Add errorf to core.

CHANGELOG.md

@@ -2,6 +2,7 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased - ???
+- Add `errorf` to core.
 - Add `lenprefix` combinator to PEGs.
 - Add `%M`, `%m`, `%N`, and `%n` formatters to formatting functions. These are the
   same as `%Q`, `%q`, `%P`, and `%p`, but will not truncate long values.

src/boot/boot.janet

@@ -141,6 +141,11 @@
   [x &opt err]
   (if x x (error (if err err "assert failure"))))
 
+(defn errorf
+  "A combination of error and string/format. Equivalent to (error (string/format fmt ;args))"
+  [fmt & args]
+  (error (string/format fmt ;args)))
+
 (defmacro default
   "Define a default value for an optional argument.
   Expands to (def sym (if (= nil sym) val sym))"