From 6192b0b2a1f4fa94d6889734b8093bac1c2f03c2 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Thu, 25 Jun 2015 22:39:11 -0700
Subject: [PATCH] refactor grammar to prevent undesirable nesting of symbols
 and so forth

---
 lalrpop/src/grammar/parse_tree.rs     |  2 +-
 lalrpop/src/normalize/tyinfer/test.rs |  4 ++--
 lalrpop/src/parser/mod.rs             | 22 +++++++++---------
 lalrpop/src/parser/test.rs            | 32 ++++++++++++++++++++++-----
 4 files changed, 42 insertions(+), 18 deletions(-)

diff --git a/lalrpop/src/grammar/parse_tree.rs b/lalrpop/src/grammar/parse_tree.rs
index 3deb352..d752b53 100644
--- a/lalrpop/src/grammar/parse_tree.rs
+++ b/lalrpop/src/grammar/parse_tree.rs
@@ -174,7 +174,7 @@ pub enum Symbol {
     // ~X
     Choose(Box<Symbol>),
 
-    // ~x:X
+    // x:X
     Name(InternedString, Box<Symbol>),
 }
 
diff --git a/lalrpop/src/normalize/tyinfer/test.rs b/lalrpop/src/normalize/tyinfer/test.rs
index c377b3c..4a124cc 100644
--- a/lalrpop/src/normalize/tyinfer/test.rs
+++ b/lalrpop/src/normalize/tyinfer/test.rs
@@ -142,7 +142,7 @@ grammar {
 
     X = {
         Y;
-        ~l:X "+" ~r:Y => l + r;
+        l:X "+" r:Y => l + r;
     };
 
     Y: i32 = "foo" => 22;
@@ -162,7 +162,7 @@ grammar {
     X = {
         Y;
         Z;
-        ~l:X \"+\" ~r:Y => l + r;
+        l:X \"+\" r:Y => l + r;
     };
 
     Y: i32 = \"foo\" => 22;
diff --git a/lalrpop/src/parser/mod.rs b/lalrpop/src/parser/mod.rs
index e4afb65..45adef1 100644
--- a/lalrpop/src/parser/mod.rs
+++ b/lalrpop/src/parser/mod.rs
@@ -102,7 +102,16 @@ rusty_peg! {
         // Symbols
 
         SYMBOL: Symbol =
-            fold(<lhs:SYMBOL0>,
+            (NAMED_SYMBOL / CHOSEN_SYMBOL / SYMBOL0);
+
+        NAMED_SYMBOL: Symbol =
+            (<l:ID> ":" <s:SYMBOL0>) => Symbol::Name(l, Box::new(s));
+
+        CHOSEN_SYMBOL: Symbol =
+            ("~" <s:SYMBOL0>) => Symbol::Choose(Box::new(s));
+
+        SYMBOL0: Symbol =
+            fold(<lhs:SYMBOL1>,
                  (<lo:POSL> <op:REPEAT_OP> <hi:POSR>) => {
                      Symbol::Repeat(Box::new(RepeatSymbol { span: Span(lo, hi),
                                                             symbol: lhs,
@@ -114,9 +123,8 @@ rusty_peg! {
         REPEAT_OP_STAR: RepeatOp = "*" => RepeatOp::Star;
         REPEAT_OP_QUESTION: RepeatOp = "?" => RepeatOp::Question;
 
-        SYMBOL0: Symbol =
-            (NAMED_SYMBOL / CHOSEN_SYMBOL / MACRO_SYMBOL / TERMINAL_SYMBOL /
-             NT_SYMBOL / ESCAPE_SYMBOL / PAREN_SYMBOL);
+        SYMBOL1: Symbol =
+            (MACRO_SYMBOL / TERMINAL_SYMBOL / NT_SYMBOL / ESCAPE_SYMBOL / PAREN_SYMBOL);
 
         MACRO_SYMBOL: Symbol =
             (<lo:POSL> <l:NONTERMINAL_ID> "<" <m:{MACRO_ARG_START}> <n:[SYMBOL]> ">" <hi:POSR>) => {
@@ -145,12 +153,6 @@ rusty_peg! {
             (<lo:POSL> <s:{SYMBOL}> <hi:POSR>) => ExprSymbol { span: Span(lo, hi),
                                                                symbols: s };
 
-        NAMED_SYMBOL: Symbol =
-            (<l:ID> ":" <s:SYMBOL>) => Symbol::Name(l, Box::new(s));
-
-        CHOSEN_SYMBOL: Symbol =
-            ("~" <s:SYMBOL>) => Symbol::Choose(Box::new(s));
-
         // TypeRef
 
         TYPE_REF: TypeRef =
diff --git a/lalrpop/src/parser/test.rs b/lalrpop/src/parser/test.rs
index 936ccbf..235126e 100644
--- a/lalrpop/src/parser/test.rs
+++ b/lalrpop/src/parser/test.rs
@@ -1,4 +1,4 @@
-use grammar::parse_tree::TypeRef;
+use grammar::parse_tree::{Symbol, TypeRef};
 
 #[test]
 fn type_ref() {
@@ -66,7 +66,7 @@ fn paren_with_plus_and_anon() {
 
 #[test]
 fn named_choice() {
-    super::parse_grammar(r#"grammar { Expr = ~n:Alt; }"#).unwrap();
+    super::parse_grammar(r#"grammar { Expr = n:Alt; }"#).unwrap();
 }
 
 #[test]
@@ -82,14 +82,14 @@ fn token_expr() {
 #[test]
 fn map1() {
     super::parse_grammar(
-        r#"grammar { Expr = ~n:Alt+ => { { foo } }; }"#).unwrap();
+        r#"grammar { Expr = n:Alt+ => { { foo } }; }"#).unwrap();
 }
 
 #[test]
 #[allow(non_snake_case)]
 fn mapN() {
     super::parse_grammar(
-        r#"grammar { Expr = { Bar => { Baz }; X ~n:Bar => { Y }; }; }"#).unwrap();
+        r#"grammar { Expr = { Bar => { Baz }; X n:Bar => { Y }; }; }"#).unwrap();
 }
 
 #[test]
@@ -101,10 +101,32 @@ fn macro_symbols() {
     super::parse_symbol(r#"Foo<"Baz"+, ("Balooga" Potato),>"#).unwrap();
 }
 
+#[test]
+fn symbol_precedence() {
+    // check that we parse this as choosing a X+
+    let sym = super::parse_symbol(r#"~X+"#).unwrap();
+    assert!(match sym {
+        Symbol::Choose(..) => true,
+        _ => false
+    });
+
+    let sym = super::parse_symbol(r#"n:X+"#).unwrap();
+    assert!(match sym {
+        Symbol::Name(..) => true,
+        _ => false
+    });
+}
+
+#[test]
+fn symbol_choose_name() {
+    // check that we can parse ~S and x:S but not both
+    assert!(super::parse_symbol(r#"~x:X+"#).is_err());
+}
+
 #[test]
 fn macro_nt() {
     super::parse_nonterminal(
-        r#"Comma<E>: Vec<E> = ~v:(~E ",")* ~e:E? => v.into_iter().chain(e.into_iter()).collect();"#)
+        r#"Comma<E>: Vec<E> = v:(~E ",")* e:E? => v.into_iter().chain(e.into_iter()).collect();"#)
         .unwrap();
 }