DOCS: Add reduce expressions to the grammar and reference.

docsite/site/content/reference/expressions.md

@@ -303,6 +303,38 @@ let tpl_filter = macro(name, val) => {
 filter tpl_filter.result test_tpl == { quux = "baz" };
 ```
 
+### Reduce expressions
+
+Reduce expressions start with the reduce keyword followed by a symbol referencing a macro a dot and the output field, then an expression for the accumulator and finally the tuple or list to process.
+
+**Tuples**
+
+```
+let test_tpl = {
+    foo = "bar",
+    quux = "baz",
+};
+let tpl_reducer = macro(acc, name, val) => {
+    result = acc{
+        keys = self.keys + [name],
+        vals = self.vals + [val],
+    },
+};
+
+reduce tpl_reducer.result {keys = [], vals = []}, test_tpl == {keys = ["foo", "quux"], vals = ["bar", "baz"]};
+```
+
+**Lists**
+
+```
+let list1 = [1, 2, 3, 4];
+let list_reducer = macro(acc, item) => {
+    result = acc + item,
+};
+
+ list_reducer.result 0, list1 == 0 + 1 + 2 + 3 + 4;
+```
+
 Include expressions
 -------------------
 

docsite/site/content/reference/grammar.md

@@ -50,6 +50,7 @@ include_keyword: "include" ;
 as_keyword: "as" ;
 macro_keyword: "macro" ;
 map_keyword: "map" ;
+reduce_keyword: "map" ;
 filter_keyword: "filter" ;
 module_keyword: "module" ;
 mod_keyword: "mod" ;
@@ -121,8 +122,8 @@ module_def: module_keyword, tuple, fatcomma, lbrace, [ { statement } ], rbrace ;
 #### Copy and Call Expression
 
 ```
-copy_expression: bareword, tuple ;
-call_expression: bareword, lparen, [arglist], rparen ;
+copy_expr: bareword, tuple ;
+call_expr: bareword, lparen, [arglist], rparen ;
 ```
 
 #### Format Expression
@@ -135,7 +136,9 @@ format_expr: str, percent, lparen, [arglist], rparen ;
 
 ```
 func_op_kind: map_keyword | filter_keyword ;
-processing_expr: func_op_kind, bareword, dot, bareword, expr ;
+map_or_filter_expr: func_op_kind, bareword, dot, bareword, expr ;
+reduce_expr: reduce_keyword, bareword, dot, bareword, expr, expr ;
+processing_expr: map_or_filter_expr | reduce_expr
 ```
 
 #### Include Expression
@@ -151,11 +154,11 @@ non_operator_expr: literal
                    | grouped
                    | macrodef
                    | module_def
-                   | format_expression
-                   | include_expression
-                   | copy_expression
+                   | format_expr
+                   | include_expr
+                   | copy_expr
                    | processing_expr
-                   | call_expression ;
+                   | call_expr ;
 ```
 
 #### Operator Expressions