compiler, language, runtime: Separate Parser Type From Instances

In P4, parsers are considered types. Those parsers are instantiated.
The instantiated parsers are values. Previously, gp4 treated a parser
type and a parser value as identical. This PR makes that difference
clear _and_ sets the stage for the future.

TODO: Make the same distinction between control and action types and
values.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/P4Compiler/Program.swift

@@ -24,18 +24,30 @@ import TreeSitterP4
 
 public struct Program {
   public static func Compile(_ source: String) -> Result<P4Lang.Program> {
-    return Program.Compile(source, withGlobalInstances: .none, withGlobalTypes: .none, withFFIs: [])
+
+    // Certain names are always in scope during compilation.
+    var globals = StaticVarValueScopes().enter()
+    globals = globals.declare(
+      identifier: accept.state().getName(),
+      withValue: (P4QualifiedType(accept.type()), P4Value(accept))
+    )
+    .declare(
+      identifier: reject.state.getName(),
+      withValue: (P4QualifiedType(reject.type()), P4Value(reject)))
+
+    return Program.Compile(
+      source, withGlobalInstances: globals, withGlobalTypes: .none, withFFIs: [])
   }
 
   public static func Compile(
-    _ source: String, withGlobalInstances globalInstances: VarTypeScopes
+    _ source: String, withGlobalInstances globalInstances: StaticVarValueScopes
   ) -> Result<P4Lang.Program> {
     return Program.Compile(
       source, withGlobalInstances: globalInstances, withGlobalTypes: .none, withFFIs: [])
   }
 
   public static func Compile(
-    _ source: String, withGlobalInstances globalInstances: VarTypeScopes?,
+    _ source: String, withGlobalInstances globalInstances: StaticVarValueScopes?,
     withGlobalTypes globalTypes: TypeTypeScopes?, withFFIs ffis: [P4FFI] = Array()
   ) -> Result<P4Lang.Program> {
 
@@ -122,8 +134,10 @@ public struct Program {
 
     // Any of the instances that are in the top-level scope should go into the program!
     program.instances = Array(
-      compilation_context.instances.map { (_, v) in
-        v
+      compilation_context.instances.filter { (_, v) in
+        v.1 != nil
+      }.map { (_, v) in
+        v.1!
       })
 
     // Any of the types that are in the top-level scope should go into the program!