ligo/gitlab-pages/docs/advanced/first-contract.md

id	title
first-contract	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 access 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 access 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 access function, we can see the operations emited - 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 access function to two entrypoints for addition and subtraction.

type action is
  Increment of int
| Decrement of int

function main (const p : action ; const s : int) : (list(operation) * int) is
  ((nil : list(operation)),
  (case p of
    | Increment (n) -> s + n
    | Decrement (n) -> s - n
   end))

type action =
| Increment of int
| Decrement of int

let main (p, s: action * int) : operation list * int =
  let result =
    match p with
    | Increment n -> s + n
    | Decrement n -> s - n
  in
  (([]: operation list), result)

type action =
| Increment(int)
| Decrement(int);

let main = (p_s: (action, int)) : (list(operation), int) => {
  let p, s = p_s;
  let result =
    switch (p) {
    | Increment(n) => s + n
    | Decrement(n) => s - n
    };
  (([]: list(operation)), result);
};

To dry-run the counter contract, we will use the main entrypoint, provide a variant parameter of 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 ;
         CAR ;
         DIP { DUP } ;
         SWAP ;
         CDR ;
         DIP { DUP } ;
         SWAP ;
         IF_LEFT
           { DUP ;
             DIP 2 { DUP } ;
             DIG 2 ;
             DIP { DUP } ;
             SUB ;
             SWAP ;
             DROP ;
             SWAP ;
             DROP }
           { DUP ;
             DIP 2 { DUP } ;
             DIG 2 ;
             DIP { DUP } ;
             ADD ;
             SWAP ;
             DROP ;
             SWAP ;
             DROP } ;
         NIL operation ;
         PAIR ;
         SWAP ;
         DROP ;
         SWAP ;
         DROP ;
         SWAP ;
         DROP } }

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