diff --git a/Makefile b/Makefile
index 6842e0ee..103c3307 100644
--- a/Makefile
+++ b/Makefile
@@ -16,7 +16,7 @@ all: $(GST_TARGET)
 ##### The core vm and runtime #####
 ###################################
 GST_CORE_SOURCES=$(addprefix core/,\
-				 compile.c disasm.c parse.c stl.c strings.c ids.c util.c\
+				 compile.c disasm.c parse.c stl.c ids.c util.c\
 				 value.c vm.c ds.c gc.c thread.c serialize.c)
 GST_CORE_OBJECTS=$(patsubst %.c,%.o,$(GST_CORE_SOURCES))
 $(GST_CORELIB): $(GST_CORE_OBJECTS) $(GST_HEADERS)
diff --git a/core/cache.h b/core/cache.h
index 2c80210b..b9ca4f9a 100644
--- a/core/cache.h
+++ b/core/cache.h
@@ -28,5 +28,6 @@
 void gst_cache_remove_string(Gst *vm, char *strmem);
 void gst_cache_remove_tuple(Gst *vm, char *tuplemem);
 void gst_cache_remove_struct(Gst *vm, char *structmem);
+void gst_cache_remove_userdata(Gst *vm, char *usermem);
 
 #endif /* end of include guard: CACHE_H_LVYZMBLR */
diff --git a/core/ds.c b/core/ds.c
index b40c9e95..61a391f8 100644
--- a/core/ds.c
+++ b/core/ds.c
@@ -159,12 +159,13 @@ GstValue gst_array_peek(GstArray *array) {
 /****/
 
 /* Create new userdata */
-void *gst_userdata(Gst *vm, uint32_t size, const GstValue *meta) {
+void *gst_userdata(Gst *vm, uint32_t size, const GstUserType *utype) {
     char *data = gst_alloc(vm, sizeof(GstUserdataHeader) + size);
     GstUserdataHeader *header = (GstUserdataHeader *)data;
     void *user = data + sizeof(GstUserdataHeader);
     header->size = size;
-    header->meta = meta;
+    header->type = utype;
+    gst_mem_tag(header, GST_MEMTAG_USER);
     return user;
 }
 
diff --git a/core/gc.c b/core/gc.c
index e6f0d377..608304b3 100644
--- a/core/gc.c
+++ b/core/gc.c
@@ -174,10 +174,8 @@ void gst_mark(Gst *vm, GstValueUnion x, GstType type) {
             if (gc_header(x.string - sizeof(GstUserdataHeader))->color != vm->black) {
                 GstUserdataHeader *userHeader = (GstUserdataHeader *)x.string - 1;
                 gc_header(userHeader)->color = vm->black;
-                GstValueUnion temp;
-                temp.st = userHeader->meta;
-                gst_mark(vm, temp, GST_STRUCT);
             }
+            break;
 
         case GST_FUNCENV:
             gst_mark_funcenv(vm, x.env);
@@ -210,6 +208,12 @@ void gst_sweep(Gst *vm) {
                     gst_cache_remove_struct(vm, (char *)(current + 1));
                 if (current->tags & GST_MEMTAG_TUPLE)
                     gst_cache_remove_tuple(vm, (char *)(current + 1));
+                if (current->tags & GST_MEMTAG_USER) {
+                    GstUserdataHeader *h = (GstUserdataHeader *)(current + 1); 
+                    if (h->type->finalize) {
+                        h->type->finalize(vm, h + 1, h->size);
+                    }
+                }
             }
             gst_raw_free(current);
         } else {
diff --git a/core/ids.c b/core/ids.c
index bc9e6601..0728ebdc 100644
--- a/core/ids.c
+++ b/core/ids.c
@@ -395,3 +395,25 @@ GstValue gst_string_cv(Gst *vm, const char *str) {
     ret.data.string = data;
     return ret;
 }
+
+/* Compares two strings */
+int gst_string_compare(const uint8_t *lhs, const uint8_t *rhs) {
+    uint32_t xlen = gst_string_length(lhs);
+    uint32_t ylen = gst_string_length(rhs);
+    uint32_t len = xlen > ylen ? ylen : xlen;
+    uint32_t i;
+    for (i = 0; i < len; ++i) {
+        if (lhs[i] == rhs[i]) {
+            continue;
+        } else if (lhs[i] < rhs[i]) {
+            return -1; /* x is less than y */
+        } else {
+            return 1; /* y is less than x */
+        }
+    }
+    if (xlen == ylen) {
+        return 0;
+    } else {
+        return xlen < ylen ? -1 : 1;
+    }
+}
diff --git a/core/serialize.c b/core/serialize.c
index 5f714bee..12265be3 100644
--- a/core/serialize.c
+++ b/core/serialize.c
@@ -38,12 +38,12 @@
  * Byte 207: Buffer  - [u32 length]*[u8... characters]
  * Byte 208: Array   - [u32 length]*[value... elements]
  * Byte 209: Tuple   - [u32 length]*[value... elements]
- * Byte 210: Thread  - [u8 state][u32 frames]*[[value callee][value env]
- *  [u32 pcoffset][u16 ret][u16 args][u16 size]*[value ...stack]
+ * Byte 210: Thread  - [value parent][u8 state][u32 frames]*[[value callee][value env]
+ *  [u32 pcoffset][u32 ret][u32 args][u32 size]*[value ...stack]]
  * Byte 211: Table   - [u32 length]*2*[value... kvs]
  * Byte 212: FuncDef - [u32 locals][u32 arity][u32 flags][u32 literallen]*[value...
  *  literals][u32 bytecodelen]*[u16... bytecode]
- * Byte 213: FunEnv  - [value thread][u32 length]*[value ...upvalues]
+ * Byte 213: FuncEnv - [value thread][u32 length]*[value ...upvalues]
  *  (upvalues is not read if thread is a thread object)
  * Byte 214: Func    - [value parent][value def][value env]
  *  (nil values indicate empty)
@@ -232,14 +232,15 @@ static const char *gst_deserialize_impl(
 
         case 210: /* Thread */
             {
-                GstValue nil;
                 GstThread *t;
                 GstValue *stack;
                 uint16_t prevsize = 0;
                 uint8_t statusbyte;
-                nil.type = GST_NIL;
-                t = gst_thread(vm, nil, 64);
-                ret.type = GST_THREAD;
+                t = gst_thread(vm, gst_wrap_nil(), 64);
+                err = gst_deserialize_impl(vm, data, end, &data, visited, &ret);
+                if (err != NULL) return err;
+                if (ret.type != GST_THREAD) return "expected thread parent to thread";
+                t->parent = ret.data.thread;
                 ret.data.thread = t;
                 deser_assert(data < end, UEB);
                 statusbyte = *data++;
@@ -258,16 +259,16 @@ static const char *gst_deserialize_impl(
                     uint16_t ret, args, size, j;
                     /* Create a new frame */
                     if (i > 0)
-                        gst_thread_beginframe(vm, t, nil, 0);
+                        gst_thread_beginframe(vm, t, gst_wrap_nil(), 0);
                     /* Read the stack */
                     err = gst_deserialize_impl(vm, data, end, &data, visited, &callee);
                     if (err != NULL) return err;
                     err = gst_deserialize_impl(vm, data, end, &data, visited, &env);
                     if (err != NULL) return err;
                     read_u32(pcoffset);
-                    read_u16(ret);
-                    read_u16(args);
-                    read_u16(size);
+                    read_u32(ret);
+                    read_u32(args);
+                    read_u32(size);
                     /* Set up the stack */
                     stack = gst_thread_stack(t);
                     if (callee.type == GST_FUNCTION) {
@@ -295,7 +296,7 @@ static const char *gst_deserialize_impl(
                 ret.type = GST_TABLE;
                 read_u32(length);
                 ret.data.table = gst_table(vm, 2 * length);
-                for (i = 0; i < length; i += 2) {
+                for (i = 0; i < length; ++i) {
                     GstValue key, value;
                     err = gst_deserialize_impl(vm, data, end, &data, visited, &key);
                     if (err != NULL) return err;
@@ -393,16 +394,8 @@ static const char *gst_deserialize_impl(
 
         case 215: /* LUdata */
             {
-                GstValue meta;
-                ret.type = GST_USERDATA;
-                err = gst_deserialize_impl(vm, data, end, &data, visited, &meta);
-                if (err != NULL) return err;
-                deser_assert(meta.type == GST_STRUCT, "userdata requires valid meta struct");
-                read_u32(length);
-                deser_datacheck(length);
-                ret.data.pointer = gst_userdata(vm, length, meta.data.st);
-                gst_memcpy(ret.data.pointer, data, length);
-                gst_array_push(vm, visited, ret);
+                /* TODO enable deserialization of userdata through registration
+                 * to names in vm. */ 
             }
             break;
 
@@ -445,7 +438,7 @@ const char *gst_deserialize(
     *out = ret;
     return NULL;
 }
-    
+
 /* Allow appending other types to buffers */
 BUFFER_DEFINE(real, GstReal)
 BUFFER_DEFINE(integer, GstInteger)
@@ -508,56 +501,124 @@ const char *gst_serialize_impl(
     /* Check reference types */
     switch (x.type) {
         default:
-           return "unable to serialize type"; 
+            return "unable to serialize type"; 
         case GST_STRING:
-           write_byte(205);
-           count = gst_string_length(x.data.string);
-           write_u32(count);
-           for (i = 0; i < count; ++i) {
-               write_byte(x.data.string[i]);
-           }
-           break;
+            write_byte(205);
+            count = gst_string_length(x.data.string);
+            write_u32(count);
+            for (i = 0; i < count; ++i) {
+                write_byte(x.data.string[i]);
+            }
+            break;
         case GST_STRUCT:
-           write_byte(206);
-           count = gst_struct_length(x.data.st);
-           write_u32(count);
-           for (i = 0; i < gst_struct_capacity(x.data.st); i += 2) {
-               if (x.data.st[i].type != GST_NIL) {
-                   err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.st[i]); 
-                   if (err != NULL) return err;
-                   err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.st[i + 1]); 
-                   if (err != NULL) return err;
-               }
-           }
-           break;
+            write_byte(206);
+            count = gst_struct_length(x.data.st);
+            write_u32(count);
+            for (i = 0; i < gst_struct_capacity(x.data.st); i += 2) {
+                if (x.data.st[i].type != GST_NIL) {
+                    err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.st[i]); 
+                    if (err != NULL) return err;
+                    err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.st[i + 1]); 
+                    if (err != NULL) return err;
+                }
+            }
+            break;
         case GST_BYTEBUFFER:
-           write_byte(207);
-           count = x.data.buffer->count;
-           write_u32(count);
-           for (i = 0; i < count; ++i) {
+            write_byte(207);
+            count = x.data.buffer->count;
+            write_u32(count);
+            for (i = 0; i < count; ++i) {
                 write_byte(x.data.buffer->data[i]);
-           }
-           break;
+            }
+            break;
         case GST_ARRAY:
-           write_byte(208);
-           count = x.data.array->count;
-           write_u32(count);
-           for (i = 0; i < count; ++i) {
-               err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.array->data[i]); 
-               if (err != NULL) return err;
-           }
-           break;
+            write_byte(208);
+            count = x.data.array->count;
+            write_u32(count);
+            for (i = 0; i < count; ++i) {
+                err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.array->data[i]); 
+                if (err != NULL) return err;
+            }
+            break;
         case GST_TUPLE:
-           write_byte(209);
-           count = gst_tuple_length(x.data.tuple);
-           write_u32(count);
-           for (i = 0; i < count; ++i) {
-               err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.tuple[i]); 
-               if (err != NULL) return err;
-           }
-           break;
-        /*case GST_THREAD:*/
-           /*break;*/
+            write_byte(209);
+            count = gst_tuple_length(x.data.tuple);
+            write_u32(count);
+            for (i = 0; i < count; ++i) {
+                err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.tuple[i]); 
+                if (err != NULL) return err;
+            }
+            break;
+        case GST_THREAD:
+            {
+                GstThread *t = x.data.thread;
+                const GstValue *stack;
+                uint32_t framecount = gst_thread_countframes(t);
+                uint32_t i;
+                err = gst_serialize_impl(vm, buffer, visited, nextId, gst_wrap_thread(t->parent));
+                if (err != NULL) return err;
+                /* Write the status byte */
+                if (t->status == GST_THREAD_PENDING) write_byte(0);
+                else if (t->status == GST_THREAD_ALIVE) write_byte(1);
+                else write_byte(2);
+                /* Write number of stack frames */
+                write_u32(framecount);
+                /* Write stack frames */
+                for (i = 0; i < framecount; ++i) {
+                    uint32_t j, size;
+                    GstValue callee = gst_frame_callee(stack);
+                    GstFuncEnv *env = gst_frame_env(stack); 
+                    err = gst_serialize_impl(vm, buffer, visited, nextId, callee); 
+                    if (err != NULL) return err;
+                    err = gst_serialize_impl(vm, buffer, visited, nextId, gst_wrap_funcenv(env));
+                    if (err != NULL) return err;
+                    if (callee.type == GST_FUNCTION) {
+                        write_u32(gst_frame_pc(stack) - callee.data.function->def->byteCode);
+                    } else {
+                        write_u32(0);
+                    }
+                    write_u32(gst_frame_ret(stack));
+                    write_u32(gst_frame_args(stack));
+                    size = gst_frame_size(stack);
+                    write_u32(size);
+                    for (j = 0; j < size; ++j) {
+                        err = gst_serialize_impl(vm, buffer, visited, nextId, stack[j]);
+                        if (err != NULL) return err;
+                    } 
+                    /* Next stack frame */
+                    stack = t->data + GST_FRAME_SIZE;
+                }
+            }
+            break;
+        case GST_TABLE:
+            write_byte(211);
+            count = x.data.table->count;
+            write_u32(count);
+            for (i = 0; i < x.data.table->capacity; i += 2) {
+                if (x.data.table->data[i].type == GST_NIL) continue;
+                err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.tuple[i]); 
+                if (err != NULL) return err;
+                err = gst_serialize_impl(vm, buffer, visited, nextId, x.data.tuple[i+1]); 
+                if (err != NULL) return err;
+            }
+            break;
+        case GST_FUNCDEF: /* Funcdef */
+            {
+                /* TODO */
+            }
+            break;
+
+        case GST_FUNCENV: /* Funcenv */
+            {
+                /* TODO */
+            }
+            break;
+
+        case GST_FUNCTION: /* Function */
+            {
+                /* TODO */
+            }
+            break;
     }
 
     /* Record reference */
diff --git a/core/stl.c b/core/stl.c
index 6fca43e0..1d1aa83d 100644
--- a/core/stl.c
+++ b/core/stl.c
@@ -328,17 +328,21 @@ int gst_stl_struct(Gst *vm) {
 /* Create a buffer */
 int gst_stl_buffer(Gst *vm) {
     uint32_t i, count;
+    const uint8_t *dat;
+    uint32_t slen;
     GstBuffer *buf = gst_buffer(vm, 10);
     count = gst_count_args(vm);
     for (i = 0; i < count; ++i) {
-        const uint8_t *string = gst_to_string(vm, gst_arg(vm, i));
-        gst_buffer_append(vm, buf, string, gst_string_length(string));
+        if (gst_chararray_view(gst_arg(vm, i), &dat, &slen))
+            gst_buffer_append(vm, buf, dat, slen);
+        else
+            gst_c_throwc(vm, GST_EXPECTED_STRING);
     }
     gst_c_return(vm, gst_wrap_buffer(buf));
 }
 
-/* Concatenate strings */
-int gst_stl_strcat(Gst *vm) {
+/* Create a string */
+int gst_stl_string(Gst *vm) {
     uint32_t j;
     uint32_t count = gst_count_args(vm);
     uint32_t length = 0;
@@ -515,7 +519,13 @@ int gst_stl_setglobal(Gst *vm) {
 /* IO */
 /****/
 
-/* TODO - add userdata to allow for manipulation of FILE pointers. */
+/* File type definition */
+static GstUserType gst_stl_filetype = {
+    "io.file",
+    NULL,
+    NULL, 
+    NULL
+};
 
 /* Open a a file and return a userdata wrapper arounf the C file API. */
 int gst_stl_open(Gst *vm) {
@@ -523,20 +533,44 @@ int gst_stl_open(Gst *vm) {
     const uint8_t *fmode = gst_to_string(vm, gst_arg(vm, 1));
     FILE *f;
     FILE **fp;
-    GstValue *st;
     if (gst_count_args(vm) < 2 || gst_arg(vm, 0).type != GST_STRING 
             || gst_arg(vm, 1).type != GST_STRING)
         gst_c_throwc(vm, "expected filename and filemode");
     f = fopen((const char *)fname, (const char *)fmode);
     if (!f)
         gst_c_throwc(vm, "could not open file");
-    st = gst_struct_begin(vm, 0);
-    fp = gst_userdata(vm, sizeof(FILE *), gst_struct_end(vm, st));
+    fp = gst_userdata(vm, sizeof(FILE *), &gst_stl_filetype);
     *fp = f;
     gst_c_return(vm, gst_wrap_userdata(fp));
 }
 
-/* Write a string to a file in one go. Overwrites an existing file. */
+/* Read an entire file into memory */
+int gst_stl_slurp(Gst *vm) {
+    GstBuffer *b;
+    long fsize;
+    FILE *f;
+    FILE **fp = gst_check_userdata(vm, 0, &gst_stl_filetype);
+    if (fp == NULL) gst_c_throwc(vm, "expected file");
+    if (!gst_check_buffer(vm, 1, &b)) b = gst_buffer(vm, 10);
+    f = *fp;
+    /* Read whole file */
+    fseek(f, 0, SEEK_END);
+    fsize = ftell(f);
+    fseek(f, 0, SEEK_SET);
+    /* Ensure buffer size */
+    gst_buffer_ensure(vm, b, b->count + fsize);
+    fread((char *)(b->data + b->count), fsize, 1, f);
+    b->count += fsize;
+    gst_c_return(vm, gst_wrap_buffer(b));
+}
+
+/* Close a file */
+int gst_stl_close(Gst *vm) {
+    FILE **fp = gst_check_userdata(vm, 0, &gst_stl_filetype);
+    if (fp == NULL) gst_c_throwc(vm, "expected file");
+    fclose(*fp);
+    gst_c_return(vm, gst_wrap_nil());
+}
 
 /****/
 /* Temporary */
@@ -576,8 +610,8 @@ static const GstModuleItem const std_module[] = {
     {"not", gst_stl_not},
     {"length", gst_stl_length},
     {"hash", gst_stl_hash},
-    {"to-integer", gst_stl_to_int},
-    {"to-real", gst_stl_to_real},
+    {"integer", gst_stl_to_int},
+    {"real", gst_stl_to_real},
     {"type", gst_stl_type},
     {"slice", gst_stl_slice},
     {"array", gst_stl_array},
@@ -585,7 +619,7 @@ static const GstModuleItem const std_module[] = {
     {"table", gst_stl_table},
     {"struct", gst_stl_struct},
     {"buffer", gst_stl_buffer},
-    {"strcat", gst_stl_strcat},
+    {"string", gst_stl_string},
     {"print", gst_stl_print},
     {"tostring", gst_stl_tostring},
     {"exit", gst_stl_exit},
@@ -601,6 +635,8 @@ static const GstModuleItem const std_module[] = {
     {"peek", gst_stl_peek},
     {"ensure", gst_stl_ensure},
     {"open", gst_stl_open},
+    {"slurp", gst_stl_slurp},
+    {"close", gst_stl_close},
     {"dasm", gst_stl_dasm},
     {NULL, NULL}
 };
diff --git a/core/strings.c b/core/strings.c
deleted file mode 100644
index a10a3d94..00000000
--- a/core/strings.c
+++ /dev/null
@@ -1,45 +0,0 @@
-/*
-* Copyright (c) 2017 Calvin Rose
-* 
-* Permission is hereby granted, free of charge, to any person obtaining a copy
-* of this software and associated documentation files (the "Software"), to
-* deal in the Software without restriction, including without limitation the
-* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
-* sell copies of the Software, and to permit persons to whom the Software is
-* furnished to do so, subject to the following conditions:
-* 
-* The above copyright notice and this permission notice shall be included in
-* all copies or substantial portions of the Software.
-* 
-* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
-* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
-* IN THE SOFTWARE.
-*/
-
-#include <gst/gst.h>
-
-/* Compares two strings */
-int gst_string_compare(const uint8_t *lhs, const uint8_t *rhs) {
-    uint32_t xlen = gst_string_length(lhs);
-    uint32_t ylen = gst_string_length(rhs);
-    uint32_t len = xlen > ylen ? ylen : xlen;
-    uint32_t i;
-    for (i = 0; i < len; ++i) {
-        if (lhs[i] == rhs[i]) {
-            continue;
-        } else if (lhs[i] < rhs[i]) {
-            return -1; /* x is less than y */
-        } else {
-            return 1; /* y is less than x */
-        }
-    }
-    if (xlen == ylen) {
-        return 0;
-    } else {
-        return xlen < ylen ? -1 : 1;
-    }
-}
diff --git a/core/thread.c b/core/thread.c
index f7551873..9cc4422f 100644
--- a/core/thread.c
+++ b/core/thread.c
@@ -173,3 +173,15 @@ GstValue *gst_thread_popframe(Gst *vm, GstThread *thread) {
     else
         return NULL;
 }
+
+/* Count the number of stack frames in a thread */
+uint32_t gst_thread_countframes(GstThread *thread) {
+    uint32_t count = 0;
+    const GstValue *stack = thread->data + GST_FRAME_SIZE;
+    const GstValue *laststack = thread->data + thread->count;
+    while (stack <= laststack) {
+        ++count;
+        stack += gst_frame_size(stack) + GST_FRAME_SIZE;
+    }
+    return count;
+}
diff --git a/core/util.c b/core/util.c
index 3062ee5b..dc3aee07 100644
--- a/core/util.c
+++ b/core/util.c
@@ -63,12 +63,27 @@ GST_WRAP_DEFINE(buffer, GstBuffer *, GST_BYTEBUFFER, buffer)
 GST_WRAP_DEFINE(function, GstFunction *, GST_FUNCTION, function)
 GST_WRAP_DEFINE(cfunction, GstCFunction, GST_CFUNCTION, cfunction)
 GST_WRAP_DEFINE(table, GstTable *, GST_TABLE, table)
-GST_WRAP_DEFINE(userdata, void *, GST_USERDATA, pointer)
 GST_WRAP_DEFINE(funcenv, GstFuncEnv *, GST_FUNCENV, env)
 GST_WRAP_DEFINE(funcdef, GstFuncDef *, GST_FUNCDEF, def)
 
 #undef GST_WRAP_DEFINE
 
+GstValue gst_wrap_userdata(void *x) {
+    GstValue ret;
+    ret.type = GST_USERDATA;
+    ret.data.pointer = x;
+    return ret;
+}
+
+void *gst_check_userdata(Gst *vm, uint32_t i, const GstUserType *type) {
+    GstValue x = gst_arg(vm, i);
+    GstUserdataHeader *h;
+    if (x.type != GST_USERDATA) return NULL;
+    h = ((GstUserdataHeader *)x.data.pointer) - 1;
+    if (h->type != type) return NULL;
+    return x.data.pointer;
+}
+
 GstValue gst_cmodule_table(Gst *vm, const GstModuleItem *mod) {
     GstTable *module = gst_table(vm, 10);
     while (mod->name != NULL) {
diff --git a/include/gst/gst.h b/include/gst/gst.h
index f4e12610..a6a35ac0 100644
--- a/include/gst/gst.h
+++ b/include/gst/gst.h
@@ -148,14 +148,13 @@ typedef struct GstTable GstTable;
 typedef struct GstThread GstThread;
 typedef int (*GstCFunction)(Gst * vm);
 
-/* Implementation details */
+/* Other structs */
 typedef struct GstUserdataHeader GstUserdataHeader;
 typedef struct GstFuncDef GstFuncDef;
 typedef struct GstFuncEnv GstFuncEnv;
 typedef union GstValueUnion GstValueUnion;
-
-/* API Types */
 typedef struct GstModuleItem GstModuleItem;
+typedef struct GstUserType GstUserType;
 
 /* C Api data types */
 struct GstModuleItem {
@@ -259,10 +258,18 @@ struct GstFunction {
     GstFunction *parent;
 };
 
+/* Defines a type for userdata */
+struct GstUserType {
+    const char *id;
+    GstValue (*serialize)(Gst *vm, void *data, uint32_t len);
+    GstValue (*deserialize)(Gst *vm, GstValue in);
+    void (*finalize)(Gst *vm, void *data, uint32_t len);
+};
+
 /* Contains information about userdata */
 struct GstUserdataHeader {
     uint32_t size;
-    const GstValue *meta;
+    const GstUserType *type;
 };
 
 /* VM return status from c function */
@@ -356,7 +363,7 @@ GstValue gst_array_peek(GstArray *array);
 /* Userdata functions */
 /****/
 
-void *gst_userdata(Gst *vm, uint32_t size, const GstValue *meta);
+void *gst_userdata(Gst *vm, uint32_t size, const GstUserType *utype);
 
 /****/
 /* Tuple functions */
@@ -409,6 +416,7 @@ void gst_thread_tuplepack(Gst *vm, GstThread *thread, uint32_t n);
 GstValue *gst_thread_beginframe(Gst *vm, GstThread *thread, GstValue callee, uint32_t arity); 
 void gst_thread_endframe(Gst *vm, GstThread *thread);
 GstValue *gst_thread_popframe(Gst *vm, GstThread *thread); 
+uint32_t gst_thread_countframes(GstThread *thread);
 
 /****/
 /* Value manipulation */
@@ -427,43 +435,13 @@ GstInteger gst_length(Gst *vm, GstValue x);
 /* Serialization */
 /****/
 
-/**
- * Data format
- * State is encoded as a string of unsigned bytes.
- *
- * Types:
- *
- * Byte 0 to 200: small integer byte - 100
- * Byte 201: Nil
- * Byte 202: True
- * Byte 203: False
- * Byte 204: Number  - double format
- * Byte 205: String  - [u32 length]*[u8... characters]
- * Byte 206: Symbol  - [u32 length]*[u8... characters]
- * Byte 207: Buffer  - [u32 length]*[u8... characters]
- * Byte 208: Array   - [u32 length]*[value... elements]
- * Byte 209: Tuple   - [u32 length]*[value... elements]
- * Byte 210: Thread  - [u8 state][u32 frames]*[[value callee][value env]
- *  [u32 pcoffset][u32 erroffset][u16 ret][u16 errloc][u16 size]*[value ...stack]
- * Byte 211: Table   - [u32 length]*2*[value... kvs]
- * Byte 212: FuncDef - [u32 locals][u32 arity][u32 flags][u32 literallen]*[value...
- *  literals][u32 bytecodelen]*[u16... bytecode]
- * Byte 213: FunEnv  - [value thread][u32 length]*[value ...upvalues]
- *  (upvalues is not read if thread is a thread object)
- * Byte 214: Func    - [value parent][value def][value env]
- *  (nil values indicate empty)
- * Byte 215: LUdata  - [value meta][u32 length]*[u8... bytes]
- * Byte 216: CFunc   - [u32 length]*[u8... idstring]
- * Byte 217: Ref     - [u32 id]
- * Byte 218: Integer - [i64 value]
- */
-
 const char *gst_deserialize(
         Gst *vm,
         const uint8_t *data,
         uint32_t len,
         GstValue *out,
         const uint8_t *nextData);
+
 const char *gst_serialize(Gst *vm, GstBuffer *buffer, GstValue x);
 
 /****/
@@ -473,6 +451,7 @@ const char *gst_serialize(Gst *vm, GstBuffer *buffer, GstValue x);
 #define GST_MEMTAG_STRING 4
 #define GST_MEMTAG_TUPLE 8
 #define GST_MEMTAG_STRUCT 16
+#define GST_MEMTAG_USER 32
 
 void gst_mark_value(Gst *vm, GstValue x);
 void gst_mark(Gst *vm, GstValueUnion x, GstType type);
@@ -539,10 +518,11 @@ int gst_check_buffer(Gst *vm, uint32_t i, GstBuffer *(*x));
 int gst_check_function(Gst *vm, uint32_t i, GstFunction *(*x));
 int gst_check_cfunction(Gst *vm, uint32_t i, GstCFunction (*x));
 int gst_check_table(Gst *vm, uint32_t i, GstTable *(*x));
-int gst_check_userdata(Gst *vm, uint32_t i, void *(*x));
 int gst_check_funcenv(Gst *vm, uint32_t i, GstFuncEnv *(*x));
 int gst_check_funcdef(Gst *vm, uint32_t i, GstFuncDef *(*x));
+void *gst_check_userdata(Gst *vm, uint32_t i, const GstUserType *type);
 
+/* Treat similar types through uniform interfaces */
 int gst_seq_view(GstValue seq, const GstValue **data, uint32_t *len);
 int gst_chararray_view(GstValue str, const uint8_t **data, uint32_t *len);
 int gst_hashtable_view(GstValue tab, const GstValue **data, uint32_t *cap);
diff --git a/libs/pp.gst b/libs/pp.gst
index a6ded9d7..be47fd13 100644
--- a/libs/pp.gst
+++ b/libs/pp.gst
@@ -14,7 +14,7 @@
         (: i (+ 1 i)))
     (if (> len 0) (pop parts))
     (push parts end)
-    (apply strcat start parts)))
+    (apply string start parts)))
 
 # Pretty print an object or struct
 (: print-struct (fn [start end s]
@@ -28,7 +28,7 @@
         (: key (next s key)))
     (if (> (length parts) 0) (pop parts))
     (push parts end)
-    (apply strcat start parts)))
+    (apply string start parts)))
 
 # Pretty