Running Varphi Programs
Once you have the Varphi Interpreter (vpi) installed, you can execute, debug, and analyze your Turing Machine programs directly from the command line.
Basic Usage
The general syntax for the interpreter is:
vpi [OPTIONS] INPUT_FILEINPUT_FILE: The path to your
.vpsource code.
Running a Program
To run a program, simply pass the filename. The interpreter will compile the code and then prompt you to enter the initial content for each tape defined in your program via standard input (stdin).
vpi example.vpExample Session:
$ vpi example.vp
Number of input tapes: 1
Tape 1: hello
————————————————————————————————————————————————————————————
HALTED at state 'accept'
Time taken: 25 steps
Space used: 11 cells
Number of tapes: 1
Tape 1: helloInput Formatting & Tape Behavior
Since Varphi allows alphanumeric characters on tapes, there are specific rules for how to represent blank cells and how the machine initializes head positions.
1. Representing Blanks (_)
_)The standard BLANK keyword used in Varphi source code programs cannot be typed easily in a terminal without confusing it for the characters "B", "L", "A", "N", and "K". When providing input, use the underscore _ to represent a blank cell.
Example Input:
abc_123Varphi will interpret this as:
['a', 'b', 'c', BLANK, '1', '2', '3']
In any output of the Varphi Interpreter, a blank tape cell will also be represented with _.
2. Head Positioning
When a program starts, the tape head, for any tape of the machine, is always stationed at the first non-blank character of your input string. Leading blanks are essentially skipped for the purpose of initialization. If there is no non-blank character on the input tape, an arbitrary blank tape cell will be chosen.
Example: If you input _____123, the tape will contain those leading blanks (obviously, because there are infinite blanks in either direction anyways), but the head will start off pointing at 1, as shown below:
3. Tape Count & Composition
The input system allows you to input more or less tapes than the machine actually needs (k), according to two rules:
Fewer Tapes: If you provide less tapes than required, the remaining tapes are automatically initialized to be blank.
More Tapes: If you provide more tapes than required, only the first k are used.
This allows convenient composition of Turing machines by piping the output of one to another, even if the two machines do not use the same number of tapes.
Note: Piping will not work well with debug mode enabled, so avoid composition/piping when --debug is used.
Execution Modes
vpi supports several flags to alter how the program is run.
1. Standard Run
By default, vpi runs silently until the machine halts. Upon completion, it prints the final state of all tapes and performance metrics (see below).
2. Debug Mode (--debug)
--debug)The --debug flag activates an interactive, step-by-step debugger. This is essential for visualizing state transitions and head movements.
Visual Head: The character currently under the tape head is enclosed in brackets (e.g.,
[1]).Stepping: Press
ENTERto advance the machine by one step.Interruption: You can stop execution at any time using
Ctrl+C.
Output Preview:
As the command line debugger requires you to input ENTER over standard input, you must not use this mode if you are redirecting the input tapes from a file, since redirection closes standard input.
3. Syntax Check (--check)
--check)If you only want to verify that your code is valid without running it, use --check. This is useful for catching syntax errors quickly.
If the code is valid, it prints OK. If there are errors, the compiler driver will point to the exact line and column of the issue. An example of a compilation error is shown below:
Understanding Performance Metrics
When a Varphi program halts, the interpreter reports Time taken and Space used. It is important to understand how these are calculated.
Time Complexity
Time taken is simply the total number of transitions (steps) the machine executed from the start state until it reached a halting configuration. You can see the exact steps by using the --debug flag.
Space Complexity (Auxiliary Space)
Space used calculates the Auxiliary Space complexity, not the total tape lengths.
The cells occupied by your initial input do not count toward space complexity.
Space is only counted when the machine writes to or visits a cell outside the range of the original input.
For example, if you input a string of length 5, and the machine only moves back and forth within those 5 cells, the Space used will be 0 cells. If it moves one step to the right of the input and reads that tape cell, the space used becomes 1 cell.
CLI Reference
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