From 74fead1eba53fd3f486590877646bb7f292f51a0 Mon Sep 17 00:00:00 2001
From: Will Hawkins <hawkinsw@obs.cr>
Date: Thu, 23 Apr 2026 06:01:31 -0400
Subject: [PATCH] grammar: Parse externs

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
---
 tree-sitter-p4/grammar.js                   |   7 +-
 tree-sitter-p4/test/corpus/declarations.txt | 109 +++++++++++++++-----
 tree-sitter-p4/test/corpus/statements.txt   |  16 ++-
 3 files changed, 95 insertions(+), 37 deletions(-)

diff --git a/tree-sitter-p4/grammar.js b/tree-sitter-p4/grammar.js
index ba28e70..c1df9ac 100644
--- a/tree-sitter-p4/grammar.js
+++ b/tree-sitter-p4/grammar.js
@@ -68,13 +68,16 @@ export default grammar({
         instantiation: $ => seq($.typeRef, '(', optional($.parameter_list), ')', $.identifier),
 
         // Declarations
-        declaration: $ => seq(choice($.parserDeclaration, $.parserTypeDeclaration, $.type_declaration, $.function_declaration, $.control_declaration)),
+        declaration: $ => seq(choice($.parserDeclaration, $.parserTypeDeclaration, $.type_declaration, $.function_declaration, $.control_declaration, $.extern_declaration)),
 
         type_declaration: $=> choice($.struct_declaration),
         struct_declaration: $ => seq($.struct, $.identifier, '{', optional($.struct_declaration_fields), '}'),
         struct_declaration_fields: $=> repeat1(seq($.variableDeclaration)),
 
-        function_declaration: $=> seq($.typeRef, $.identifier, $.parameters, $.statement),
+        extern_declaration: $=> seq($.extern, $.declaration),
+
+        // The body is truly optional only in the extern case. This check is handled by the compiler.
+        function_declaration: $=> seq($.typeRef, $.identifier, $.parameters, optional($.blockStatement)),
 
         // Make separate productions for the parser type and the parser type declaration because the latter can have type parameters.
         parserTypeDeclaration: $ => seq(optional($.annotations), $.parser, field('parser_name', $.identifier), optional($.typeParameters), $.parameters),
diff --git a/tree-sitter-p4/test/corpus/declarations.txt b/tree-sitter-p4/test/corpus/declarations.txt
index 3e189af..9412c9c 100644
--- a/tree-sitter-p4/test/corpus/declarations.txt
+++ b/tree-sitter-p4/test/corpus/declarations.txt
@@ -282,18 +282,16 @@ bool functionb() {
    )
    (identifier)
    (parameters)
-   (statement
-    (blockStatement
-     (statements
-      (statement
-       (variableDeclaration
-        (typeRef
-         (baseType
-          (string)
-         )
+   (blockStatement
+    (statements
+     (statement
+      (variableDeclaration
+       (typeRef
+        (baseType
+         (string)
         )
-       (identifier)
        )
+      (identifier)
       )
      )
     )
@@ -340,18 +338,16 @@ bool functionb(bool a, int b) {
      )
     )
    )
-   (statement
-    (blockStatement
-     (statements
-      (statement
-       (variableDeclaration
-        (typeRef
-         (baseType
-          (string)
-         )
+   (blockStatement
+    (statements
+     (statement
+      (variableDeclaration
+       (typeRef
+        (baseType
+         (string)
         )
-       (identifier)
        )
+      (identifier)
       )
      )
     )
@@ -416,17 +412,78 @@ bool functionb(in bool a, out int b, inout string c) {
      )
     )
    )
-   (statement
-    (blockStatement
-     (statements
-      (statement
-       (variableDeclaration
+   (blockStatement
+    (statements
+     (statement
+      (variableDeclaration
+       (typeRef
+        (baseType
+         (string)
+        )
+       )
+      (identifier)
+      )
+     )
+    )
+   )
+  )
+ )
+)
+=========================
+Extern Function Declaration
+=========================
+extern bool functionb(in bool a, out int b, inout string c);
+---
+(p4program
+ (declaration
+  (extern_declaration
+   (extern)
+   (declaration
+    (function_declaration
+     (typeRef
+      (baseType
+       (bool)
+      )
+     )
+     (identifier)
+     (parameters
+      (parameter_list
+       (parameter_list
+        (parameter_list
+         (parameter
+          (direction
+           (in)
+          )
+          (typeRef
+           (baseType
+           (bool)
+           )
+          )
+          (identifier)
+         )
+        )
+        (parameter
+         (direction
+          (out)
+         )
+         (typeRef
+          (baseType
+           (int)
+          )
+         )
+         (identifier)
+        )
+       )
+       (parameter
+        (direction
+         (inout)
+        )
         (typeRef
          (baseType
           (string)
          )
         )
-       (identifier)
+        (identifier)
        )
       )
      )
diff --git a/tree-sitter-p4/test/corpus/statements.txt b/tree-sitter-p4/test/corpus/statements.txt
index a1ce8a2..57d2a09 100644
--- a/tree-sitter-p4/test/corpus/statements.txt
+++ b/tree-sitter-p4/test/corpus/statements.txt
@@ -43,15 +43,13 @@ int fun() {
    )
    (identifier)
    (parameters)
-   (statement
-    (blockStatement
-     (statements
-      (statement
-       (return_statement
-        (expression
-         (simple_expression
-          (integer)
-         )
+   (blockStatement
+    (statements
+     (statement
+      (return_statement
+       (expression
+        (simple_expression
+         (integer)
         )
        )
       )