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Launcher

This extension provides an alternative entry point for Shen. The alternative entry point handles command line arguments, providing defaults for some common cases, ports can choose to extend the list of accepted arguments. Compared to the default entry-point provided in the kernel (shen.repl, which launches the REPL), the entry-point in this extension implements basic script running, expression evaluation and file loading.

Instructions

Instead of using shen.repl as an entry point, use either shen.x.launcher.main or shen.x.launcher.launch-shen if more control is wanted.

shen.x.launcher.main accepts as an argument a list containing all the command line arguments, with the program name as the first argument. It provides default behaviour for processing those arguments and handling the result.

For example:

my-shen eval -e "(+ 1 2)"

should be represented by the list:

["my-shen" "eval" "-e" "(+ 1 2)"]

and:

/path/to/shen repl

by the list:

["/path/to/shen" "repl"]

shen.x.launcher.launch-shen takes the same input as shen.x.launcher.main but returns a value so that the caller is able perform custom actions (like for example processing extra arguments, or extending the help message).

(shen.x.launcher.launch-shen ArgList) will return one of the following lists:

  • [success]: All arguments processed without errors.
  • [success Message]: Success, but with a message to print.
  • [error Message]: There was an error (described by Message) when processing the arguments. The port should print this error and exit with an error status code if the platform supports it.
  • [launch-repl | Args]: Request to launch the repl, can be done by invoking (shen.repl). Args are extra arguments the port may want to do something with.
  • [show-help HelpText]: Request to print the help text. HelpText contains the default help text, the port may add more.
  • [unknown-arguments Exe UnknownCommandOrFlag | Args]: Returned when there are arguments that could not be processed. The port can handle those arguments if it knows how, otherwise it should let the user know aand exit with an error status code if the platform supports it.

The shen.x.launcher.default-handle-result implements a portable default behaviour for these results. Ports can use this directly or process the results they are interested in and pass the rest to this function.

Example (default behaviour with correct exit codes):

(define my-handle-result
  [error Message] -> (do (shen.x.launcher.default-handle-result
                            [error Message])
                         (scm.exit 1))
  [unknown-arguments | Rest]
  -> (do (shen.x.launcher.default-handle-result
            [unknown-arguments | Rest])
         (scm.exit 1))
  Other -> (shen.x.launcher.default-handle-result Other))

(my-handle-result (shen.x.launcher.launch-shen
                      ["my-shen" "eval" "-e" "(+ 1 2)"]))