Support using functions with :out dynamic binding

src/core/io.c

@@ -483,6 +483,19 @@ static Janet cfun_io_print_impl_x(int32_t argc, Janet *argv, int newline,
                 janet_buffer_push_u8(buf, '\n');
             return janet_wrap_nil();
         }
+        case JANET_FUNCTION: {
+            /* Special case function */
+            JanetFunction *fun = janet_unwrap_function(x);
+            JanetBuffer *buf = janet_buffer(0);
+            for (int32_t i = offset; i < argc; ++i) {
+                janet_to_string_b(buf, argv[i]);
+            }
+            if (newline)
+                janet_buffer_push_u8(buf, '\n');
+            Janet args[1] = { janet_wrap_buffer(buf) };
+            janet_call(fun, 1, args);
+            return janet_wrap_nil();
+        }
         case JANET_NIL:
             f = dflt_file;
             if (f == NULL) janet_panic("cannot print to nil");

test/suite0007.janet

@@ -168,6 +168,16 @@
 (assert (= (string out-buf) "Hello\nhi") "print and prin to buffer 1")
 (assert (= (string err-buf) "Sup\nnot much.") "eprint and eprin to buffer 1")
 
+# Printing to functions
+(def out-buf @"")
+(defn prepend [x]
+  (with-dyns [:out out-buf]
+    (prin "> " x)))
+(with-dyns [:out prepend]
+  (print "Hello world"))
+
+(assert (= (string out-buf) "> Hello world\n") "print to buffer via function")
+
 (assert (= (string '()) (string [])) "empty bracket tuple literal")
 
 # with-vars