# NEO-GO smart contract compiler The neo-go compiler compiles Go programs to bytecode that the NEO virtual machine can understand. ## Language compatibility The compiler is mostly compatible with regular Go language specification, but there are some important deviations that you need to be aware of that make it a dialect of Go rather than a complete port of the language: * `new()` is not supported, most of the time you can substitute structs with composite literals * `make()` is supported for maps and slices with elements of basic types * `copy()` is supported only for byte slices, because of underlying `MEMCPY` opcode * pointers are supported only for struct literals, one can't take an address of an arbitrary variable * there is no real distinction between different integer types, all of them work as big.Int in Go with a limit of 256 bit in width, so you can use `int` for just about anything. This is the way integers work in Neo VM and adding proper Go types emulation is considered to be too costly. * goroutines, channels and garbage collection are not supported and will never be because emulating that aspects of Go runtime on top of Neo VM is close to impossible * `defer` and `recover` are supported except for cases where panic occurs in `return` statement, because this complicates implementation and imposes runtime overhead for all contracts. This can easily be mitigated by first storing values in variables and returning the result. * lambdas are supported, but closures are not. * maps are supported, but valid map keys are booleans, integers and strings with length <= 64 ## VM API (interop layer) Compiler translates interop function calls into NEO VM syscalls or (for custom functions) into NEO VM instructions. [Refer to pkg.go.dev](https://pkg.go.dev/github.com/nspcc-dev/neo-go/pkg/interop) for full API documentation. In general it provides the same level of functionality as Neo .net Framework library. Compiler provides some helpful builtins in `util` and `convert` packages. Refer to them for detailed documentation. `_deploy()` function has a special meaning and is executed when contract is deployed. It should return no value and accept single bool argument which will be true on contract update. `_deploy()` functions are called for every imported package in the same order as `init()`. ## Quick start ### Compiling ``` ./bin/neo-go contract compile -i mycontract.go ``` By default the filename will be the name of your .go file with the .nef extension, the file will be located in the same directory where your Go contract is. If you want another location for your compiled contract: ``` ./bin/neo-go contract compile -i mycontract.go --out /Users/foo/bar/contract.nef ``` If you contract is split across multiple files, you must provide a path to the directory where package files are contained instead of a single Go file: ``` ./bin/neo-go contract compile -i ./path/to/contract ``` ### Debugging You can dump the opcodes generated by the compiler with the following command: ``` ./bin/neo-go contract inspect -i mycontract.go -c ``` This will result in something like this: ``` INDEX OPCODE PARAMETER 0 INITSLOT 0500 ("\x05\x00") << 3 PUSH0 4 REVERSEN 5 SYSCALL "\x9a\x1f\x19J" 10 NOP 11 STLOC0 12 LDLOC0 13 PUSH1 14 REVERSEN 15 PUSH1 16 PACK 17 SYSCALL "\x05\a\x92\x16" 22 NOP 23 PUSH0 24 REVERSEN 25 SYSCALL "E\x99Z\\" 30 NOP 31 STLOC1 32 LDLOC1 33 PUSH1 34 REVERSEN 35 PUSH1 36 PACK 37 SYSCALL "\x05\a\x92\x16" 42 NOP 43 PUSH0 44 REVERSEN 45 SYSCALL "\x87\xc3\xd2d" 50 NOP 51 STLOC2 52 LDLOC2 53 PUSH1 54 REVERSEN 55 PUSH1 56 PACK 57 SYSCALL "\x05\a\x92\x16" 62 NOP 63 PUSH0 64 REVERSEN 65 SYSCALL "\x1dY\xe1\x19" 70 NOP 71 STLOC3 72 LDLOC3 73 PUSH1 74 REVERSEN 75 PUSH1 76 PACK 77 SYSCALL "\x05\a\x92\x16" 82 NOP 83 PUSH1 84 RET ``` #### Neo Smart Contract Debugger support It's possible to debug contracts written in Go using standard [Neo Smart Contract Debugger](https://github.com/neo-project/neo-debugger/) which is a part of [Neo Blockchain Toolkit](https://github.com/neo-project/neo-blockchain-toolkit/). To do that you need to generate debug information using `--debug` option, like this: ``` $ ./bin/neo-go contract compile -i contract.go -o contract.nef --debug contract.debug.json ``` This file can then be used by debugger and set up to work just like for any other supported language. ### Deploying Deploying a contract to blockchain with neo-go requires a configuration file with contract's metadata in YAML format, like the following: ``` project: author: Jack Smith email: jack@example.com version: 1.0 name: 'Smart contract' description: 'Even smarter than Jack himself' hasstorage: true hasdynamicinvocation: false ispayable: false returntype: ByteArray parameters: ['String', 'Array'] ``` It's passed to the `deploy` command via `-c` option: ``` $ ./bin/neo-go contract deploy -i contract.nef -c contract.yml -r http://localhost:20331 -w wallet.json -g 0.001 ``` Deployment works via an RPC server, an address of which is passed via `-r` option and should be signed using a wallet from `-w` option. More details can be found in `deploy` command help. #### Neo Express support It's possible to deploy contracts written in Go using [Neo Express](https://github.com/neo-project/neo-express) which is a part of [Neo Blockchain Toolkit](https://github.com/neo-project/neo-blockchain-toolkit/). To do that you need to generate a different metadata file using YAML written for deployment with neo-go. It's done in the same step with compilation via `--config` input parameter and `--abi` output parameter, combined with debug support the command line will look like this: ``` $ ./bin/neo-go contract compile -i contract.go --config contract.yml -o contract.nef --debug contract.debug.json --abi contract.abi.json ``` This file can then be used by toolkit to deploy contract the same way contracts in other languagues are deployed. ### Invoking You can import your contract into the standalone VM and run it there (see [VM documentation](vm.md) for more info), but that only works for simple contracts that don't use blockchain a lot. For more real contracts you need to deploy them first and then do test invocations and regular invocations with `contract testinvokefunction` and `contract invokefunction` commands (or their variants, see `contract` command help for more details. They all work via RPC, so it's a mandatory parameter. Example call (contract `f84d6a337fbc3d3a201d41da99e86b479e7a2554` with method `balanceOf` and method's parameter `AK2nJJpJr6o664CWJKi1QRXjqeic2zRp8y` using given RPC server and wallet and paying 0.00001 GAS for this transaction): ``` $ ./bin/neo-go contract invokefunction -r http://localhost:20331 -w my_wallet.json -g 0.00001 f84d6a337fbc3d3a201d41da99e86b479e7a2554 balanceOf AK2nJJpJr6o664CWJKi1QRXjqeic2zRp8y ``` ## Smart contract examples Some examples are provided in the [examples directory](../examples). ### Check if the invoker of the contract is the owning address ```Golang package mycontract import ( "github.com/nspcc-dev/neo-go/pkg/interop/runtime" "github.com/nspcc-dev/neo-go/pkg/interop/util" ) var owner = util.FromAddress("AJX1jGfj3qPBbpAKjY527nPbnrnvSx9nCg") func Main() bool { isOwner := runtime.CheckWitness(owner) if isOwner { runtime.Log("invoker is the owner") return true } return false } ``` ### Simple token ```Golang package mytoken import ( "github.com/nspcc-dev/neo-go/pkg/interop/runtime" "github.com/nspcc-dev/neo-go/pkg/interop/storage" ) var owner = util.FromAddress("AJX1jGfj3qPBbpAKjY527nPbnrnvSx9nCg") type Token struct { Name string Symbol string TotalSupply int Owner []byte } func (t Token) AddToCirculation(amount int) bool { ctx := storage.Context() var inCirc int val := storage.Get(ctx, "in_circ") if val != nil { inCirc = val.(int) } inCirc += amount storage.Put(ctx, "in_circ", inCirc) return true } func newToken() Token { return Token{ Name: "your awesome NEO token", Symbol: "YANT", TotalSupply: 1000, Owner: owner, } } func Main(operation string, args []interface{}) bool { token := newToken() trigger := runtime.GetTrigger() if trigger == runtime.Verification { isOwner := runtime.CheckWitness(token.Owner) if isOwner { return true } return false } if trigger == runtime.Application { if operation == "mintTokens" { token.AddToCirculation(100) } } return true } ``` ## How to report compiler bugs 1. Make a proper testcase (example testcases can be found in the tests folder) 2. Create an issue on Github 3. Make a PR with a reference to the created issue, containing the testcase that proves the bug 4. Either you fix the bug yourself or wait for patch that solves the problem