Allow redeclaring values in REPL and give temporary names to expressions that are not assigned

[afroozeh]

I've been working a bit with the Rascal REPL in the last weeks, and compared to Scala/Node.JS REPLs, I miss two features:

• Redefining values: Rascal throws an error that the variable is already declared. I can imagine that because of immutability this is desired in the language, but for REPL, I'd consider it a bit of restriction. Especially when experimenting, it's hard to restart the REPL or choose a different variable name each time.

  

• Giving names to expressions that are not assigned. Another handy feature that I miss is giving a temporary name to expressions. This is also useful for experimentation.

  

[jurgenvinju]

This makes total sense. Toplevel behavior should be different in the REPL. The same holds for functions. Sometimes you just want to replace a function definition with a new version rather than adding a new (non-deterministic) alternative.

[jurgenvinju]

the implicit variable name feature is also cool. maybe we could solve this by binding it with the latest result, just like $? in bash?

[afroozeh]

I think in other REPLs the implicit variable names all stay in the scope when the REPL session is valid, so they should go to the top-level environment. As far as I know, $? will refer to the last executed one, so it will be rewritten. Maybe you can just bind it to an implicit auto-incremented name that is put in the env?

[afroozeh]

Also would be good to drop semicolon requirement for statements as well. It's particularly noticeable when importing modules. This screenshot is from the Java REPL (JShell in Java 11).

[jurgenvinju]

agreed; we locked the grammar while working on the compiler, but this is a possible and useful feature.