStream = inner.collect();
+                        quote! {
+                            (#expr).into_iter().map(::std::convert::Into::<usize>::into)
+                        }
+                    } else {
+                        let expr = group.stream();
+                        quote! { (#expr).into_iter() }
+                    };
+                    let chained = parse_chain_suffix(tokens, ctx, base)?;
                     stmts.push(quote! {
-                        let #temp: Vec<usize> = (#expr).into_iter()
-                            .map(::std::convert::Into::<usize>::into)
-                            .collect();
+                        let #temp: Vec<usize> = #chained.collect();
                     });
                     // An empty splice means the field is absent — skip it
                     // entirely rather than emitting an empty named field.
@@ -472,6 +484,58 @@ fn parse_direct_node_inner(tokens: &mut Tokens, ctx: &Ident) -> Result<TokenStre
     })
 }
 
+/// Parse a chain of `.method(args)` suffixes after a `{expr}` or `{..expr}`
+/// placeholder in tree templates. Currently supports:
+///
+/// ```text
+/// .map(param -> template)   -- iterator map: produces Vec<usize>
+/// ```
+///
+/// The chain may be empty (returns `base` unchanged). Multiple chained calls
+/// are supported, e.g. `.map(p -> ...).map(q -> ...)`.
+///
+/// Each call expects the receiver to be an iterator. The `base` argument
+/// should therefore already be an iterator (use `.into_iter()` on it before
+/// calling this function).
+fn parse_chain_suffix(
+    tokens: &mut Tokens,
+    ctx: &Ident,
+    base: TokenStream,
+) -> Result<TokenStream> {
+    let mut current = base;
+    while matches!(tokens.peek(), Some(TokenTree::Punct(p)) if p.as_char() == '.') {
+        tokens.next(); // consume .
+        let method = expect_ident(tokens, "expected method name after `.`")?;
+        let method_str = method.to_string();
+        let args_group = expect_group(tokens, Delimiter::Parenthesis)?;
+        match method_str.as_str() {
+            "map" => {
+                let mut inner = args_group.stream().into_iter().peekable();
+                let param = expect_ident(&mut inner, "expected lambda parameter name")?;
+                expect_punct(&mut inner, '-', "expected `->` after lambda parameter")?;
+                expect_punct(&mut inner, '>', "expected `->` after lambda parameter")?;
+                let body = parse_direct_node(&mut inner, ctx)?;
+                if let Some(tok) = inner.next() {
+                    return Err(syn::Error::new_spanned(
+                        tok,
+                        "unexpected token after lambda body",
+                    ));
+                }
+                current = quote! {
+                    #current.map(|#param| #body)
+                };
+            }
+            _ => {
+                return Err(syn::Error::new_spanned(
+                    method,
+                    format!("unknown builtin method `.{method_str}()`"),
+                ));
+            }
+        }
+    }
+    Ok(current)
+}
+
 /// Parse the top-level list of a `trees!` template.
 /// Each item is a node template or `{expr}` splice.
 fn parse_direct_list(tokens: &mut Tokens, ctx: &Ident) -> Result<Vec<TokenStream>> {
@@ -492,18 +556,27 @@ fn parse_direct_list(tokens: &mut Tokens, ctx: &Ident) -> Result<Vec<TokenStream
             continue;
         }
 
-        // {expr} or {..expr} — single node or splice
+        // {expr} or {..expr} (with optional .chain) — single node or splice
         if peek_is_group(tokens, Delimiter::Brace) {
             let group = expect_group(tokens, Delimiter::Brace)?;
+            let has_chain = matches!(tokens.peek(), Some(TokenTree::Punct(p)) if p.as_char() == '.');
             let mut inner = group.stream().into_iter().peekable();
-            if peek_is_dotdot(&inner) {
-                inner.next(); // consume first .
-                inner.next(); // consume second .
-                let expr: TokenStream = inner.collect();
-                items.push(quote! {
-                    __nodes.extend(
+            let is_splice = peek_is_dotdot(&inner);
+            if is_splice || has_chain {
+                let base: TokenStream = if is_splice {
+                    inner.next(); // consume first .
+                    inner.next(); // consume second .
+                    let expr: TokenStream = inner.collect();
+                    quote! {
                         (#expr).into_iter().map(::std::convert::Into::<usize>::into)
-                    );
+                    }
+                } else {
+                    let expr = group.stream();
+                    quote! { (#expr).into_iter() }
+                };
+                let chained = parse_chain_suffix(tokens, ctx, base)?;
+                items.push(quote! {
+                    __nodes.extend(#chained);
                 });
             } else {
                 let expr = group.stream();