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Pragmas

Pragmas are source forms evaluated by the Epsil parser rather than at run time. A pragma is replaced by its value while the program is being read, before execution begins.

Environment Variables​

Environment variables are defined in the host process when Epsil is parsed under Node.js. In Unix, they are set using a shell-specific syntax (export VARIABLE=value in bash shells, for example).

Environment variables are not normally available when parsing takes place in a browser.

Use #env() to read an environment variable:

#env("DEBUG")

Some common environment variables include:

  • NO_COLOR: if set, color output to the terminal should be avoided
  • TERM: describe the capabilities of the output terminal, e.g. xterm-256color
  • HOME: path to the user home directory
  • TEMP: path to a temporary file directory

#env() reads host state and is therefore disabled by default: without opting in, it produces a host-pragma-disabled diagnostic and the value Nothing. A trusted host can enable it.

Navigator properties are available when parsing takes place in a browser.

Use #navigator() to read a property of the browser's navigator object. Like #env(), it is disabled unless the host opts in. It returns Nothing when the browser property is not available.

#navigator("userAgent")

Parser Messages​

#error() stops parsing, and reports an error-directive diagnostic:

#error("File cannot be compiled")

#warning() does not write to the console and does not add a diagnostic. It evaluates at parse time to its message string, allowing parsing to continue:

#warning("TODO: Implement function")

Other Pragmas​

The following pragmas are replaced with the indicated value:

  • #line: the current source line number. The first line is line 1.
  • #column: the current column number. The first column is column 1.
  • #url: the source URL the host supplied for the program, or Nothing when none was.
  • #filename: the final path component of the source URL, or Nothing when no URL was supplied.
  • #date: the current date in the YYYY-MM-DD format.
  • #time: the current time in the HH:MM:SS format.

These six pragmas are always available. Epsil does not currently implement a pragma for overriding the source location.