Basic IO

All Charm scripts automatically import a library called world.ch. This contains some commands and types that let you perform basic IO.

It does this in ways that are entirely extensible and customizable: you can use Charm to talk to whatever applications or devices your OS can talk to. We will discuss how to write libraries for such purposes in the section on Advanced Charm. For now, let's look at the contents of the built-in world library.

get

get commands perform input. They typically take the form get <variable name> from <source>. Apart from the commands for handling SQL and micro services, the following commands are available:

• get x from File(<string>) sets x equal to the contents of the given file.
• get x from File(<string>, <type>) sets x equal to the contents of the given file as a string if the type is string, and broken into a list of strings at the newlines of the file if the type is list.
• get x from FileExists(<string>) sets x equal to true or false depending on whether the file does in fact exist.
• get x from Input(<string>) sets x equal to the response we get when prompting the end-user with the given string.
• get x from Random(<integer>) sets x equal to a random integer less than the supplied integer.
• get x from Random(<list>) sets x equal to a random element of the list.
• get x from UnixClock(<TimeUnit>) sets x equal to the Unix epoch in the given unit, where TimeUnit is an enum with values SECONDS, MILLISECONDS, and NANOSECONDS.

put, post, and delete

These commands are used to perform output. Their meanings are based on HTTP:

• put represents output which would have the same effect if you did it several times as if you did it once, such as writing a file to disc.
• post represents output which has a distinct effect each time you do it, such as writing text to the terminal.
• delete is always implemented so that if the thing to be deleted already doesn't exist, this is counted as a success rather than an error.

Apart from the commands for handling SQL and microservices, the following commands are available:

• post (<tuple>) to Output() posts the values given to the current output.
• post (<tuple>) posts to Output() by default.
• post (<tuple>) to Terminal() posts the values given to the terminal. For now you may see little difference between that and post (<tuple>), but the difference will become apparent later.
• put (<int>) into RandomSeed() seeds the random number generator.
• put (<string>) into File(<filename>) puts the given string into the file, creating it if it doesn't exist.
• delete File (<filename>) deletes the given file.