Add edefer.

Also improve error messages from vm internal errors.
(Show bad value, not its type).

CHANGELOG.md

@@ -2,6 +2,7 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased - ???
+- Add `edefer` macro to core.
 - A struct/table literal/constructor with duplicate keys will use the last value given.
   Previously, this was inconsistent between tables and structs, literals and constructor functions.
 - Add debugger to core. The debugger functions are only available

src/boot/boot.janet

@@ -301,7 +301,19 @@
        ,form
        (if (= (,fiber/status ,f) :dead)
          ,r
-         (propagate ,r ,f)))))
+         (,propagate ,r ,f)))))
+
+(defmacro edefer
+  "Run form after body in the case that body terminates abnormally (an error or user signal 0-4).
+  Otherwise, return last form in body."
+  [form & body]
+  (with-syms [f r]
+    ~(do
+       (def ,f (,fiber/new (fn [] ,;body) :ti))
+       (def ,r (,resume ,f))
+       (if (= (,fiber/status ,f) :dead)
+         ,r
+         (do ,form (,propagate ,r ,f))))))
 
 (defmacro prompt
   "Set up a checkpoint that can be returned to. Tag should be a value
@@ -314,7 +326,7 @@
        (def [,target ,payload] ,res)
        (if (,= ,tag ,target)
          ,payload
-         (propagate ,res ,fib)))))
+         (,propagate ,res ,fib)))))
 
 (defmacro chr
   "Convert a string of length 1 to its byte (ascii) value at compile time."

src/core/vm.c

@@ -107,13 +107,13 @@ JANET_THREAD_LOCAL jmp_buf *janet_vm_jmp_buf = NULL;
 #define vm_assert_type(X, T) do { \
     if (!(janet_checktype((X), (T)))) { \
         vm_commit(); \
-        janet_panicf("expected %T, got %t", (1 << (T)), (X)); \
+        janet_panicf("expected %T, got %v", (1 << (T)), (X)); \
     } \
 } while (0)
 #define vm_assert_types(X, TS) do { \
     if (!(janet_checktypes((X), (TS)))) { \
         vm_commit(); \
-        janet_panicf("expected %T, got %t", (TS), (X)); \
+        janet_panicf("expected %T, got %v", (TS), (X)); \
     } \
 } while (0)
 
@@ -910,7 +910,7 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in) {
         if (janet_indexed_view(stack[D], &vals, &len)) {
             janet_fiber_pushn(fiber, vals, len);
         } else {
-            janet_panicf("expected %T, got %t", JANET_TFLAG_INDEXED, stack[D]);
+            janet_panicf("expected %T, got %v", JANET_TFLAG_INDEXED, stack[D]);
         }
     }
     stack = fiber->data + fiber->frame;