Make exists prefix

Author: zdevito

docs/source/inference.rst

@@ -278,7 +278,7 @@ Constant fill using an exists clause
 .. code::
 
     def constant_fill(float(N) A, float c) -> B {
-      B(i) = c where A(i) exists
+      B(i) = c where exists A(i)
     }
 
 An :code:`exists` clause allows you to add additional expressions to the range

include/tc/lang/parser.h

@@ -162,8 +162,8 @@ struct Parser {
     } else if (lookahead.kind == TK_IN) {
       return parseRangeConstraint();
     } else {
-      auto exp = parseExp();
       L.expect(TK_EXISTS);
+      auto exp = parseExp();
       return Exists::create(exp->range(), {exp});
     }
   }

include/tc/lang/sema.h

@@ -487,12 +487,11 @@ struct Sema {
     // if we redefined an input, it is no longer valid for range expressions
     live_input_names.erase(stmt.ident().name());
 
-    auto equivalent_statement_ =
-        stmt.equivalent().map([&](Equivalent eq) {
-          auto indices_ = eq.accesses().map(
-              [&](TreeRef index) { return checkExp(index, true); });
-          return Equivalent::create(eq.range(), eq.name(), indices_);
-        });
+    auto equivalent_statement_ = stmt.equivalent().map([&](Equivalent eq) {
+      auto indices_ = eq.accesses().map(
+          [&](TreeRef index) { return checkExp(index, true); });
+      return Equivalent::create(eq.range(), eq.name(), indices_);
+    });
 
     TreeRef assignment = stmt.assignment();
     // For semantic consistency we allow overwriting reductions like +=!

test/test_corner_cases.cc

@@ -216,7 +216,7 @@ TEST(TestCornerCases, E16){
 TEST(TestCornerCases, E17) {
   auto r = F(1);
   Succeed(
-      "def f(float(1) a) -> (b) { b(i) = 4.0 where a(i) exists }", {F(1)}, {r});
+      "def f(float(1) a) -> (b) { b(i) = 4.0 where exists a(i) }", {F(1)}, {r});
   CHECK_EQ(at::Scalar(r[0]).toFloat(), 4);
 }