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Using the REPL

desirepl is a small interactive prompt for trying out Desi — expressions, a few bindings, a short function. It is deliberately simple in 0.1.0: it is a scratchpad, not a development environment. See Limitations before you settle in.

Desi is a compiled language, so there is no interpreter behind the prompt. The REPL keeps track of what you have entered, assembles a real program from it, and compiles and runs it with desic. That means the REPL can never disagree with a real build — the trade-off is roughly half a second per input.

Starting the REPL

desirepl

desirepl 0.1.0
Type expressions or statements. :help for commands, :quit to exit.

>>>

Quick Tour

Expressions show their value

>>> 1 + 2
3
>>> 5 > 3
true

Bindings persist across lines

>>> let x = 10
>>> x * 4
40
>>> let name = "Desi"
>>> print(f"Hello, {name}!")
Hello, Desi!

Collections

>>> let nums = [10, 20, 30]
>>> nums[0]
10
>>> len(nums)
3
>>> let info = {"lang": "desi"}
>>> info["lang"]
desi

Functions

A line ending in : starts a block. Keep typing, then finish with a blank line:

>>> def double(n: int) -> int:
...     n * 2
...
>>> double(21)
42

Errors do not end the session

Type errors are reported and the offending input is discarded — everything you entered before it is still there:

>>> let x: str = 42
  [DTE0004] type error: cannot assign 'int' to 'str'
>>> 1 + 1
2

REPL Commands

Command Description
:help or :h Show available commands
:session Show everything currently in the session
:reset Clear the session
:ast Show the AST for the next input
:quit or :q Exit the REPL
quit or exit Exit the REPL (convenience)

Viewing the AST

>>> :ast
(will show AST for next input)
>>> let z = 42
Module("<repl>")
    Func __repl()
        Block
            Let z = Int(42)

How it works

Each input is classified and stored:

  • Imports and declarations (import, def, class, enum, …) are kept and placed at the top of the generated program.
  • Statements (let, assignments, if/while blocks) are kept and replayed inside main().
  • Expressions are wrapped in print(...) so you see the value, and are not kept — they have already been shown.

Because retained statements are replayed with every later input, a statement with a side effect runs again each time. Output you have already seen is not reprinted, but something like let f = open_file(...) in the session will be re-executed. Use :reset to start over.

Limitations

desirepl is intentionally minimal in 0.1.0. It is not an IDE like Python's IDLE, and it is not as capable as python's own REPL:

  • No command history or line editing. Arrow keys do not recall previous input — there is no readline support yet. This is the roughest edge.
  • No tab completion and no syntax highlighting.
  • No editor window, no debugger, no inspector.
  • About half a second per input, because each one is compiled and run.
  • Retained statements re-run, as described above.

For anything beyond a quick experiment, write a file and run it:

# Run a file
desic run hello.desi

# Run a project (uses desi.mod)
desic run

The REPL is expected to grow — history and line editing first. Its current scope is deliberately narrow rather than half-finished in a way that misleads you about what it can do.

See Also