# Go {#sec-language-go}

## Building Go modules with `buildGoModule` {#ssec-language-go}

The function `buildGoModule` builds Go programs managed with Go modules. It builds [Go Modules](https://go.dev/wiki/Modules) through a two phase build:

- An intermediate fetcher derivation called `goModules`. This derivation will be used to fetch all the dependencies of the Go module.
- A final derivation will use the output of the intermediate derivation to build the binaries and produce the final output.

### Example for `buildGoModule` {#ex-buildGoModule}

The following is an example expression using `buildGoModule`:

```nix
{
  pet = buildGoModule (finalAttrs: {
    pname = "pet";
    version = "0.3.4";

    src = fetchFromGitHub {
      owner = "knqyf263";
      repo = "pet";
      tag = "v${finalAttrs.version}";
      hash = "sha256-Gjw1dRrgM8D3G7v6WIM2+50r4HmTXvx0Xxme2fH9TlQ=";
    };

    vendorHash = "sha256-ciBIR+a1oaYH+H1PcC8cD8ncfJczk1IiJ8iYNM+R6aA=";

    meta = {
      description = "Simple command-line snippet manager, written in Go";
      homepage = "https://github.com/knqyf263/pet";
      license = lib.licenses.mit;
      maintainers = with lib.maintainers; [ kalbasit ];
    };
  });
}
```

## Attributes of `buildGoModule` {#buildgomodule-parameters}

Many attributes [controlling the build phase](#variables-controlling-the-build-phase) are respected by `buildGoModule`. Note that `buildGoModule` reads the following attributes also when building the `vendor/` goModules fixed output derivation as well:

- [`sourceRoot`](#var-stdenv-sourceRoot)
- [`prePatch`](#var-stdenv-prePatch)
- [`patches`](#var-stdenv-patches)
- [`patchFlags`](#var-stdenv-patchFlags)
- [`postPatch`](#var-stdenv-postPatch)
- [`preBuild`](#var-stdenv-preBuild)
- `env`: useful for passing down variables such as `GOWORK`.

To control test execution of the build derivation, the following attributes are of interest:

- [`checkInputs`](#var-stdenv-checkInputs)
- [`preCheck`](#var-stdenv-preCheck)
- [`checkFlags`](#var-stdenv-checkFlags)

In addition to the above attributes, and the many more variables respected also by `stdenv.mkDerivation`, `buildGoModule` respects Go-specific attributes that tweak them to behave slightly differently:

### `vendorHash` {#var-go-vendorHash}

Hash of the output of the intermediate fetcher derivation (the dependencies of the Go modules).

`vendorHash` can be set to `null`.
In that case, rather than fetching the dependencies, the dependencies already vendored in the `vendor` directory of the source repo will be used.

To avoid updating this field when dependencies change, run `go mod vendor` in your source repo and set `vendorHash = null;`.
You can read more about [vendoring in the Go documentation](https://go.dev/ref/mod#vendoring).

To obtain the hash, set `vendorHash = lib.fakeHash;` and run the build. ([more details here](#sec-source-hashes)).
Another way is to use use `nix-prefetch` to obtain the hash. The following command gets the value of `vendorHash` for package `pet`:


```sh
cd path/to/nixpkgs
nix-prefetch -E "{ sha256 }: ((import ./. { }).my-package.overrideAttrs { vendorHash = sha256; }).goModules"
```

`vendorHash` can be overridden with `overrideAttrs`. Override the above example like this:

```nix
{
  pet_0_4_0 = pet.overrideAttrs (
    finalAttrs: previousAttrs: {
      version = "0.4.0";
      src = fetchFromGitHub {
        inherit (previousAttrs.src) owner repo;
        rev = "v${finalAttrs.version}";
        hash = "sha256-gVTpzmXekQxGMucDKskGi+e+34nJwwsXwvQTjRO6Gdg=";
      };
      vendorHash = "sha256-dUvp7FEW09V0xMuhewPGw3TuAic/sD7xyXEYviZ2Ivs=";
    }
  );
}
```

### `proxyVendor` {#var-go-proxyVendor}

If `true`, the intermediate fetcher downloads dependencies from the
[Go module proxy](https://go.dev/ref/mod#module-proxy) (using `go mod download`) instead of vendoring them. The resulting
[module cache](https://go.dev/ref/mod#module-cache) is then passed to the final derivation.

This is useful if your code depends on C code and `go mod tidy` does not include the needed sources to build or