Writing To Handles¶

All of the printing functions support writing to handles (any kind of output handle, file, string etc.).

Hiding in plain sight has been that printf has been implicitly writing to the “current output handle”. Here, printf ... is a simple wrapper to

hprintf (current-output-handle) ...

where hprintf is like fprintf(3) in that it takes a handle as a parameter. Similarly, eprintf sends its output to the current error handle.

write-char writes one Unicode code point to the handle
write writes the reader form of its argument to the handle

This only affects strings and Unicode code points. The normal “display” form is the UTF-8 encoding of the string or code point. The reader form is "foo" or #\a suitable for the reader to read back in.
puts writes a single string to the handle
display writes the display form of a value to the handle

The display form of many values might be an invalid reader form, for example, a handle:
```
#<H ofw!iF   1:"*stdout*":1:0>
```
printf and siblings implement formatted values with a familiar feel

writing-handles.idio¶

fh := open-file "/tmp/words.txt" "r+"

while #t {
  ;; remember pos so we can seek correctly
  pos := handle-pos fh
  cp := read-char fh

  (cond
   ((eof? cp) {
     (break)
   })
   ((Uppercase? cp) {
     ;; cp is not a fixnum so no comparators
     cv := unicode->integer cp

     ;; Only ASCII in case the Uppercase UTF-8 encoding is a different
     ;; number of bytes
     if (cv lt #x80) {
       seek-handle fh pos
       write-char (->Lowercase cp) fh
     }
   }))
}

close-handle fh

$ echo -e "\u0126eLLo\nWorld" > /tmp/words.txt
$ cat /tmp/words.txt
ĦeLLo
World

$ idio writing-handles
$ cat /tmp/words.txt
Ħello
world

Last built at 2025-11-14T07:11:19Z+0000 from 77077af (dev) for Idio 0.3