Acton REPL

acton repl starts an interactive shell for trying Acton code without creating a normal project by hand.

$ acton repl
Welcome to InterActon, the interactive Acton REPL shell. Type :help for commands, :quit to exit.
acton> 1 + 2
3
acton> :quit

The REPL is still compiler-backed. It writes a small scratch project, compiles the code you enter, and runs a small generated executable. This makes it slower than a bytecode interpreter, but it uses the same parser, type checker, code generator, and runtime as normal Acton programs.

Session source

The REPL keeps a source model of the current interactive session.

Imports, signatures, functions, classes, actors, and other declarations are retained as session source:

acton> def greet(name: str) -> str:
...     return "hello " + name
...
acton> greet("Ada")
hello Ada

If you redefine a retained top-level name, the previous definition for that name is removed from the session before the next compile:

acton> def greet(name: str) -> str:
...     return "hi " + name
...
acton> greet("Ada")
hi Ada

Top-level assignments are handled differently. They are replayed as setup code inside the implicit evaluation procedure before each evaluation, instead of being emitted at module top level. This allows effectful setup, including actor creation:

acton> actor Foo():
...     print("hello from Foo")
...
acton> f = Foo()
hello from Foo

Use :show to inspect the retained session source and :reset to clear it:

acton> :show
def greet(name: str) -> str:
    return "hi " + name

acton> :reset
Session reset.
acton> :show
<empty>

Expressions and statements

Plain expressions are evaluated and non-None results are printed automatically:

acton> "ab" + "cd"
abcd

The generated eval procedure stores the expression result in a local variable and prints it only when it is not None. Calls that already print or otherwise return None therefore do not produce an extra None line:

acton> proc def log():
...     print("logged")
...
acton> log()
logged

Inputs that are not a single expression compile as statement blocks and report compiler errors in the usual way.

Actor construction returns an actor reference, so a direct constructor expression displays that reference after starting the actor. Use a setup assignment if you want to start the actor without displaying the reference:

acton> f = Foo()

Scratch project

By default, the REPL uses a cache-backed scratch directory so the generated project outputs can be reused by the normal warmup build across REPL starts. The scratch project contains three generated Acton modules:

• repl_session contains retained imports and declarations.
• repl_eval renders retained imports, imports repl_session, and defines a fixed proc def eval() entry point with setup replay and the current input.
• repl_main renders retained imports and is the executable runner that calls repl_eval.eval() and exits.

repl_session and repl_eval are built as dynamic libraries. Most evaluations therefore rebuild only the changed evaluation module instead of relinking the runner executable.

If you pass --tempdir DIR, Acton treats DIR as a parent directory and writes the scratch project under DIR/.acton-repl. This avoids overwriting a real project's Build.act or src directory.

Dependencies and projects

In a standalone REPL, use :dep add NAME to add a library package from the online package index:

acton> :dep add textfsm
Added dependency textfsm

Use :dep list to show REPL dependencies and :dep rm NAME to remove one. The REPL intentionally does not expose local path dependency commands.

When started inside an Acton project, the REPL adds that project as a local scratch dependency after the first accepted import. The generated scratch project stays outside the project source tree.

Commands and editing

The interactive shell supports readline-style editing, including common cursor movement keys such as Ctrl-a and Ctrl-e. Tab completion covers REPL commands and :dep subcommands, not filesystem paths.

Available commands:

:help          Show REPL help
:quit, :q      Exit
:reset         Clear retained imports, definitions, setup, and dependencies
:show          Show retained session source
:dep list      Show online package dependencies
:dep add NAME  Add an online package dependency
:dep rm NAME   Remove an online package dependency

For multi-line definitions, enter the header line ending in : and then continue at the ... prompt. A blank line ends the block.