compiler: Refactor Transition Statement Compilation

Make it follow the standard protocol(s). Unfortunately that means
that some additional information will have to be carried in
the compilation context. It seems like a decent tradeoff -- but
it may be revisited in the future.

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

Sources/P4Compiler/SpecialCompilers.swift

@@ -28,88 +28,102 @@ protocol AnyCompilable {
   ) -> Common.Result<(Any, CompilerContext)>
 }
 
-public struct SpecialCompilers {
-  public struct TransitionStatement {
-    static func Compile(
-      node: Node, forState state_identifier: Common.Identifier,
-      withStatements stmts: [P4Statement], withContext context: CompilerContext
-    ) -> Result<ParserState> {
+extension TransitionStatement: Compilable {
+  public static func Compile(
+    node: Node, withContext context: CompilerContext
+  ) -> Result<ParserState> {
 
-      #RequireNodeType<Node, P4Statement>(
-        node: node, type: "parserTransitionStatement", nice_type_name: "parser transition statement"
-      )
+    guard let state_identifier = context.lexical_context_name else {
+      return .Error(
+        ErrorWithLocation(
+          sourceLocation: node.toSourceLocation(),
+          withError: "Cannot parse a transition statement without the name of the containing state."))
+    }
 
-      guard let tse_node = node.child(at: 1),
-        tse_node.nodeType! == "transitionSelectionExpression"
-      else {
-        return .Error(
-          ErrorWithLocation(
-            sourceLocation: node.toSourceLocation(),
-            withError: "Could not find transition select expression"))
-      }
+    guard let stmts = context.lexical_context_statements else {
+      return .Error(
+        ErrorWithLocation(
+          sourceLocation: node.toSourceLocation(),
+          withError: "Cannot parse a transition statement without statements of the containing state."))
+    }
 
-      guard let next_node = tse_node.child(at: 0) else {
-        return .Error(
-          ErrorWithLocation(
-            sourceLocation: node.toSourceLocation(),
-            withError: "Could not find the next token in a transition selection expression"))
-      }
 
-      // If the next node is an identifier, we have the simple form ...
-      if next_node.nodeType == "identifier" {
-        let maybe_parsed_next_state_id = Identifier.Compile(
-          node: next_node, withContext: context)
-        if case .Ok(let next_state_id) = maybe_parsed_next_state_id {
-          if case .Ok(let next_state) = context.instances.lookup(identifier: next_state_id) {
-            switch next_state {
-            case (_, .some(let instance)):
-              return .Ok(
-                ParserStateDirectTransition(
-                  name: state_identifier,
-                  withNextState: instance.dataValue() as! InstantiatedParserState,
-                  withStatements: stmts,
-                )
+    #RequireNodeType<Node, P4Statement>(
+      node: node, type: "parserTransitionStatement", nice_type_name: "parser transition statement"
+    )
+
+    guard let tse_node = node.child(at: 1),
+      tse_node.nodeType! == "transitionSelectionExpression"
+    else {
+      return .Error(
+        ErrorWithLocation(
+          sourceLocation: node.toSourceLocation(),
+          withError: "Could not find transition select expression"))
+    }
+
+    guard let next_node = tse_node.child(at: 0) else {
+      return .Error(
+        ErrorWithLocation(
+          sourceLocation: node.toSourceLocation(),
+          withError: "Could not find the next token in a transition selection expression"))
+    }
+
+    // If the next node is an identifier, we have the simple form ...
+    if next_node.nodeType == "identifier" {
+      let maybe_parsed_next_state_id = Identifier.Compile(
+        node: next_node, withContext: context)
+      if case .Ok(let next_state_id) = maybe_parsed_next_state_id {
+        if case .Ok(let next_state) = context.instances.lookup(identifier: next_state_id) {
+          switch next_state {
+          case (_, .some(let instance)):
+            return .Ok(
+              ParserStateDirectTransition(
+                name: state_identifier,
+                withNextState: instance.dataValue() as! InstantiatedParserState,
+                withStatements: stmts,
               )
-            case (_, .none):
-              return .Ok(
-                ParserStateDirectTransition(
-                  name: state_identifier,
-                  withNextStateIdentifier: next_state_id, withStatements: stmts)
-              )
-            }
-          } else {
-
-            return .Error(
-              Error(
-                withMessage:
-                  "\(next_state_id) does not name a parser state in scope"
-              ))
+            )
+          case (_, .none):
+            return .Ok(
+              ParserStateDirectTransition(
+                name: state_identifier,
+                withNextStateIdentifier: next_state_id, withStatements: stmts)
+            )
           }
         } else {
+
           return .Error(
             Error(
               withMessage:
-                "Could not parse the next state in a transition statement: \(maybe_parsed_next_state_id.error()!)"
+                "\(next_state_id) does not name a parser state in scope"
             ))
         }
-      }
-
-      // We know that the next node is a select expression.
-      return
-        switch SelectExpression.compile(node: next_node, withContext: context)
-      {
-      case .Ok(let tse):
-        .Ok(
-          ParserStateSelectTransition(
-            name: state_identifier, withTransitionExpression: tse as! SelectExpression,
-            withStatements: stmts,
-          )
-        )
-      case .Error(let e): .Error(e)
+      } else {
+        return .Error(
+          Error(
+            withMessage:
+              "Could not parse the next state in a transition statement: \(maybe_parsed_next_state_id.error()!)"
+          ))
       }
     }
-  }
 
+    // We know that the next node is a select expression.
+    return
+      switch SelectExpression.compile(node: next_node, withContext: context)
+    {
+    case .Ok(let tse):
+      .Ok(
+        ParserStateSelectTransition(
+          name: state_identifier, withTransitionExpression: tse as! SelectExpression,
+          withStatements: stmts,
+        )
+      )
+    case .Error(let e): .Error(e)
+    }
+  }
+}
+
+public struct SpecialCompilers {
   public struct Statements {
     static func Compile(
       node: Node, withContext context: CompilerContext