Miden REPL

The Miden Read–eval–print loop (REPL) is a Miden shell that allows for quick and easy debugging of Miden assembly. After the REPL gets initialized, you can execute any Miden instruction, undo executed instructions, check the state of the stack and memory at a given point, and do many other useful things! When the REPL is exited, a history.txt file is saved. One thing to note is that all the REPL native commands start with an ! to differentiate them from regular assembly instructions.

Miden REPL can be started via the CLI repl command like so:

./target/optimized/miden repl

It is also possible to initialize REPL with libraries. To create it with Miden standard library you need to specify -s or --stdlib subcommand, it is also possible to add a third-party library by specifying -l or --libraries subcommand with paths to .masl library files. For example:

./target/optimized/miden repl -s -l example/library.masl

Miden assembly instruction

All Miden instructions mentioned in the Miden Assembly sections are valid. One can either input instructions one by one or multiple instructions in one input.

For example, the below two commands will result in the same output.

>> push.1
>> push.2
>> push.3
push.1 push.2 push.3

To execute a control flow operation, one must write the entire statement in a single line with spaces between individual operations.


The above example should be written as follows in the REPL tool:

repeat.20 pow2 end


The !help command prints out all the available commands in the REPL tool.


The !program command prints out the entire Miden program being executed. E.g., in the below scenario:

>> push.
>> repeat.16 pow2 end
>> u32wrapping_add

>> !program
    repeat.16 pow2 end


The !stack command prints out the state of the stack at the last executed instruction. Since the stack always contains at least 16 elements, 16 or more elements will be printed out (even if all of them are zeros).

>> push.1 push.2 push.3 push.4 push.5
>> exp
>> u32wrapping_mul
>> swap
>> eq.2
>> assert

The !stack command will print out the following state of the stack:

>> !stack
3072 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0


The !mem command prints out the contents of all initialized memory locations. For each such location, the address, along with its memory values, is printed. Recall that four elements are stored at each memory address.

If the memory has at least one value that has been initialized:

>> !mem
7: [1, 2, 0, 3]
8: [5, 7, 3, 32]
9: [9, 10, 2, 0]

If the memory is not yet been initialized:

>> !mem
The memory has not been initialized yet


The !mem[addr] command prints out memory contents at the address specified by addr.

If the addr has been initialized:

>> !mem[9]
9: [9, 10, 2, 0]

If the addr has not been initialized:

>> !mem[87]
Memory at address 87 is empty


The !use command prints out the list of all modules available for import.

If the stdlib was added to the available libraries list !use command will print all its modules:

>> !use
Modules available for importing:

Using the !use command with a module name will add the specified module to the program imports:

>> !use std::math::u64

>> !program




The !undo command reverts to the previous state of the stack and memory by dropping off the last executed assembly instruction from the program. One could use !undo as often as they want to restore the state of a stack and memory instructions ago (provided there are instructions in the program). The !undo command will result in an error if no remaining instructions are left in the Miden program.

>> push.1 push.2 push.3
>> push.4
>> !stack
4 3 2 1 0 0 0 0 0 0 0 0 0 0 0 0

>> push.5
>> !stack
5 4 3 2 1 0 0 0 0 0 0 0 0 0 0 0

>> !undo
4 3 2 1 0 0 0 0 0 0 0 0 0 0 0 0

>> !undo
3 2 1 0 0 0 0 0 0 0 0 0 0 0 0 0