Support for initial commands in REPL

[lwronski]

Is your feature request related to a problem? Please describe.
Support for running initial commands for REPL

Describe the solution you'd like

Adding a flag to specify initial commands. It can be supported in two ways:

• accept string like this: "import Hello.*"
• accept *.sc files which should be executed before opening REPL

[alexarchambault]

I'm not sure the Scala 3 REPL allows to specify initial code like this though (might be the same with the Scala 2 one).

[zmerr]

I am reading the documents for ammonite. It’s just that I don’t understand what initial commands are. Do you want scala-cli to support something like magic imports in ammonite ?

[zmerr]

You mean something like

scala-cli repl -ic “import Hello.*”
or
scala-cli repl -bo Hello.sc
?

[lwronski]

Hi @zmerr, I thinking about similar feature as in SBT.

Yea, It would be great if Scala CLI could be able to accept both -ic “import Hello.*” and scala-cli repl -bo Hello.sc

[zmerr]

Hi @lwronski so you mean using those commands for configuring scala-cli to import or open your preferred files in the current execution of repl or some other initial command in the string? Is it what you want me to implement?

[alexarchambault]

Don't know if that's what @lwronski is thinking about, but I think we'd want to use the --predef-code option of Ammonite (which is mentioned on its website). It basically allows to run a bit of code right before the repl session. I guess Scala CLI could accept such code via a flag like… --run-before maybe? or --predef-code like Ammonite?

For *.sc files, maybe we should either read them ourselves, and pass them to Ammonite via --predef-code too, or use Ammonite's --predef option (that accepts paths to *.sc files).

[lwronski]

@zmerr If you run scala-cli repl Hello.scala you have access to Hello object

VL-D-0317:scala-demo lwronski$ scala-cli repl Hello.scala 
Welcome to Scala 3.1.1 (17.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                
scala> println(Hello.hello)
Hello World

so it would be great if it worked like this:

VL-D-0317:scala-demo lwronski$ scala-cli repl Hello.scala --run-before "import Hello.*"
Welcome to Scala 3.1.1 (17.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                
scala> println(hello)
Hello World

if that's not possible for Scala 2/3 REPL, then we can add this option only for Ammonite REPL

VL-D-0317:scala-demo lwronski$ scala-cli repl Hello.scala  --ammonite --run-before "import Hello.*"
Welcome to the Ammonite Repl 2.5.1-6-5fce97fb (Scala 3.0.0 Java 17.0.1)
@ println(hello)
Hello World

[zmerr]

@lwronski Awesome. Now I got what it is that you actually want me to do. I am just curious, why are you still adding Hello.scala to the command, when you are already importing Hello.*? Sorry I am new to this.

[zmerr]

@lwronski I mean why not this:

scala-cli repl --ammonite --run-before "import Hello.*”

or

scala-cli repl --run-before "import Hello.*”

[lwronski]

If you run REPL without the source files, you have REPL without any loaded classes.

scala-cli repl --ammonite --run-before "import Hello.*” - it doesn't make sense, because there is no exists Hello object.

[zmerr]

@lwronski thanks. So making it work that way, requires making change to repl as well.

[prolativ]

In scala 3 if you write your init script like this

export Hello.*

and you run scala-cli repl InitScript.scala
then in REPL you can already directly access everything that was defined in Hello.
Does this solve our problem?
The only thing I can think of that doesn't work out of the box currently is instant execution of side effects from scripts. So even if I write my init script like this

val xxx = println("Hello")

nothing will be printed until I type xxx in the REPL. But maybe that's actually good that starting a REPL doesn't cause any side effects under the hood?

[prolativ]

I've found one limitation of my solution: wildcard exports work correctly with objects but not with packages.

[bjornregnell]

@prolativ @Gedochao I tried to find among issues in dotty a discussion about export not working for packages. I think it really should, if possible, as I think that feature is very useful. Do you perhaps know where to find any potentially existing discussions about exporting top-level package members also, not just object members? This will allow init repl stuff to be used via scala-cli repl --scala-snippet as discussed in #1853