diff --git a/Makefile b/Makefile
index dbe0c538..2368be0c 100644
--- a/Makefile
+++ b/Makefile
@@ -31,7 +31,7 @@ PREFIX=/usr/local
 DST_TARGET=dst
 DST_XXD=xxd
 DEBUGGER=lldb
-DST_INTERNAL_HEADERS=$(addprefix core/,symcache.h opcodes.h)
+DST_INTERNAL_HEADERS=$(addprefix core/,symcache.h opcodes.h strtod.h)
 DST_HEADERS=$(addprefix include/dst/,dst.h)
 
 #############################
@@ -59,7 +59,7 @@ $(DST_XXD): libs/xxd.c
 ###################################
 
 DST_CORE_SOURCES=$(addprefix core/,\
-				 array.c asm.c buffer.c fiber.c func.c gc.c parse.c string.c\
+				 array.c asm.c buffer.c fiber.c func.c gc.c parse.c string.c strtod.c\
 				 struct.c symcache.c syscalls.c table.c tuple.c userdata.c util.c\
 				 value.c vm.c wrap.c)
 DST_CORE_OBJECTS=$(patsubst %.c,%.o,$(DST_CORE_SOURCES))
@@ -79,7 +79,7 @@ CCU_FLAGS = $(CFLAGS) -DDST_UNIT_TEST
 
 DST_UNIT_BINARIES=$(addprefix unittests/,\
 				  asm_test.out array_test.out buffer_test.out fiber_test.out \
-				  parse_test.out table_test.out)
+				  parse_test.out strtod_test.out table_test.out)
 
 %.out: %.c $(DST_CORE_OBJECTS) $(DST_ALL_HEADERS) unittests/unit.h
 	$(CC) $(CCU_FLAGS) $(DST_CORE_OBJECTS) $< -o $@
@@ -90,6 +90,7 @@ unit: $(DST_UNIT_BINARIES)
 	unittests/buffer_test.out
 	unittests/fiber_test.out
 	unittests/parse_test.out
+	unittests/strtod_test.out
 	unittests/table_test.out
 
 ###################
diff --git a/core/asm.c b/core/asm.c
index 92e38490..131b5944 100644
--- a/core/asm.c
+++ b/core/asm.c
@@ -92,6 +92,7 @@ struct DstAssembler {
     DstFuncDef *def;
     jmp_buf on_error;
     const uint8_t *errmessage;
+    const DstValue *errmap;
 
     int32_t environments_capacity;
     int32_t bytecode_count; /* Used for calculating labels */
@@ -229,21 +230,27 @@ static void dst_asm_deinit(DstAssembler *a) {
 }
 
 /* Throw some kind of assembly error */
-static void dst_asm_error(DstAssembler *a, const char *message) {
+static void dst_asm_error(DstAssembler *a, const DstValue *map, const char *message) {
     a->errmessage = dst_cstring(message);
+    a->errmap = map;
     longjmp(a->on_error, 1);
 }
-#define dst_asm_assert(a, c, m) do { if (!(c)) dst_asm_error((a), (m)); } while (0)
+#define dst_asm_assert(a, c, map, m) do { if (!(c)) dst_asm_error((a), (map), (m)); } while (0)
 
 /* Throw some kind of assembly error */
-static void dst_asm_errorv(DstAssembler *a, const uint8_t *m) {
+static void dst_asm_errorv(DstAssembler *a, const DstValue *map, const uint8_t *m) {
     a->errmessage = m;
+    a->errmap = map;
     longjmp(a->on_error, 1);
 }
 
 /* Parse an argument to an assembly instruction, and return the result as an
  * integer. This integer will need to be trimmed and bound checked. */
-static int32_t doarg_1(DstAssembler *a, DstOpArgType argtype, DstValue x) {
+static int32_t doarg_1(
+        DstAssembler *a,
+        const DstValue *map,
+        DstOpArgType argtype,
+        DstValue x) {
     int32_t ret = -1;
     DstTable *c;
     switch (argtype) {
@@ -281,7 +288,7 @@ static int32_t doarg_1(DstAssembler *a, DstOpArgType argtype, DstValue x) {
                 int32_t i = 0;
                 ret = 0;
                 for (i = 0; i < dst_tuple_length(t); i++) {
-                    ret |= doarg_1(a, DST_OAT_SIMPLETYPE, t[i]);
+                    ret |= doarg_1(a, map, DST_OAT_SIMPLETYPE, t[i]);
                 }
             } else {
                 goto error;
@@ -299,14 +306,14 @@ static int32_t doarg_1(DstAssembler *a, DstOpArgType argtype, DstValue x) {
                         ret = dst_unwrap_integer(result);
                     }
                 } else {
-                    dst_asm_errorv(a, dst_formatc("unknown name %q", x));
+                    dst_asm_errorv(a, map, dst_formatc("unknown name %q", x));
                 }
             } else if (argtype == DST_OAT_TYPE || argtype == DST_OAT_SIMPLETYPE) {
                 int32_t index = strsearch(dst_unwrap_symbol(x), dst_type_names);
                 if (index != -1) {
                     ret = index;
                 } else {
-                    dst_asm_errorv(a, dst_formatc("unknown type %q", x));
+                    dst_asm_errorv(a, map, dst_formatc("unknown type %q", x));
                 }
             } else {
                 goto error;
@@ -319,7 +326,7 @@ static int32_t doarg_1(DstAssembler *a, DstOpArgType argtype, DstValue x) {
     return ret;
 
     error:
-    dst_asm_errorv(a, dst_formatc("error parsing instruction argument %v", x));
+    dst_asm_errorv(a, map, dst_formatc("error parsing instruction argument %v", x));
     return 0;
 }
 
@@ -327,99 +334,104 @@ static int32_t doarg_1(DstAssembler *a, DstOpArgType argtype, DstValue x) {
  * try to convert arguments to bit patterns */
 static uint32_t doarg(
         DstAssembler *a,
+        const DstValue *map,
         DstOpArgType argtype,
         int nth,
         int nbytes,
         int hassign,
         DstValue x) {
-    int32_t arg = doarg_1(a, argtype, x);
+    int32_t arg = doarg_1(a, map, argtype, x);
     /* Calculate the min and max values that can be stored given
      * nbytes, and whether or not the storage is signed */
     int32_t min = (-hassign) << ((nbytes << 3) - 1);
     int32_t max = ~((-1) << ((nbytes << 3) - hassign));
     if (arg < min)
-        dst_asm_errorv(a, dst_formatc("instruction argument %v is too small, must be %d byte%s",
+        dst_asm_errorv(a, map, dst_formatc("instruction argument %v is too small, must be %d byte%s",
                     x, nbytes, nbytes > 1 ? "s" : ""));
     if (arg > max)
-        dst_asm_errorv(a, dst_formatc("instruction argument %v is too large, must be %d byte%s",
+        dst_asm_errorv(a, map, dst_formatc("instruction argument %v is too large, must be %d byte%s",
                     x, nbytes, nbytes > 1 ? "s" : ""));
     return ((uint32_t) arg) << (nth << 3);
 }
 
 /* Provide parsing methods for the different kinds of arguments */
-static uint32_t read_instruction(DstAssembler *a, const DstInstructionDef *idef, const DstValue *argt) {
+static uint32_t read_instruction(
+        DstAssembler *a, 
+        const DstValue *map,
+        const DstInstructionDef *idef,
+        const DstValue *argt) {
     uint32_t instr = idef->opcode;
     switch (idef->type) {
         case DIT_0:
         {
             if (dst_tuple_length(argt) != 1)
-                dst_asm_error(a, "expected 0 arguments: (op)");
+                dst_asm_error(a, map, "expected 0 arguments: (op)");
             break;
         }
         case DIT_S:
         {
             if (dst_tuple_length(argt) != 2)
-                dst_asm_error(a, "expected 1 argument: (op, slot)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 3, 0, argt[1]);
+                dst_asm_error(a, map, "expected 1 argument: (op, slot)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 3, 0, argt[1]);
             break;
         }
         case DIT_L:
         {
             if (dst_tuple_length(argt) != 2)
-                dst_asm_error(a, "expected 1 argument: (op, label)");
-            instr |= doarg(a, DST_OAT_LABEL, 1, 3, 1, argt[1]);
+                dst_asm_error(a, map, "expected 1 argument: (op, label)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_LABEL, 1, 3, 1, argt[1]);
             break;
         }
         case DIT_SS:
         {
             if (dst_tuple_length(argt) != 3)
-                dst_asm_error(a, "expected 2 arguments: (op, slot, slot)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_SLOT, 2, 2, 0, argt[2]);
+                dst_asm_error(a, map, "expected 2 arguments: (op, slot, slot)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_SLOT, 2, 2, 0, argt[2]);
             break;
         }
         case DIT_SL:
         {
             if (dst_tuple_length(argt) != 3)
-                dst_asm_error(a, "expected 2 arguments: (op, slot, label)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_LABEL, 2, 2, 1, argt[2]);
+                dst_asm_error(a, map, "expected 2 arguments: (op, slot, label)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_LABEL, 2, 2, 1, argt[2]);
             break;
         }
         case DIT_ST:
         {
             if (dst_tuple_length(argt) != 3)
-                dst_asm_error(a, "expected 2 arguments: (op, slot, type)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_TYPE, 2, 2, 0, argt[2]);
+                dst_asm_error(a, map, "expected 2 arguments: (op, slot, type)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_TYPE, 2, 2, 0, argt[2]);
             break;
         }
         case DIT_SI:
         case DIT_SU:
         {
             if (dst_tuple_length(argt) != 3)
-                dst_asm_error(a, "expected 2 arguments: (op, slot, integer)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_INTEGER, 2, 2, idef->type == DIT_SI, argt[2]);
+                dst_asm_error(a, map, "expected 2 arguments: (op, slot, integer)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_INTEGER, 2, 2, idef->type == DIT_SI, argt[2]);
             break;
         }
         case DIT_SSS:
         {
             if (dst_tuple_length(argt) != 4)
-                dst_asm_error(a, "expected 3 arguments: (op, slot, slot, slot)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_SLOT, 2, 1, 0, argt[2]);
-            instr |= doarg(a, DST_OAT_SLOT, 3, 1, 0, argt[3]);
+                dst_asm_error(a, map, "expected 3 arguments: (op, slot, slot, slot)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_SLOT, 2, 1, 0, argt[2]);
+            instr |= doarg(a, dst_parse_submap_index(map, 3), DST_OAT_SLOT, 3, 1, 0, argt[3]);
             break;
         }
         case DIT_SSI:
         case DIT_SSU:
         {
             if (dst_tuple_length(argt) != 4)
-                dst_asm_error(a, "expected 3 arguments: (op, slot, slot, integer)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_SLOT, 2, 1, 0, argt[2]);
-            instr |= doarg(a, DST_OAT_INTEGER, 3, 1, idef->type == DIT_SSI, argt[3]);
+                dst_asm_error(a, map, "expected 3 arguments: (op, slot, slot, integer)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_SLOT, 2, 1, 0, argt[2]);
+            instr |= doarg(a, dst_parse_submap_index(map, 3), DST_OAT_INTEGER, 3, 1, idef->type == DIT_SSI, argt[3]);
             break;
         }
         case DIT_SES:
@@ -427,24 +439,24 @@ static uint32_t read_instruction(DstAssembler *a, const DstInstructionDef *idef,
             DstAssembler *b = a;
             uint32_t env;
             if (dst_tuple_length(argt) != 4)
-                dst_asm_error(a, "expected 3 arguments: (op, slot, environment, envslot)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            env = doarg(a, DST_OAT_ENVIRONMENT, 0, 1, 0, argt[2]);
+                dst_asm_error(a, map, "expected 3 arguments: (op, slot, environment, envslot)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            env = doarg(a, dst_parse_submap_index(map, 2), DST_OAT_ENVIRONMENT, 0, 1, 0, argt[2]);
             instr |= env << 16;
             for (env += 1; env > 0; env--) {
                 b = b->parent;
                 if (NULL == b)
-                    dst_asm_error(a, "invalid environment index");
+                    dst_asm_error(a, dst_parse_submap_index(map, 2), "invalid environment index");
             }
-            instr |= doarg(b, DST_OAT_SLOT, 3, 1, 0, argt[3]);
+            instr |= doarg(b, dst_parse_submap_index(map, 3), DST_OAT_SLOT, 3, 1, 0, argt[3]);
             break;
         }
         case DIT_SC:
         {
             if (dst_tuple_length(argt) != 3)
-                dst_asm_error(a, "expected 2 arguments: (op, slot, constant)");
-            instr |= doarg(a, DST_OAT_SLOT, 1, 1, 0, argt[1]);
-            instr |= doarg(a, DST_OAT_CONSTANT, 2, 2, 0, argt[2]);
+                dst_asm_error(a, map, "expected 2 arguments: (op, slot, constant)");
+            instr |= doarg(a, dst_parse_submap_index(map, 1), DST_OAT_SLOT, 1, 1, 0, argt[1]);
+            instr |= doarg(a, dst_parse_submap_index(map, 2), DST_OAT_CONSTANT, 2, 2, 0, argt[2]);
             break;
         }
     }
@@ -513,23 +525,33 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
     a.errmessage = NULL;
     a.environments_capacity = 0;
     a.bytecode_count = 0;
+    a.errmap = NULL;
     dst_table_init(&a.labels, 10);
     dst_table_init(&a.constants, 10);
     dst_table_init(&a.slots, 10);
     dst_table_init(&a.envs, 10);
 
+    /* Initialize result */
+    result.error_start = -1;
+    result.error_end = -1;
+
     /* Set error jump */
     if (setjmp(a.on_error)) {
-        dst_asm_deinit(&a);
         if (NULL != a.parent) {
+            dst_asm_deinit(&a);
             longjmp(a.parent->on_error, 1);
         }
         result.result.error = a.errmessage;
         result.status = DST_ASSEMBLE_ERROR;
+        if (a.errmap != NULL) {
+            result.error_start = dst_unwrap_integer(a.errmap[0]);
+            result.error_end = dst_unwrap_integer(a.errmap[1]);
+        }
+        dst_asm_deinit(&a);
         return result;
     }
 
-    dst_asm_assert(&a, dst_checktype(opts.source, DST_STRUCT), "expected struct for assembly source");
+    dst_asm_assert(&a, dst_checktype(opts.source, DST_STRUCT), opts.sourcemap, "expected struct for assembly source");
 
     /* Set function arity */
     x = dst_struct_get(st, dst_csymbolv("arity"));
@@ -538,20 +560,24 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
     /* Create slot aliases */
     x = dst_struct_get(st, dst_csymbolv("slots"));
     if (dst_seq_view(x, &arr, &count)) {
+        const DstValue *slotmap = 
+            dst_parse_submap_value(opts.sourcemap, dst_csymbolv("slots"));
         for (i = 0; i < count; i++) {
+            const DstValue *imap = dst_parse_submap_index(slotmap, i);
             DstValue v = arr[i];
             if (dst_checktype(v, DST_TUPLE)) {
                 const DstValue *t = dst_unwrap_tuple(v);
                 int32_t j; 
                 for (j = 0; j < dst_tuple_length(t); j++) {
+                    const DstValue *tjmap = dst_parse_submap_index(imap, j);
                     if (!dst_checktype(t[j], DST_SYMBOL))
-                        dst_asm_error(&a, "slot names must be symbols");
+                        dst_asm_error(&a, tjmap, "slot names must be symbols");
                     dst_table_put(&a.slots, t[j], dst_wrap_integer(i));
                 }
             } else if (dst_checktype(v, DST_SYMBOL)) {
                 dst_table_put(&a.slots, v, dst_wrap_integer(i));
             } else {
-                dst_asm_error(&a, "slot names must be symbols or tuple of symbols");
+                dst_asm_error(&a, imap, "slot names must be symbols or tuple of symbols");
             }
         }
     }
@@ -559,10 +585,13 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
     /* Create environment aliases */
     x = dst_struct_get(st, dst_csymbolv("environments"));
     if (dst_seq_view(x, &arr, &count)) {
+        const DstValue *emap =
+            dst_parse_submap_value(opts.sourcemap, dst_csymbolv("environments"));
         for (i = 0; i < count; i++) {
-            dst_asm_assert(&a, dst_checktype(arr[i], DST_SYMBOL), "environment must be a symbol");
+            const DstValue *imap = dst_parse_submap_index(emap, i);
+            dst_asm_assert(&a, dst_checktype(arr[i], DST_SYMBOL), imap, "environment must be a symbol");
             if (dst_asm_addenv(&a, arr[i]) < 0) {
-                dst_asm_error(&a, "environment not found");
+                dst_asm_error(&a, imap, "environment not found");
             }
         }
     }
@@ -570,12 +599,15 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
     /* Parse constants */
     x = dst_struct_get(st, dst_csymbolv("constants"));
     if (dst_seq_view(x, &arr, &count)) {
+        const DstValue *cmap =
+            dst_parse_submap_value(opts.sourcemap, dst_csymbolv("constants"));
         def->constants_length = count;
         def->constants = malloc(sizeof(DstValue) * count);
         if (NULL == def->constants) {
             DST_OUT_OF_MEMORY;
         }
         for (i = 0; i < count; i++) {
+            const DstValue *imap = dst_parse_submap_index(cmap, i);
             DstValue ct = arr[i];
             if (dst_checktype(ct, DST_TUPLE) &&
                 dst_tuple_length(dst_unwrap_tuple(ct)) > 1 &&
@@ -591,7 +623,7 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
                     def->constants[i] = t[2];
                     dst_table_put(&a.constants, t[1], dst_wrap_integer(i));
                 } else {
-                    dst_asm_errorv(&a, dst_formatc("could not parse constant \"%v\"", ct));
+                    dst_asm_errorv(&a, imap, dst_formatc("could not parse constant \"%v\"", ct));
                 }
                 /* Todo - parse nested funcdefs */
             } else {
@@ -606,16 +638,19 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
     /* Parse bytecode and labels */
     x = dst_struct_get(st, dst_csymbolv("bytecode"));
     if (dst_seq_view(x, &arr, &count)) {
+        const DstValue *bmap =
+            dst_parse_submap_value(opts.sourcemap, dst_csymbolv("bytecode"));
         /* Do labels and find length */
         int32_t blength = 0;
         for (i = 0; i < count; ++i) {
+            const DstValue *imap = dst_parse_submap_index(bmap, i);
             DstValue instr = arr[i];
             if (dst_checktype(instr, DST_SYMBOL)) {
                 dst_table_put(&a.labels, instr, dst_wrap_integer(blength));
             } else if (dst_checktype(instr, DST_TUPLE)) {
                 blength++;
             } else {
-                dst_asm_error(&a, "expected assembly instruction");
+                dst_asm_error(&a, imap, "expected assembly instruction");
             }
         }
         /* Allocate bytecode array */
@@ -626,6 +661,7 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
         }
         /* Do bytecode */
         for (i = 0; i < count; ++i) {
+            const DstValue *imap = dst_parse_submap_index(bmap, i);
             DstValue instr = arr[i];
             if (dst_checktype(instr, DST_SYMBOL)) {
                 continue;
@@ -633,23 +669,23 @@ static DstAssembleResult dst_asm1(DstAssembler *parent, DstAssembleOptions opts)
                 uint32_t op;
                 const DstInstructionDef *idef;
                 const DstValue *t;
-                dst_asm_assert(&a, dst_checktype(instr, DST_TUPLE), "expected tuple");
+                dst_asm_assert(&a, dst_checktype(instr, DST_TUPLE), imap, "expected tuple");
                 t = dst_unwrap_tuple(instr);
                 if (dst_tuple_length(t) == 0) {
                     op = 0;
                 } else {
-                    dst_asm_assert(&a, dst_checktype(t[0], DST_SYMBOL),
+                    dst_asm_assert(&a, dst_checktype(t[0], DST_SYMBOL), imap,
                             "expected symbol in assembly instruction");
                     idef = dst_findi(dst_unwrap_symbol(t[0]));
                     if (NULL == idef)
-                        dst_asm_errorv(&a, dst_formatc("unknown instruction %v", instr));
-                    op = read_instruction(&a, idef, t);
+                        dst_asm_errorv(&a, imap, dst_formatc("unknown instruction %v", instr));
+                    op = read_instruction(&a, imap, idef, t);
                 }
                 def->bytecode[a.bytecode_count++] = op;
             }
         }
     } else {
-        dst_asm_error(&a, "bytecode expected");
+        dst_asm_error(&a, opts.sourcemap, "bytecode expected");
     }
 
     /* Finish everything and return funcdef */
diff --git a/core/compile.c b/core/compile.c
index f06e29e3..49db206b 100644
--- a/core/compile.c
+++ b/core/compile.c
@@ -29,12 +29,42 @@ typedef struct DstCompiler DstCompiler;
 typedef struct FormOptions FormOptions;
 typedef struct SlotTracker SlotTracker;
 typedef struct DstScope DstScope;
+typedef struct DstCFunctionOptimizer DstCFunctionOptimizer;
+
+#define DST_SLOT_CONSTANT 1
+#define DST_SLOT_TEMP 2
+#define DST_SLOT_RETURNED 4
+#define DST_SLOT_NIL 8
 
 /* A stack slot */
 struct DstSlot {
     int32_t index;
     uint32_t flags;
     uint32_t types; /* bit set of possible primitive types */
+    DstValue constant; /* If the slot has a constant value */
+}
+
+/* Most forms that return a constant will not generate any bytecode */
+
+/* Special forms that need support */
+/* cond
+ * while (continue, break)
+ * quote
+ * fn
+ * def
+ * var
+ * do
+ */
+
+#define DST_OPTIMIZER_CONSTANTS 1
+#define DST_OPTIMIZER_BYTECODE 2
+#define DST_OPTIMIZER_PARTIAL_CONSTANTS 4
+
+/* A grouping of optimization on a cfunction given certain conditions
+ * on the arguments (such as all constants, or some known types). The appropriate
+ * optimizations should be tried before compiling a normal function call. */
+struct DstCFunctionOptimizer {
+    uint32_t flags; /* Indicate what kind of optimizations can be performed */
 }
 
 /* A lexical scope during compilation */
@@ -61,6 +91,6 @@ struct DstFormOptions {
     DstCompiler *compiler;
     DstValue x;
     uint32_t flags;
-    uint32_t types; /* bit set of accepeted primitive types */
+    uint32_t types; /* bit set of accepted primitive types */
     int32_t target_slot;
 };
diff --git a/core/parse.c b/core/parse.c
index 6ad520fd..ec8298aa 100644
--- a/core/parse.c
+++ b/core/parse.c
@@ -21,89 +21,7 @@
 */
 
 #include <dst/dst.h>
-
-/* Get an integer power of 10 */
-static double exp10(int power) {
-    if (power == 0) return 1;
-    if (power > 0) {
-        double result = 10;
-        int currentPower = 1;
-        while (currentPower * 2 <= power) {
-            result = result * result;
-            currentPower *= 2;
-        }
-        return result * exp10(power - currentPower);
-    } else {
-        return 1 / exp10(-power);
-    }
-}
-
-/* Read an integer */
-static int read_integer(const uint8_t *string, const uint8_t *end, int64_t *ret) {
-    int sign = 1, x = 0;
-    int64_t accum = 0;
-    if (*string == '-') {
-        sign = -1;
-        ++string;
-    } else if (*string == '+') {
-        ++string;
-    }
-    if (string >= end) return 0;
-    while (string < end) {
-        x = *string;
-        if (x < '0' || x > '9') return 0;
-        x -= '0';
-        accum = accum * 10 + x;
-        ++string;
-    }
-    *ret = accum * sign;
-    return 1;
-}
-
-/* Read a real from a string. Returns if successfuly
- * parsed a real from the enitre input string.
- * If returned 1, output is int ret.
- * TODO - consider algorithm that does not lose precision. */
-static int read_real(const uint8_t *string, const uint8_t *end, double *ret, int forceInt) {
-    int sign = 1, x = 0;
-    double accum = 0, exp = 1, place = 1;
-    /* Check the sign */
-    if (*string == '-') {
-        sign = -1;
-        ++string;
-    } else if (*string == '+') {
-        ++string;
-    }
-    if (string >= end) return 0;
-    while (string < end) {
-        if (*string == '.' && !forceInt) {
-            place = 0.1;
-        } else if (!forceInt && (*string == 'e' || *string == 'E')) {
-            /* Read the exponent */
-            ++string;
-            if (string >= end) return 0;
-            if (!read_real(string, end, &exp, 1))
-                return 0;
-            exp = exp10(exp);
-            break;
-        } else {
-            x = *string;
-            if (x < '0' || x > '9') return 0;
-            x -= '0';
-            if (place < 1) {
-                accum += x * place;
-                place *= 0.1;
-            } else {
-                accum *= 10;
-                accum += x;
-            }
-        }
-        ++string;
-    }
-    *ret = accum * sign * exp;
-    return 1;
-}
-
+#include "strtod.h"
 
 /* Checks if a string slice is equal to a string constant */
 static int check_str_const(const char *ref, const uint8_t *start, const uint8_t *end) {
@@ -236,17 +154,14 @@ static const uint8_t *parse_recur(
         default: 
         atom:
         {
-            double real;
-            int64_t integer;
+            DstValue numcheck;
             const uint8_t *tokenend = src;
             if (!is_symbol_char(*src)) goto unexpected_character;
             while (tokenend < end && is_symbol_char(*tokenend))
                 tokenend++;
-            if (tokenend >= end) goto unexpected_eos;
-            if (read_integer(src, tokenend, &integer)) {
-                ret = dst_wrap_integer(integer);
-            } else if (read_real(src, tokenend, &real, 0)) {
-                ret = dst_wrap_real(real);
+            numcheck = dst_scan_number(src, tokenend - src);
+            if (!dst_checktype(numcheck, DST_NIL)) {
+                ret = numcheck;
             } else if (check_str_const("nil", src, tokenend)) {
                 ret = dst_wrap_nil();
             } else if (check_str_const("false", src, tokenend)) {
@@ -509,10 +424,10 @@ DstParseResult dst_parse(const uint8_t *src, int32_t len) {
 
     if (args.errmsg) {
         res.result.error = dst_cstring(args.errmsg);
-        res.map = dst_wrap_nil();
+        res.map = NULL;
     } else {
         res.result.value = dst_array_pop(&args.stack);
-        res.map = dst_array_pop(&args.mapstack);
+        res.map = dst_unwrap_tuple(dst_array_pop(&args.mapstack));
     }
 
     dst_array_deinit(&args.stack);
@@ -527,3 +442,56 @@ DstParseResult dst_parsec(const char *src) {
     while (src[len]) ++len;
     return dst_parse((const uint8_t *)src, len);
 }
+
+/* Get the sub source map by indexing a value. Used to traverse
+ * into arrays and tuples */
+const DstValue *dst_parse_submap_index(const DstValue *map, int32_t index) {
+    if (NULL != map && dst_tuple_length(map) >= 3) {
+        const DstValue *seq;
+        int32_t len;
+        if (dst_seq_view(map[2], &seq, &len)) {
+            if (index >= 0 && index < len) {
+                if (dst_checktype(seq[index], DST_TUPLE)) {
+                    const DstValue *ret = dst_unwrap_tuple(seq[index]);
+                    if (dst_tuple_length(ret) >= 2 &&
+                        dst_checktype(ret[0], DST_INTEGER) &&
+                        dst_checktype(ret[1], DST_INTEGER)) {
+                        return ret;
+                    }
+                }
+            }
+        }
+    }
+    return NULL;
+}
+
+/* Traverse into tables and structs */
+static const DstValue *dst_parse_submap_kv(const DstValue *map, DstValue key, int kv) {
+    if (NULL != map && dst_tuple_length(map) >= 3) {
+        DstValue kvpair = dst_get(map[2], key);
+        if (dst_checktype(kvpair, DST_TUPLE)) {
+            const DstValue *kvtup = dst_unwrap_tuple(kvpair);
+            if (dst_tuple_length(kvtup) >= 2) {
+                if (dst_checktype(kvtup[kv], DST_TUPLE)) {
+                    const DstValue *ret = dst_unwrap_tuple(kvtup[kv]);
+                    if (dst_tuple_length(ret) >= 2 &&
+                        dst_checktype(ret[0], DST_INTEGER) &&
+                        dst_checktype(ret[1], DST_INTEGER)) {
+                        return ret;
+                    }
+                }
+            }
+        }
+    }
+    return NULL;
+}
+
+/* Traverse into a key of a table or struct */
+const DstValue *dst_parse_submap_key(const DstValue *map, DstValue key) {
+    return dst_parse_submap_kv(map, key, 0);
+}
+
+/* Traverse into a value of a table or struct */
+const DstValue *dst_parse_submap_value(const DstValue *map, DstValue key) {
+    return dst_parse_submap_kv(map, key, 1);
+}
diff --git a/core/string.c b/core/string.c
index 55416605..bce9294a 100644
--- a/core/string.c
+++ b/core/string.c
@@ -21,6 +21,7 @@
 */
 
 #include <dst/dst.h>
+#include "strtod.h"
 
 /* Begin building a string */
 uint8_t *dst_string_begin(int32_t length) {
@@ -102,7 +103,7 @@ const uint8_t *dst_cstring(const char *str) {
 #define DST_BUFSIZE 36
 
 static int32_t real_to_string_impl(uint8_t *buf, double x) {
-    int count = snprintf((char *) buf, DST_BUFSIZE, "%.21g", x);
+    int count = snprintf((char *) buf, DST_BUFSIZE, "%.17g", x);
     return (int32_t) count;
 }
 
@@ -323,7 +324,7 @@ void dst_short_description_b(DstBuffer *buffer, DstValue x) {
 /* Helper structure for stringifying nested structures */
 typedef struct DstPrinter DstPrinter;
 struct DstPrinter {
-    DstBuffer buffer;
+    DstBuffer *buffer;
     DstTable seen;
     uint32_t flags;
     uint32_t state;
@@ -344,7 +345,7 @@ static void dst_print_indent(DstPrinter *p) {
     int32_t i, len;
     len = p->indent_size * p->indent;
     for (i = 0; i < len; i++) {
-        dst_buffer_push_u8(&p->buffer, ' ');
+        dst_buffer_push_u8(p->buffer, ' ');
     }
 }
 
@@ -400,15 +401,15 @@ static void dst_print_hashtable_inner(DstPrinter *p, const DstValue *data, int32
         }
     }
     if (doindent) {
-        dst_buffer_push_u8(&p->buffer, '\n');
+        dst_buffer_push_u8(p->buffer, '\n');
         p->indent++;
         for (i = 0; i < cap; i += 2) {
             if (!dst_checktype(data[i], DST_NIL)) {
                 dst_print_indent(p);
                 dst_description_helper(p, data[i]);
-                dst_buffer_push_u8(&p->buffer, ' ');
+                dst_buffer_push_u8(p->buffer, ' ');
                 dst_description_helper(p, data[i + 1]);
-                dst_buffer_push_u8(&p->buffer, '\n');
+                dst_buffer_push_u8(p->buffer, '\n');
             }
         }
         p->indent--;
@@ -420,9 +421,9 @@ static void dst_print_hashtable_inner(DstPrinter *p, const DstValue *data, int32
                 if (isfirst)
                     isfirst = 0;
                 else
-                    dst_buffer_push_u8(&p->buffer, ' ');
+                    dst_buffer_push_u8(p->buffer, ' ');
                 dst_description_helper(p, data[i]);
-                dst_buffer_push_u8(&p->buffer, ' ');
+                dst_buffer_push_u8(p->buffer, ' ');
                 dst_description_helper(p, data[i + 1]);
             }
         }
@@ -446,12 +447,12 @@ static void dst_print_seq_inner(DstPrinter *p, const DstValue *data, int32_t len
         }
     }
     if (doindent) {
-        dst_buffer_push_u8(&p->buffer, '\n');
+        dst_buffer_push_u8(p->buffer, '\n');
         p->indent++;
         for (i = 0; i < len; ++i) {
             dst_print_indent(p);
             dst_description_helper(p, data[i]);
-            dst_buffer_push_u8(&p->buffer, '\n');
+            dst_buffer_push_u8(p->buffer, '\n');
         }
         p->indent--;
         dst_print_indent(p);
@@ -459,7 +460,7 @@ static void dst_print_seq_inner(DstPrinter *p, const DstValue *data, int32_t len
         for (i = 0; i < len; ++i) {
             dst_description_helper(p, data[i]);
             if (i != len - 1)
-                dst_buffer_push_u8(&p->buffer, ' ');
+                dst_buffer_push_u8(p->buffer, ' ');
         }
     }
 }
@@ -475,11 +476,11 @@ static void dst_description_helper(DstPrinter *p, DstValue x) {
     switch (dst_type(x)) {
         default:
             if (p->flags & DST_PRINTFLAG_COLORIZE) {
-                dst_buffer_push_cstring(&p->buffer, dst_type_colors[dst_type(x)]);
-                dst_short_description_b(&p->buffer, x);
-                dst_buffer_push_cstring(&p->buffer, "\x1B[0m");
+                dst_buffer_push_cstring(p->buffer, dst_type_colors[dst_type(x)]);
+                dst_short_description_b(p->buffer, x);
+                dst_buffer_push_cstring(p->buffer, "\x1B[0m");
             } else {
-                dst_short_description_b(&p->buffer, x);
+                dst_short_description_b(p->buffer, x);
             }
             p->depth++;
             return;
@@ -502,21 +503,21 @@ static void dst_description_helper(DstPrinter *p, DstValue x) {
     }
     check = dst_table_get(&p->seen, x);
     if (dst_checktype(check, DST_INTEGER)) {
-        dst_buffer_push_cstring(&p->buffer, "<cycle ");
-        integer_to_string_b(&p->buffer, dst_unwrap_integer(check));
-        dst_buffer_push_cstring(&p->buffer, ">");
+        dst_buffer_push_cstring(p->buffer, "<cycle ");
+        integer_to_string_b(p->buffer, dst_unwrap_integer(check));
+        dst_buffer_push_cstring(p->buffer, ">");
         return;
     }
     dst_table_put(&p->seen, x, dst_wrap_integer(p->next++));
-    dst_buffer_push_cstring(&p->buffer, open);
+    dst_buffer_push_cstring(p->buffer, open);
     if (p->depth == 0) {
-        dst_buffer_push_cstring(&p->buffer, "...");
+        dst_buffer_push_cstring(p->buffer, "...");
     } else if (dst_hashtable_view(x, &data, &len, &cap)) {
         dst_print_hashtable_inner(p, data, len, cap);
     } else if (dst_seq_view(x, &data, &len)) {
         dst_print_seq_inner(p, data, len);
     }
-    dst_buffer_push_cstring(&p->buffer, close);
+    dst_buffer_push_cstring(p->buffer, close);
     /* Remove from seen as we know that printing completes, we
      * can print in multiple times and we know we are not recursing */
     dst_table_remove(&p->seen, x);
@@ -538,15 +539,17 @@ const uint8_t *dst_description(DstValue x) {
     DstPrinter printer;
     const uint8_t *ret;
 
+    DstBuffer buffer;
     dst_printer_defaults(&printer);
     printer.state = 0;
-    dst_buffer_init(&printer.buffer, 0);
+    dst_buffer_init(&buffer, 0);
+    printer.buffer = &buffer;
 
     /* Only print description up to a depth of 4 */
     dst_description_helper(&printer, x);
-    ret = dst_string(printer.buffer.data, printer.buffer.count);
+    ret = dst_string(buffer.data, buffer.count);
 
-    dst_buffer_deinit(&printer.buffer);
+    dst_buffer_deinit(&buffer);
     if (printer.state)
         dst_table_deinit(&printer.seen);
     return ret;
@@ -572,7 +575,8 @@ const uint8_t *dst_formatc(const char *format, ...) {
     int32_t i;
     const uint8_t *ret;
     DstPrinter printer;
-    DstBuffer *bufp = &printer.buffer;
+    DstBuffer buffer;
+    DstBuffer *bufp = &buffer;
     printer.state = 0;
     
     /* Calculate length */
@@ -580,6 +584,7 @@ const uint8_t *dst_formatc(const char *format, ...) {
 
     /* Initialize buffer */
     dst_buffer_init(bufp, len);
+    printer.buffer = bufp;
 
     /* Start args */
     va_start(args, format);
@@ -646,8 +651,8 @@ const uint8_t *dst_formatc(const char *format, ...) {
 
     va_end(args);
 
-    ret = dst_string(printer.buffer.data, printer.buffer.count);
-    dst_buffer_deinit(&printer.buffer);
+    ret = dst_string(buffer.data, buffer.count);
+    dst_buffer_deinit(&buffer);
     if (printer.state)
         dst_table_deinit(&printer.seen);
     return ret;
diff --git a/core/strtod.c b/core/strtod.c
new file mode 100644
index 00000000..fb6ffc10
--- /dev/null
+++ b/core/strtod.c
@@ -0,0 +1,242 @@
+/*
+* 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.
+*/
+
+/* Use a custom double parser instead of libc's strtod for better portability
+ * and control. Also, uses a less strict rounding method than ieee to not incur
+ * the cost of 4000 loc and dependence on arbitary precision arithmetic.  There
+ * is no plan to use arbitrary precision arithmetic for parsing numbers, and a
+ * formal rounding mode has yet to be chosen (round towards 0 seems
+ * reasonable).
+ *
+ * This version has been modified for much greater flexibility in parsing, such
+ * as choosing the radix, supporting integer output, and returning DstValues
+ * directly. 
+ *
+ * Numbers are of the form [-+]R[rR]I.F[eE&][-+]X where R is the radix, I is
+ * the integer part, F is the fractional part, and X is the exponent. All
+ * signs, radix, decimal point, fractional part, and exponent can be ommited.
+ * The number will be considered and integer if the there is no decimal point
+ * and no exponent. Any number greater the 2^32-1 or less than -(2^32) will be
+ * coerced to a double. If there is an error, the function dst_scan_number will
+ * return a dst nil. The radix is assumed to be 10 if omitted, and the E
+ * separator for the exponent can only be used when the radix is 10. This is
+ * because E is a vaid digit in bases 15 or greater. For bases greater than 10,
+ * the letters are used as digitis. A through Z correspond to the digits 10
+ * through 35, and the lowercase letters have the same values. The radix number
+ * is always in base 10. For example, a hexidecimal number could be written
+ * '16rdeadbeef'. dst_scan_number also supports some c style syntax for
+ * hexidecimal literals. The previous number could also be written
+ * '0xdeadbeef'. Note that in this case, the number will actually be a double
+ * as it will not fit in the range for a signed 32 bit integer. The string
+ * '0xbeef' would parse to an integer as it is in the range of an int32_t. */
+
+/* TODO take down missle defence */
+
+#include <dst/dst.h>
+#include <math.h>
+
+/* Lookup table for getting values of characters when parsing numbers. Handles
+ * digits 0-9 and a-z (and A-Z). A-Z have values of 10 to 35. */
+static uint8_t digit_lookup[128] = {
+    0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
+    0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
+    0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
+    0,1,2,3,4,5,6,7,8,9,0xff,0xff,0xff,0xff,0xff,0xff,
+    0xff,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,
+    25,26,27,28,29,30,31,32,33,34,35,0xff,0xff,0xff,0xff,0xff,
+    0xff,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,
+    25,26,27,28,29,30,31,32,33,34,35,0xff,0xff,0xff,0xff,0xff
+};
+
+/* Read in a mantissa and exponent of a certain base, and give
+ * back the double value. Should properly handle 0s, Inifinties, and
+ * denormalized numbers. (When the exponent values are too large) */
+static double dst_convert_mantissa_exp(
+        int negative,
+        uint64_t mantissa,
+        int32_t base,
+        int32_t exponent) {
+
+    int32_t exponent2 = 0;
+
+    /* Short circuit zero and huge numbers */
+    if (mantissa == 0)
+        return 0.0;
+    if (exponent > 1022)
+        return negative ? -1.0/0.0 : 1.0/0.0;
+
+    /* TODO add fast paths */
+
+    /* Convert exponent on the base into exponent2, the power of
+     * 2 the will be used. Modify the mantissa as we convert. */
+    if (exponent > 0) {
+        /* Make the mantissa large enough so no precision is lost */
+        while (mantissa <= 0x03ffffffffffffffULL && exponent > 0) {
+            mantissa *= base;
+            exponent--;
+        }
+        while (exponent > 0) {
+            /* Allow 6 bits of room when multiplying. This is because
+             * the largest base is 36, which is 6 bits. The space of 6 should
+             * prevent overflow.*/
+            mantissa >>= 1;
+            exponent2++;
+            if (mantissa <= 0x03ffffffffffffffULL) {
+                mantissa *= base;
+                exponent--;
+            }
+        }
+    } else {
+        while (exponent < 0) {
+            mantissa <<= 1;
+            exponent2--;
+            /* Ensure that the last bit is set for minimum error
+             * before dividing by the base */
+            if (mantissa > 0x7fffffffffffffffULL) {
+                mantissa /= base;
+                exponent++;
+            }
+        }
+    }
+
+    /* Build the number to return */
+    return ldexp(mantissa, exponent2);
+}
+
+/* Get the mantissa and exponent of decimal number. The
+ * mantissa will be stored in a 64 bit unsigned integer (always positive).
+ * The exponent will be in a signed 32 bit integer. Will also check if 
+ * the decimal point has been seen. Returns -1 if there is an invalid
+ * number. */
+DstValue dst_scan_number(
+        const uint8_t *str, 
+        int32_t len) {
+
+    const uint8_t *end = str + len;
+    int32_t seenpoint = 0;
+    uint64_t mant = 0;
+    int32_t neg = 0;
+    int32_t ex = 0;
+    int foundExp = 0;
+
+    /* Set some constants */
+    int base = 10;
+
+    /* Prevent some kinds of overflow bugs relating to the exponent
+     * overflowing.  For example, if a string was passed 2GB worth of 0s after
+     * the decimal point, exponent could wrap around and become positive. It's
+     * easier to reject ridiculously large inputs than to check for overflows.
+     * */
+    if (len > INT32_MAX / base) goto error;
+
+    /* Get sign */
+    if (str >= end) goto error;
+    if (*str == '-') {
+        neg = 1;
+        str++;
+    } else if (*str == '+') {
+        str++;
+    }
+
+    /* Skip leading zeros */
+    while (str < end && (*str == '0' || *str == '.')) {
+        if (seenpoint) ex--;
+        if (*str == '.') {
+            if (seenpoint) goto error;
+            seenpoint = 1;
+        }
+        str++;
+    }
+
+    /* Parse significant digits */
+    while (str < end) {
+        if (*str == '.') {
+            if (seenpoint) goto error;
+            seenpoint = 1;
+        } else if (*str == '&') {
+            foundExp = 1;
+            break;
+        } else if (base == 10 && (*str == 'E' || *str == 'e')) {
+            foundExp = 1;
+            break;
+        } else if (*str == 'x' || *str == 'X') {
+            if (seenpoint || mant > 0) goto error;
+            base = 16;
+            mant = 0;
+        } else if (*str == 'r' || *str == 'R')  {
+            if (seenpoint) goto error;
+            if (mant < 2 || mant > 36) goto error;
+            base = mant;
+            mant = 0;
+        } else if (*str == '_')  {
+            ;
+            /* underscores are ignored - can be used for separator */
+        } else {
+            int digit = digit_lookup[*str & 0x7F];
+            if (digit >= base) goto error;
+            if (seenpoint) ex--;
+            if (mant > 0x00ffffffffffffff)
+                ex++;
+            else
+                mant = base * mant + digit;
+        }
+        str++;
+    }
+
+    /* Read exponent */
+    if (str < end && foundExp) {
+        int eneg = 0;
+        int ee = 0;
+        str++;
+        if (str >= end) goto error;
+        if (*str == '-') {
+            eneg = 1;
+            str++;
+        } else if (*str == '+') {
+            str++;
+        }
+        /* Skip leading 0s in exponent */
+        while (str < end && *str == '0') str++;
+        while (str < end && ee < (INT32_MAX / base - base)) {
+            int digit = digit_lookup[*str & 0x7F];
+            if (digit >= base) goto error;
+            ee = base * ee + digit;
+            str++;
+        }
+        if (eneg) ex -= ee; else ex += ee;
+    } else if (!seenpoint) {
+        /* Check for integer literal */
+        int64_t i64 = neg ? -mant : mant;
+        if (i64 <= INT32_MAX && i64 >= INT32_MIN)
+            return dst_wrap_integer((int32_t) i64);
+    } else if (str < end) {
+        goto error;
+    }
+    
+    /* Convert mantissa and exponent into double */
+    return dst_wrap_real(dst_convert_mantissa_exp(neg, mant, base, ex));
+
+    error:
+    return dst_wrap_nil();
+
+}
+
diff --git a/core/strtod.h b/core/strtod.h
new file mode 100644
index 00000000..91e879d7
--- /dev/null
+++ b/core/strtod.h
@@ -0,0 +1,30 @@
+/*
+* 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.
+*/
+
+#ifndef DST_STRTOD_H_defined
+#define DST_STRTOD_H_defined
+
+DstValue dst_scan_number(
+        const uint8_t *str, 
+        int32_t len);
+
+#endif
diff --git a/core/syscalls.c b/core/syscalls.c
index 9bf85ce0..8ce59e54 100644
--- a/core/syscalls.c
+++ b/core/syscalls.c
@@ -46,7 +46,9 @@ int dst_sys_asm(DstValue *argv, int32_t argn) {
         return 1;
     }
     opts.source = argv[0];
-    opts.parsemap = argn >= 2 ? argv[1] : dst_wrap_nil();
+    opts.sourcemap = (argn >= 2 && dst_checktype(argv[1], DST_TUPLE))
+        ? dst_unwrap_tuple(argv[1])
+        : NULL;
     opts.flags = 0;
     res = dst_asm(opts);
     if (res.status == DST_ASSEMBLE_OK) {
diff --git a/core/util.c b/core/util.c
index d1576932..e1fa564d 100644
--- a/core/util.c
+++ b/core/util.c
@@ -115,3 +115,4 @@ int dst_hashtable_view(DstValue tab, const DstValue **data, int32_t *len, int32_
     }
     return 0;
 }
+
diff --git a/core/vm.c b/core/vm.c
index ba310dba..0032f101 100644
--- a/core/vm.c
+++ b/core/vm.c
@@ -50,11 +50,6 @@ static int dst_update_fiber() {
     return 0;
 }
 
-/* Eventually use computed gotos for more effient vm loop. */
-#define vm_next() continue
-#define vm_checkgc_next() dst_maybe_collect(); continue
-
-
 /* Start running the VM from where it left off. */
 int dst_continue() {
 
@@ -63,6 +58,10 @@ int dst_continue() {
     uint32_t *pc;
     DstFunction *func;
 
+/* Eventually use computed gotos for more effient vm loop. */
+#define vm_next() continue
+#define vm_checkgc_next() dst_maybe_collect(); continue
+
     /* Used to extract bits from the opcode that correspond to arguments.
      * Pulls out unsigned integers */
 #define oparg(shift, mask) (((*pc) >> ((shift) << 3)) & (mask))
diff --git a/core/wrap.c b/core/wrap.c
index 963369a8..e713133a 100644
--- a/core/wrap.c
+++ b/core/wrap.c
@@ -28,7 +28,9 @@ void *dst_nanbox_to_pointer(DstValue x) {
     /* We need to do this shift to keep the higher bits of the pointer
      * the same as bit 47 as required by the x86 architecture. We may save
      * an instruction if we do x.u64 & DST_NANBOX_POINTERBITS, but this 0s
-     * the high bits, and may make the pointer non-canocial on x86. */
+     * the high bits, and may make the pointer non-canocial on x86. If we switch
+     * to 47 bit pointers (which is what userspace uses on Windows, we can use
+     * the single mask rather than two shifts. */
     x.i64 = (x.i64 << 16) >> 16;
     return x.pointer;
 }
diff --git a/dsts/minimal.dsts b/dsts/minimal.dsts
index 0cb42c60..62899eed 100644
--- a/dsts/minimal.dsts
+++ b/dsts/minimal.dsts
@@ -1,5 +1,5 @@
+# A fairly minimal example of a dst assembly file
 {
-  bork 'boop
   bytecode [
     (load-integer 0 15)
     (load-integer 1 0)
@@ -8,9 +8,7 @@
     :label
     (equals 2 1 0)
     (jump-if 2 :done) 
-    (push 0)
     (add-immediate 0 0 -1)
-    (syscall 2 0)
     (get 2 3 0)
     (push 2)
     (syscall 2 0)
@@ -18,6 +16,9 @@
 
     :done
     (return-nil)
+    
+    :extra
+    (push 2r1010101010101010)
   ]
   constants [
     (def lookup "0123456789abcdef")
diff --git a/include/dst/dst.h b/include/dst/dst.h
index b4627324..ad9a9d49 100644
--- a/include/dst/dst.h
+++ b/include/dst/dst.h
@@ -599,10 +599,12 @@ struct DstAssembleResult {
         DstFuncDef *def;
         const uint8_t *error;
     } result;
+    int32_t error_start;
+    int32_t error_end;
     DstAssembleStatus status;
 };
 struct DstAssembleOptions {
-    DstValue parsemap;
+    const DstValue *sourcemap;
     DstValue source;
     uint32_t flags;
 };
@@ -648,12 +650,15 @@ struct DstParseResult {
         DstValue value;
         const uint8_t *error;
     } result;
-    DstValue map;
+    const DstValue *map;
     int32_t bytes_read;
     DstParseStatus status;
 };
 DstParseResult dst_parse(const uint8_t *src, int32_t len);
 DstParseResult dst_parsec(const char *src);
+const DstValue *dst_parse_submap_index(const DstValue *map, int32_t index);
+const DstValue *dst_parse_submap_key(const DstValue *map, DstValue key);
+const DstValue *dst_parse_submap_value(const DstValue *map, DstValue key);
 
 /* VM functions */
 int dst_init();
@@ -662,9 +667,6 @@ int dst_continue();
 int dst_run(DstValue callee);
 DstValue dst_transfer(DstFiber *fiber, DstValue x);
 
-/* Wrap data in DstValue */
-
-
 /* GC */
 
 /* The metadata header associated with an allocated block of memory */
diff --git a/unittests/asm_test.c b/unittests/asm_test.c
index f234c3e5..1aa23c63 100644
--- a/unittests/asm_test.c
+++ b/unittests/asm_test.c
@@ -26,21 +26,20 @@ int main() {
     free(string);
 
     if (pres.status == DST_PARSE_ERROR) {
-        dst_puts(dst_formatc("parse error at %d: %s\n", pres.bytes_read, pres.result.error));
+        dst_puts(dst_formatc("parse error at %d: %S\n", pres.bytes_read, pres.result.error));
         return 1;
     }
     assert(pres.status == DST_PARSE_OK);
     dst_puts(dst_formatc("\nparse result: %v\n\n", pres.result.value));
-    dst_puts(dst_formatc("\nparse map result: %v\n\n", pres.map));
 
-    /*
     opts.flags = 0;
     opts.source = pres.result.value;
-    opts.parsemap = dst_wrap_nil();
+    opts.sourcemap = pres.map;
 
     ares = dst_asm(opts);
     if (ares.status == DST_ASSEMBLE_ERROR) {
-        dst_puts(dst_formatc("assembly error: %s\n", ares.result.error));
+        dst_puts(dst_formatc("assembly error: %S\n", ares.result.error));
+        dst_puts(dst_formatc("error location: %d, %d\n", ares.error_start, ares.error_end));
         return 1;
     }
     assert(ares.status == DST_ASSEMBLE_OK);
@@ -49,7 +48,6 @@ int main() {
     
     dst_run(dst_wrap_function(func));
     dst_puts(dst_formatc("result: %v\n", dst_vm_fiber->ret));
-    */
 
     dst_deinit();
 
diff --git a/unittests/strtod_test.c b/unittests/strtod_test.c
new file mode 100644
index 00000000..6c4b364e
--- /dev/null
+++ b/unittests/strtod_test.c
@@ -0,0 +1,122 @@
+/*
+* 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.
+*/
+
+/* Use a custom double parser instead of libc's strtod for better portability
+ * and control. Also, uses a less strict rounding method than ieee to not incur
+ * the cost of 4000 loc and dependence on arbitary precision arithmetic.  There
+ * is no plan to use arbitrary precision arithmetic for parsing numbers, and a
+ * formal rounding mode has yet to be chosen (round towards 0 seems
+ * reasonable).
+ *
+ * This version has been modified for much greater flexibility in parsing, such
+ * as choosing the radix, supporting integer output, and returning DstValues
+ * directly. 
+ *
+ * Numbers are of the form [-+]R[rR]I.F[eE&][-+]X where R is the radix, I is
+ * the integer part, F is the fractional part, and X is the exponent. All
+ * signs, radix, decimal point, fractional part, and exponent can be ommited.
+ * The number will be considered and integer if the there is no decimal point
+ * and no exponent. Any number greater the 2^32-1 or less than -(2^32) will be
+ * coerced to a double. If there is an error, the function dst_scan_number will
+ * return a dst nil. The radix is assumed to be 10 if omitted, and the E
+ * separator for the exponent can only be used when the radix is 10. This is
+ * because E is a vaid digit in bases 15 or greater. For bases greater than 10,
+ * the letters are used as digitis. A through Z correspond to the digits 10
+ * through 35, and the lowercase letters have the same values. The radix number
+ * is always in base 10. For example, a hexidecimal number could be written
+ * '16rdeadbeef'. dst_scan_number also supports some c style syntax for
+ * hexidecimal literals. The previous number could also be written
+ * '0xdeadbeef'. Note that in this case, the number will actually be a double
+ * as it will not fit in the range for a signed 32 bit integer. The string
+ * '0xbeef' would parse to an integer as it is in the range of an int32_t. */
+
+#include "unit.h"
+#include <dst/dst.h>
+#include <math.h>
+
+DstValue dst_scan_number(const uint8_t *str, int32_t len);
+
+const char *valid_test_strs[] = {
+    "0",
+    "-0.0",
+    "+0",
+    "123",
+    "-123",
+    "aaaaaa",
+    "+a123",
+    "0.12312",
+    "89.12312",
+    "-123.01231",
+    "123e10",
+    "1203412347981232379183.13013248723478932478923478e12",
+    "120341234798123237918313013248723478932478923478",
+    "999_999_999_999",
+    "8r777",
+    "",
+    "----",
+    "   ",
+    "--123",
+    "0xff",
+    "0xff.f",
+    "0xff&-1",
+    "0xfefefe",
+    "1926.4823e11",
+    "0xff_ff_ff_ff",
+    "0xff_ff_ff_ff_ff_ff",
+    "2r1010",
+    "2r10101010001101",
+    "123a",
+    "0.1e510",
+    "4.123123e-308",
+    "4.123123e-320",
+    "1e-308",
+    "1e-309",
+    "9e-308",
+    "9e-309",
+    "919283691283e-309",
+    "9999e302",
+    "123.12312.123",
+    "90.e0.1",
+    "90.e1",
+    ".e1"
+};
+
+int main() {
+    dst_init();
+    unsigned i;
+    for (i = 0; i < (sizeof(valid_test_strs) / sizeof(void *)); i++) {
+        DstValue out;
+        double refout;
+        const uint8_t *str = (const uint8_t *) valid_test_strs[i];
+        int32_t len = 0; while (str[len]) len++;
+
+        refout = strtod(valid_test_strs[i], NULL);
+        out = dst_scan_number(str, len);
+        dst_puts(dst_formatc("literal: %s, out: %v, refout: %v\n", 
+                    valid_test_strs[i], out, dst_wrap_real(refout)));
+
+    }
+        uint64_t x = 0x07FFFFFFFFFFFFFF;
+        uint64_t y = 36;
+
+        printf("%llu, %llu\n", x, (x * y) / y);
+}