Version: 0.3.0

First contract

So far so good, we have learned enough of the LIGO language, we are confident enough to write out first smart contract.

We will be implementing a counter contract.

Dry-running a Contract

Testing a contract can be quite easy if we utilize LIGO's built-in dry run feature. Dry-run works by simulating the main function execution, as if it were deployed on a real chain. You need to provide the following:

  • file - contract to run
  • entrypoint - name of the function to execute
  • parameter - parameter passed to the main function (in a theoretical invocation operation)
  • storage - a mock storage value, as if it were stored on a real chain

Here is a full example:

ligo dry-run src/basic.ligo main Unit Unit
// Outputs:
// tuple[ list[]
// Unit
// ]

Output of the dry-run is the return value of our main function, we can see the operations emitted (in our case an empty list, and the new storage value being returned) which in our case is still Unit.

A Counter Contract

Our counter contract will store a single int as it's storage, and will accept an action variant in order to re-route our single main function to two entrypoints for add (addition) and sub (subtraction).

type parameter is
Increment of int
| Decrement of int
type storage is int
type return is list (operation) * storage
function add (const n : int; const store : storage) : storage is store + n
function sub (const n : int; const store : storage) : storage is store - n
function main (const action : parameter; const store : storage) : return is
((nil : list(operation)),
case action of
Increment (n) -> add (n, store)
| Decrement (n) -> sub (n, store)
end)

To dry-run the counter contract, we will provide the main function with a variant parameter of value Increment (5) and an initial storage value of 5.

ligo dry-run src/counter.ligo main "Increment(5)" 5
// tuple[ list[]
// 10
// ]

Our contract's storage has been successfuly incremented to 10.

Deploying and interacting with a contract on a live-chain

In order to deploy the counter contract to a real Tezos network, we'd have to compile it first, this can be done with the help of the compile-contract CLI command:

ligo compile-contract src/counter.ligo main

Command above will output the following Michelson code:

{ parameter (or (int %decrement) (int %increment)) ;
storage int ;
code { DUP ;
CDR ;
DIP { DUP } ;
SWAP ;
CAR ;
IF_LEFT
{ DUP ;
DIP { DIP { DUP } ; SWAP } ;
PAIR ;
DUP ;
CDR ;
DIP { DUP ; CAR } ;
SUB ;
DIP { DROP 2 } }
{ DUP ;
DIP { DIP { DUP } ; SWAP } ;
PAIR ;
DUP ;
CDR ;
DIP { DUP ; CAR } ;
ADD ;
DIP { DROP 2 } } ;
NIL operation ;
PAIR ;
DIP { DROP 2 } } }

However in order to originate a Michelson contract on Tezos, we also need to provide the initial storage value, we can use compile-storage to compile the LIGO representation of the storage to Michelson.

ligo compile-storage src/counter.ligo main 5
// Outputs: 5

In our case the LIGO storage value maps 1:1 to its Michelson representation, however this will not be the case once the parameter is of a more complex data type, like a record.

Invoking a LIGO contract

Same rules apply for parameters, as apply for translating LIGO storage values to Michelson. We will need to use compile-parameter to compile our action variant into Michelson, here's how:

ligo compile-parameter src/counter.ligo main 'Increment(5)'
// Outputs: (Right 5)

Now we can use (Right 5) which is a Michelson value, to invoke our contract - e.g., via tezos-client