Running Scala CLI without any arguments should start a REPL

Is your feature request related to a problem? Please describe.

This issue was created based on the comments in Scala CLI SIP.

The current scala command starts repl with no arguments provided but with scala-cli repl subcommand is needed. I think we should change the behaviour to match what scala is doing now: running repl when no commands are provided.

Describe the solution you’d like

When running scala-cli without any command, a reply should be started.

Additional context

We would need to find a way to display help for the whole scala-cli' tool. With the default changed, user running scala-cli --helpmay expect either help for the wholescala-clior therepl` command. I can see 2 options:

• scala-cli --help prints the help for the whole tool with the mention that to see a help dedicated for repl command run scala-cli repl --help
• scala-cli --help prints the help for repl command with the mention that to see the whole help run e.g. scala-cli --help --all

I am personally prefer the former mechanism.

[Gedochao]

CLI tools typically print the help text on bad usage. We can keep only the “traditional” -h and --help. Even if you type nonexistent switch (like -help), you will still be show the help text.

FYI we already have a ticket for this: https://github.com/VirtusLab/scala-cli/issues/1240

[bjornregnell]

It would be good to make sure that it also works as before with scala-cli to give the -S or --scala-version options (and perhaps other relevant options to scala-cli). So instead of:

$ scala-cli repl -S 2
Welcome to Scala 2.13.8 (OpenJDK 64-Bit Server VM, Java 17.0.3).
Type in expressions for evaluation. Or try :help.

scala> 

$ scala-cli repl -S 3
Welcome to Scala 3.1.3 (17.0.3, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> 

We can write just:

$ scala -S 2
Welcome to Scala 2.13.8 (OpenJDK 64-Bit Server VM, Java 17.0.3).
Type in expressions for evaluation. Or try :help.

scala> 

$ scala -S 3
Welcome to Scala 3.1.3 (17.0.3, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala>