CSE Machine: Overhaul environment handling at the start of programs (#…

…1642)

* fix env push condition

* Overhaul environment handling at the start of programs

* code cleanup

src/cse-machine/interpreter.ts

@@ -396,48 +396,28 @@ const cmdEvaluators: { [type: string]: CmdEvaluator } = {
     stash: Stash,
     isPrelude: boolean
   ) {
-    // Create and push the environment only if it is non empty.
-    if (hasDeclarations(command) || hasImportDeclarations(command)) {
-      const environment = createProgramEnvironment(context, isPrelude)
-      pushEnvironment(context, environment)
-      evaluateImports(command as unknown as es.Program, context)
-      declareFunctionsAndVariables(context, command, environment)
+    // After execution of a program, the current environment might be a local one.
+    // This can cause issues (for example, during execution of consecutive REPL programs)
+    // This piece of code will reset the current environment to either a global one, a program one or a prelude one.
+    while (
+      currentEnvironment(context).name != 'global' &&
+      currentEnvironment(context).name != 'programEnvironment' &&
+      currentEnvironment(context).name != 'prelude'
+    ) {
+      popEnvironment(context)
     }
 
-    // A strange bug occurs here when successive REPL commands are run, as they
-    // are each evaluated as separate programs. This causes the environment to be
-    // pushed multiple times.
-
-    // As such, we need to "append" the tail environment to the current environment
-    // if and only if the tail environment is a previous program environment.
-
-    const currEnv = currentEnvironment(context)
-    if (
-      currEnv &&
-      currEnv.name === 'programEnvironment' &&
-      currEnv.tail &&
-      currEnv.tail.name === 'programEnvironment'
-    ) {
-      // we need to take that tail environment and append its items to the current environment
-      const oldEnv = currEnv.tail
-
-      // separate the tail environment from the environments list
-      currEnv.tail = oldEnv.tail
-
-      // we will recycle the old environment's item list
-      // add the items from the current environment to the tail environment
-      // this is fine, especially as the older program will never
-      // need to use the old environment's items again
-      for (const key in currEnv.head) {
-        oldEnv.head[key] = currEnv.head[key]
+    // If the program has outer declarations:
+    // - Create the program environment (if none exists yet), and
+    // - Declare the functions and variables in the program environment.
+    if (hasDeclarations(command) || hasImportDeclarations(command)) {
+      if (currentEnvironment(context).name != 'programEnvironment') {
+        const programEnv = createProgramEnvironment(context, isPrelude)
+        pushEnvironment(context, programEnv)
       }
-
-      // set the current environment to the old one
-      // this will work across successive programs as well
-
-      // this will also allow continuations to read newer program
-      // values from their "outdated" program environment
-      currEnv.head = oldEnv.head
+      const environment = currentEnvironment(context)
+      evaluateImports(command as unknown as es.Program, context)
+      declareFunctionsAndVariables(context, command, environment)
     }
 
     if (command.body.length == 1) {
@@ -457,8 +437,10 @@ const cmdEvaluators: { [type: string]: CmdEvaluator } = {
     // we do not need to push another one
     // The no declarations case is handled at the transform stage, so no blockStatement node without declarations should end up here.
     const next = control.peek()
-    // Push ENVIRONMENT instruction if needed
-    if (!next || !(isInstr(next) && next.instrType === InstrType.ENVIRONMENT)) {
+
+    // Push ENVIRONMENT instruction if needed - if next control stack item
+    // exists and is not an environment instruction
+    if (next && !(isInstr(next) && next.instrType === InstrType.ENVIRONMENT)) {
       control.push(instr.envInstr(currentEnvironment(context), command))
     }