diff --git a/src/interpreter.c b/src/interpreter.c
index 2548776..e904467 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -11,7 +11,7 @@
 
 int currentInstruction = 0;
 
-void runtimeError(GroundRuntimeError error, char* what, GroundInstruction* where, int whereLine) {
+[[noreturn]] void runtimeError(GroundRuntimeError error, char* what, GroundInstruction* where, int whereLine) {
     printf("Ground runtime error:\n  ErrorType: ");
     switch (error) {
         case ARG_TYPE_MISMATCH: {
@@ -46,6 +46,18 @@ void runtimeError(GroundRuntimeError error, char* what, GroundInstruction* where
             printf("MathError");
             break;
         }
+        case RETURN_TYPE_MISMATCH: {
+            printf("ReturnTypeMismatch");
+            break;
+        }
+        case PREMATURE_EOF: {
+            printf("PrematureEof");
+            break;
+        }
+        case INVALID_INSTRUCTION: {
+            printf("InvalidInstruction");
+            break;
+        }
         default:
         case FIXME: {
             printf("FIXME (please report issue to https://chsp.au/ground/cground)");
@@ -173,7 +185,7 @@ GroundDebugInstruction parseDebugInstruction(char* in) {
         }
     }
 
-    if (spacepos == -1) {
+    if (spacepos == (size_t) -1) {
         spacepos = insize;
     }
 
@@ -234,8 +246,25 @@ void groundAddNativeFunction(GroundScope* scope, char* name, NativeGroundFunctio
     addVariable(scope->variables, name, createFunctionGroundValue(gf));
 }
 
-GroundStruct parseStruct(GroundProgram* in) {
+GroundStruct parseStruct(GroundProgram* in, size_t errorOffset) {
     GroundStruct gstruct = createStruct();
+    for (size_t i = 0; i < in->size; i++) {
+        switch (in->instructions[i].type) {
+            case SET: {
+                break;
+            }
+            case INIT: {
+                break;
+            }
+            case FUN: {
+                break;
+            }
+            default: {
+                runtimeError(INVALID_INSTRUCTION, "Unsupported instruction while inside struct", &in->instructions[i], errorOffset + i);
+            }
+
+        }
+    }
     return gstruct;
 }
 
@@ -252,7 +281,7 @@ GroundValue interpretGroundProgram(GroundProgram* in, GroundScope* inScope) {
         scope.variables = &variables;
     }
     // Preprocess all labels, structs and functions
-    for (int i = 0; i < in->size; i++) {
+    for (size_t i = 0; i < in->size; i++) {
         if (in->instructions[i].type == CREATELABEL) {
             addLabel(scope.labels, in->instructions[i].args.args[0].value.refName, i);
         }
@@ -309,7 +338,8 @@ GroundValue interpretGroundProgram(GroundProgram* in, GroundScope* inScope) {
             i++;
 
             size_t counter = 1;
-            GroundProgram gp;
+            GroundProgram gp = createGroundProgram();
+            size_t errorOffset = i;
             while (counter > 0) {
                 if (i >= in->size) {
                     runtimeError(PREMATURE_EOF, "Reached end of scope before struct definition ended", &in->instructions[i - 1], i - 1);
@@ -321,12 +351,13 @@ GroundValue interpretGroundProgram(GroundProgram* in, GroundScope* inScope) {
                     counter--;
                 }
                 addInstructionToProgram(&gp, in->instructions[i]);
+                i++;
             }
 
-            GroundStruct gs = parseStruct(&gp);
+            GroundStruct gs = parseStruct(&gp, errorOffset);
         }
     }
-    for (int i = 0; i < in->size; i++) {
+    for (size_t i = 0; i < in->size; i++) {
         if (in->instructions[i].type == FUN) {
             int count = 1;
             while (count > 0) {
@@ -463,7 +494,7 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
     GroundInstruction* in = &copied_inst;
 
     // Insert variables prefixed with $
-    for (int i = 0; i < in->args.length; i++) {
+    for (size_t i = 0; i < in->args.length; i++) {
         if (in->args.args[i].type == VALREF) {
             GroundVariable* variable = findVariable(*scope->variables, in->args.args[i].value.refName);
             if (variable) {
@@ -476,9 +507,11 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
         }
     }
     switch (in->type) {
-        // We can safely ignore any CREATELABEL, FUN, and ENDFUN instructions, these have been preprocessed
+        // We can safely ignore these instructions, as they have been preprocessed
         case FUN:
         case ENDFUN:
+        case STRUCT:
+        case ENDSTRUCT:
         case CREATELABEL: {
             break;
         }
@@ -551,7 +584,7 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
             if (in->args.length < 1) {
                 runtimeError(TOO_FEW_ARGS, "Expecting 1 or more args", in, currentInstruction);
             }
-            for (int i = 0; i < in->args.length; i++) {
+            for (size_t i = 0; i < in->args.length; i++) {
                 if (i != 0) printf(" ");
                 printGroundArg(&in->args.args[i]);
             }
@@ -561,7 +594,7 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
             if (in->args.length < 1) {
                 runtimeError(TOO_FEW_ARGS, "Expecting 1 or more args", in, currentInstruction);
             }
-            for (int i = 0; i < in->args.length; i++) {
+            for (size_t i = 0; i < in->args.length; i++) {
                 if (i != 0) printf(" ");
                 printGroundArg(&in->args.args[i]);
             }
@@ -653,9 +686,15 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
                 }
                 case FUNCTION: {
                     addVariable(scope->variables, in->args.args[1].value.refName, createStringGroundValue("function"));
+                    break;
+                }
+                case STRUCTVAL: {
+                    addVariable(scope->variables, in->args.args[1].value.refName, createStringGroundValue("struct"));
+                    break;
                 }
                 case NONE: {
                     addVariable(scope->variables, in->args.args[1].value.refName, createStringGroundValue("none"));
+                    break;
                 }
             }
 
@@ -690,7 +729,7 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
                 runtimeError(ARG_TYPE_MISMATCH, "Expecting a DirectRef for arg 1", in, currentInstruction);
             }
             List newList = createList();
-            for (int i = 1; i < in->args.length; i++) {
+            for (size_t i = 1; i < in->args.length; i++) {
                 if (in->args.args[i].type != VALUE) {
                     runtimeError(ARG_TYPE_MISMATCH, "Expecting a Value for all args after arg 1", in, currentInstruction);
                 }
@@ -1167,7 +1206,7 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
                 runtimeError(ARG_TYPE_MISMATCH, "Expecting a DirectRef for arg 3", in, currentInstruction);
             }
             char* str = in->args.args[0].value.value.data.stringVal;
-            int64_t idx = in->args.args[1].value.value.data.intVal;
+            size_t idx = in->args.args[1].value.value.data.intVal;
             if (idx < strlen(str)) {
                 addVariable(scope->variables, in->args.args[2].value.refName, createCharGroundValue(str[idx]));
             } else {
diff --git a/src/interpreter.h b/src/interpreter.h
index fd6f990..83ce97e 100644
--- a/src/interpreter.h
+++ b/src/interpreter.h
@@ -7,7 +7,7 @@
 #include "include/uthash.h"
 
 typedef enum GroundRuntimeError {
-    ARG_TYPE_MISMATCH, TOO_FEW_ARGS, TOO_MANY_ARGS, UNKNOWN_LABEL, UNKNOWN_VARIABLE, LIST_ERROR, STRING_ERROR, MATH_ERROR, RETURN_TYPE_MISMATCH, PREMATURE_EOF, FIXME
+    ARG_TYPE_MISMATCH, TOO_FEW_ARGS, TOO_MANY_ARGS, UNKNOWN_LABEL, UNKNOWN_VARIABLE, LIST_ERROR, STRING_ERROR, MATH_ERROR, RETURN_TYPE_MISMATCH, PREMATURE_EOF, INVALID_INSTRUCTION, FIXME
 } GroundRuntimeError;
 
 typedef enum GroundDebugInstructionType {
@@ -36,9 +36,11 @@ typedef struct GroundDebugInstruction {
     char* arg;
 } GroundDebugInstruction;
 
-GroundStruct parseStruct(GroundProgram* in); 
+GroundStruct parseStruct(GroundProgram* in, size_t errorOffset); 
 GroundValue interpretGroundProgram(GroundProgram* in, GroundScope* inScope);
 GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scope);
 
 
+
+
 #endif
diff --git a/src/types.c b/src/types.c
index 50271aa..d5b294f 100644
--- a/src/types.c
+++ b/src/types.c
@@ -74,7 +74,7 @@ GroundValue copyGroundValue(const GroundValue* gv) {
             break;
         case LIST: {
             List newList = createList();
-            for (int i = 0; i < gv->data.listVal.size; i++) {
+            for (size_t i = 0; i < gv->data.listVal.size; i++) {
                 // Recursive call to handle nested lists and other types
                 appendToList(&newList, copyGroundValue(&gv->data.listVal.values[i]));
             }
@@ -93,6 +93,10 @@ GroundValue copyGroundValue(const GroundValue* gv) {
             // FIXME
             newGv.data.customVal = gv->data.customVal;
             break;
+        case NONE:
+        default: {
+
+        }
     }
     return newGv;
 }
@@ -125,7 +129,7 @@ void printGroundValue(GroundValue* gv) {
         }
         case LIST: {
             printf("[");
-            for (int i = 0; i < gv->data.listVal.size; i++) {
+            for (size_t i = 0; i < gv->data.listVal.size; i++) {
                 printGroundValue(&gv->data.listVal.values[i]);
                 if (i < gv->data.listVal.size - 1) {
                     printf(", ");
@@ -152,7 +156,7 @@ void freeGroundValue(GroundValue* gv) {
     }
     if (gv->type == LIST && gv->data.listVal.values != NULL) {
         List* list = &gv->data.listVal;
-        for (int i = 0; i < list->size; i++) {
+        for (size_t i = 0; i < list->size; i++) {
             freeGroundValue(&list->values[i]);
         }
         free(list->values);
@@ -394,7 +398,7 @@ void printGroundInstruction(GroundInstruction* gi) {
             break;
     }
     if (gi->type != CREATELABEL) printf(" ");
-    for (int i = 0; i < gi->args.length; i++) {
+    for (size_t i = 0; i < gi->args.length; i++) {
         if (gi->args.args[i].type == VALUE && gi->args.args[i].value.value.type == STRING) {
             printf("\"");
             printGroundArg(&gi->args.args[i]);
@@ -428,7 +432,7 @@ void appendToList(List* list, GroundValue value) {
     list->values[list->size - 1] = value;
 }
 
-ListAccess getListAt(List* list, int idx) {
+ListAccess getListAt(List* list, size_t idx) {
     if (list == NULL) {
         printf("Expecting a List ptr, got a null pointer instead.\nThis is likely not an error with your Ground program.\nPlease report this issue to https://chsp.au/ground/cground\n");
         exit(EXIT_FAILURE);
@@ -446,7 +450,7 @@ ListAccess getListAt(List* list, int idx) {
     }
 }
 
-ListAccessStatus setListAt(List* list, int idx, GroundValue value) {
+ListAccessStatus setListAt(List* list, size_t idx, GroundValue value) {
     if (list == NULL) {
         printf("Expecting a List ptr, got a null pointer instead.\nThis is likely not an error with your Ground program.\nPlease report this issue to https://chsp.au/ground/cground\n");
         exit(EXIT_FAILURE);
diff --git a/src/types.h b/src/types.h
index f5ac32e..b1803ee 100644
--- a/src/types.h
+++ b/src/types.h
@@ -234,10 +234,10 @@ void appendToList(List* list, GroundValue value);
 
 // Gets item at index (idx) from list (list). If there is an error, it
 // will be indicated in the status field.
-ListAccess getListAt(List* list, int idx);
+ListAccess getListAt(List* list, size_t idx);
 
 // Sets an item in list (list) at index (idx) to GroundValue (value).
-ListAccessStatus setListAt(List* list, int idx, GroundValue value);
+ListAccessStatus setListAt(List* list, size_t idx, GroundValue value);
 
 // Creates a Ground struct
 GroundStruct createStruct();