Michelson onchain views

Context

Currently, there is no direct way to get output from another contract. This MR adds a mechanism to declare a view that takes a value and the current storage of the contract as input and returns a value as output.

• TZIP:
  • version 0: https://gitlab.com/tzip/tzip/-/merge_requests/108
  • version 1: tzip!169 (merged)
• Tezos Agora: https://forum.tezosagora.org/t/views-tzip/2560/14

A new toplevel keyword view is introduced for declaring views in Michelson scripts; each script can declare 0, 1, or several views.

Views are named using strings, in a given script views should have distinct names.

The syntax of a view declaration is:

• view name 'arg_ty 'return_ty { (instr) ; ... }: This view should be defined in the top-level. Like the main Michelson program, it maintains a stack and its initial value contains a pair in which the first element is an input value and the second element is the contract storage.

To call views from other contracts, a new instruction VIEW is introduced.

• VIEW name 'arg_ty 'return_ty: It allows to use of predefined view in top-level and a result can be obtained immediately. If the given address or name is nonexistent, None will be returned. Otherwise, Some a will be returned.

    VIEW name 'arg_ty 'return_ty :: 'arg_ty : address : 'S -> option 'return_ty : 'S

Here is an example of contracts: In the first contract, it defines two view s in top-level which named add_v and mul_v.

    { parameter nat;
      storage nat;
      code { CAR; NIL operation ; PAIR };
      view "add_v" nat nat { UNPAIR; ADD };
      view "mul_v" nat nat { UNPAIR; MUL };
    }

In this contract, it calls the add_v of the above contract and obtains a result immediately.

    { parameter (pair nat address) ;
      storage nat ;
      code { CAR ; UNPAIR; VIEW "add_v" nat nat ;
             IF_SOME { } { FAIL }; NIL operation; PAIR }; 
    }

reference: https://forum.tezosagora.org/t/adding-read-only-calls/1227

Manually testing the MR

The following is for manually running test cases of this change.

poetry run pytest tests_alpha/test_contract.py -k testView

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• Add item in the Development Version section of CHANGES.md (only for new features and bug fixes).

Reviewers

@galfour