How to echo user input in Scala REPL?

Question

I'm teaching an introductory programming class, using Scala. We are starting with the REPL. The REPL has a bug in that, when the student enters a readLine command, their input is not echoed. Is there some workaround that I can suggest or provide?

I don't have this trouble when using Eclipse, but it will be some weeks before I introduce Eclipse to my students.

@som-snytt reader.readLine doesn't work with 2.10 for some weird reason. I am guessing that's the problem. — gourlaysama
– gourlaysama, Commented Aug 31, 2013 at 21:43

gourlaysama · Accepted Answer · 2013-08-31 21:45:50Z

4

You can use power mode to get access to the REPL's reader; it will give you a fully working readLine:

scala> :power
** Power User mode enabled - BEEP WHIR GYVE **
** :phase has been set to 'typer'.          **
** scala.tools.nsc._ has been imported      **
** global._, definitions._ also imported    **
** Try  :help, :vals, power.<tab>           **

scala> repl.in.readLine("enter something: ")
enter something: hello world
res0: String = hello world

scala>

Edit: as @som-snytt pointed out, in 2.11 you can use reader instead of repl.in in the above code, which is both shorter and easier to remember.

edited Aug 31, 2013 at 21:45

answered Aug 31, 2013 at 21:21

gourlaysama

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1 Comment

David Matuszek Over a year ago

Thanks! repl.in.readLine works perfectly; reader.readLine does not. <br> On my Mac, I get error: not found: value reader<br> On Windows, I get error: value reader in class Global cannot be accessed in scala.tools.nsc.Global <br> repl.in.readLine is probably better for my students in any case, as it emphasizes that this is only for the REPL.

Marth · Accepted Answer · 2015-08-07 14:30:41Z

2

Use scala -Xnojline :

scala> val l = readLine
test
l: String = test

This does however break some things, in particular arrow keys, so you can't modify previous commands.

If available, you can use rlwrap scala -Xnojline (should be available on cygwin too) to restore those functionalities.

Full credit to this post.

edited Aug 7, 2015 at 14:30

answered Aug 31, 2013 at 18:50

Marth

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