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Scilla: A Smart Contract Intermediate Level Language

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Scilla short for Smart Contract Intermediate-Level LAnguage is an intermediate-level smart contract language being developed for Zilliqa. Scilla has been designed as a principled language with smart contract safety in mind.

Scilla imposes a structure on smart contracts that will make applications less vulnerable to attacks by eliminating certain known vulnerabilities directly at the language-level. Furthermore, the principled structure of Scilla will make applications inherently more secure and amenable to formal verification.

Zilliqa - the underlying blockchain platform on which Scilla contracts are run, has been designed to be scalable. It employs the idea of sharding to validate transactions in parallel. Zilliqa has an intrinsic token named Zilling, ZIL for short that are required to run smart contracts on Zilliqa.

Language Reference

A comprehensive documentation on Scilla, its features and constructs can be found here

Building Scilla

If you don't want to setup and build Scilla from source, skip this section to follow the opam installation instructions.

1. Cloning source code

We suggest users to use the latest release of Scilla available here.

If you'd like to hack on Scilla, clone it with all of its submodules:

git clone --jobs 4 --recurse-submodules

2. Build prerequisites

Platform specific instructions for setting up your system for building Scilla can be found in

3. Compiling

To build the project from the root folder:



Scilla can be installed into your opam switch as

make install

and can similarly be uninstalled as

make uninstall

Installing Scilla with opam

Scilla can be installed using OCaml's package manager opam.

Installing Scilla from GitHub

To install the development version of Scilla package make sure you are using the correct opam switch and execute the following

opam pin add scilla git+ --yes

Installing Scilla from your local repo

cd <scilla-repo>
# It is important to pick the right git branch because opam pins the package to the current branch
git checkout master
opam install ./scilla.opam

If you are using a local opam switch (see here) in you local Scilla repo (~/path/to/scilla), then most likely you will want to reuse the same local switch for your Scilla-based project. To do that create a symlink _opam as follows:

cd <scilla-based-project-repo>
ln -s ~/path/to/scilla/_opam _opam

Running the binary

Once the project is built you can try the following things:

Evaluating a standalone closed expression:

From the project root, execute

eval-runner -gaslimit 10000 -libdir src/stdlib tests/eval/good/let.scilexp

Instead of let.scilla you might want to try any different file in tests/eval. The second argument, which is a path to the Scilla standard library can alternatively be specified in the environment variable SCILLA_STDLIB_PATH. This must be an absolute path (or a list of paths separated with : (or ; on Windows).

Type-checking a contract

From the project root, execute

scilla-checker -gaslimit 10000 -libdir src/stdlib tests/contracts/auction.scilla

Instead of auction.scilla you might want to try any different file in tests/contracts with a complete implementation of a contract, or your own contract code. The second argument, which is a path to the Scilla standard library can alternatively be specified in the environment variable SCILLA_STDLIB_PATH. As above, this must be an absolute path(s).

If the checker only returns the contract structure in JSON format, it means that the contract has no type errors. Otherwise, a type error trace is provided.

The checker can be run with the following optional flags:

  • -cf to enable the cashflow checker and print its results.

Executing a simple transition

From the project root, execute

scilla-runner -init tests/runner/crowdfunding/init.json -istate tests/runner/crowdfunding/state_4.json -iblockchain tests/runner/crowdfunding/blockchain_4.json -imessage tests/runner/crowdfunding/message_4.json -o tests/runner/crowdfunding/output_4.json -i tests/contracts/crowdfunding.scilla -libdir src/stdlib -gaslimit 8000


scilla-runner -init tests/runner/zil-game/init.json -istate tests/runner/zil-game/state_5.json -iblockchain tests/runner/zil-game/blockchain_5.json -imessage tests/runner/zil-game/message_5.json -o tests/runner/zil-game/output_5.json -i tests/contracts/zil-game.scilla -libdir src/stdlib -gaslimit 8000

If you'd like to see the output produced by the aforementioned commands, check the file specified by -o path/to/file.json argument.

Alternatively, use the script as below:

./ crowdfunding 1

where n is a number 0-5 for the number of "steps" to execute the protocol (the messages and blockchain states are provided for only so many steps in the simulation).

Using Scilla as a service

A scilla-server is provided that provides the functionality of scilla-runner and scilla-checker as a JSON-RPC server. The scilla-server process accepts contract execution requests and executes the contract, providing a JSON output within the server process itself.

More details on the protocol can be found here.

For local testing and experiments, a scilla-client is also provided on development builds (make dev). This can interact with scilla-server, achieving the same effect as scilla-runner and scilla-client.

Start scilla-server without any arguments. Examples for checking a contract and running a transition via scilla-server are provided below. They are to be run on a separate shell (while scilla-server continues to run).

scilla-client run -argv " -init tests/runner/crowdfunding/init.json -istate tests/runner/crowdfunding/state_4.json -iblockchain tests/runner/crowdfunding/blockchain_4.json -imessage tests/runner/crowdfunding/message_4.json -o tests/runner/crowdfunding/output_4.json -i tests/contracts/crowdfunding.scilla -libdir src/stdlib -gaslimit 8000"

scilla-client check -argv " -libdir src/stdlib -gaslimit 8000 tests/contracts/helloWorld.scilla"

Where to find binaries

  • The runnables are put into the folder

Running the testsuite

The testsuite is based on the OUnit2 framework and is driven by the main module in tests/ There are several types of tests run in the testsuite. For instance, contracts tests run a full transition on a contract with all input data provided, and eval tests only test expression evaluation. To add more tests of either of these kinds, look for the corresponding .ml files in their tests/directory and add accordingly.

To run the testsuite:

make test

(this makes the Dune build system invoke testsuite executable with -print-diff true option to print colored diffs between error messages).

To run the testsuite executable manually using dune exec tests/testsuite.exe, you have to provide the parameters -bin-dir and -tests-dir, which must be absolute paths to the directory containing eval-runner, type-checker, scilla-runner, scilla-checker and the tests/ directory containing the tests. Relative paths may not work. Parameters to testsuite executable can be passed like so:

dune exec tests/testsuite.exe -- <space-separated-parameters>

To obtain a list of tests available:

dune exec tests/testsuite.exe -- -list-test

Running an individual test

To run an individual test(s), for example tests:4:checker:0:good:1:exptests:5:one-accept.scilla, (it's one of the tests from the list obtained via dune exec -- tests/testsuite.exe -list-test, this needs to be run from the project's root):

dune exec tests/testsuite.exe -- -only-test tests:4:checker:0:good:1:exptests:5:one-accept.scilla -print-cli true

The optional -print-cli true argument is to produce the command line that has been used to run the test.

Running a group of tests

If you'd like to run a group of tests, for instance, the typechecking tests which are assigned the name checker, execute the following from the project's root:

dune exec -- tests/testsuite.exe -only-test tests:4:checker

If you need to update the so-called gold-files which keep the expected output for the checker tests, run the following command:

dune exec -- tests/testsuite.exe -only-test tests:4:checker -update-gold true

Formatting and linting the codebase

Our CI checks that the source code is formatted properly. Use

make fmt

to ensure your code adheres to the style guide. Note that the command will automatically change ("promote") your source code. You will need the ocamlformat opam package for the command above to work.

To make sure you are good to go, before sending PR run

make lint

to check if there are any issues with your contribution. In addition to the ocamlformat package, make lint uses opam and shellcheck.


To debug scilla-checker or scilla-runner, you must build make debug, which will generate the OCaml bytecode versions of the binaries. These can be debugged using ocamldebug. Executing a bytecode executable also requires the environment variable LD_LIBRARY_PATH to be set to _build/default/src/base/cpp. An example debug command line is provided below.

LD_LIBRARY_PATH=${PWD}/_build/default/src/base/cpp ocamldebug _build/default/src/runners/scilla_checker.bc -libdir src/stdlib -gaslimit 10000 tests/contracts/helloworld.scilla

Developer Tools

Emacs mode

An emacs major mode for editing Scilla contracts is provided. Add the following line to your .emacs file to load this mode for files ending with .scilla and .scillib. For enabling flycheck mode for Scilla (see When scilla-checker is available, type reporting is also supported. The key binding C-c C-t will print the type of the variable on which the cursor currently is.

;; For enabling flycheck mode for Scilla.
(setq scilla-root "/path/to/scilla/root")
;; Scilla mode
(load-file "/path/to/scilla-mode.el")

Vim plugin

A vim plugin for editing Scilla contracts is provided.

You can install the vim config files through Pathogen by:

git clone ~/.vim/bundle/vim-scilla

Or through Vundle by adding the following line to your ~/.vimrc:

Plugin 'edisonljh/vim-scilla'

Repo: vim-scilla.

VSCode Plugin

Visual Studio Code support for Scilla is avaiable. Github Source You can install it through:

Credits: as1ndu

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