io

err -- effect providing an output stream

out -- effect providing an output stream

unit type to group buffered io

dir -- a unit type that contains effects related to file system
directories

file -- a unit type that contains effects related to file I/O

print_effect -- effect providing an output stream

This is used as heir feature for effects such as io.out and io.err.

short-hand for accessing err effect in current environment

short-hand for accessing out effect in current environment

short-hand to return an io.reader with a read provider
allowing to read from stdin.

usage example:
```
line_or_error := io.stdin.with ()->
io.buffered.read_line
```

unit type to indicate an `end of file`
or the depletion of a source