Writing Custom Linter in Go

Write a custom linter in Go. Writing linters is simple. I was surprised how it’s easy to write a Go linter. Today, we’ll write a linter that will calculate the cyclomatic complexity of the Golang code. We have a fully functional Go linter! Your linter to golangci-lint. Here’s an example of the linter we’ve built.

Writing linters is simple. I was surprised how it’s easy to write a Go linter. Today, we’ll write a linter that will calculate the cyclomatic complexity of the Go code.

What is cyclomatic complexity?

Cyclomatic complexity is a software metric used to indicate the complexity of a program. ref

The idea is simple — every time we find any control flow statements we increase the complexity by one. I know I oversimplified it a bit but I don’t want to overwhelm you with unnecessary details.

There are a few steps we should follow to write our custom linter. Firstly, we can create a test that will check if our linter works or not. Let’s put him into pkg/analyzer/analyzer_test.go file.

package analyzer

import (
    "os"
    "path/filepath"
    "testing"

    "golang.org/x/tools/go/analysis/analysistest"
)

func TestAll(t *testing.T) {
    wd, err := os.Getwd()
    if err != nil {
        t.Fatalf("Failed to get wd: %s", err)
    }

    testdata := filepath.Join(filepath.Dir(filepath.Dir(wd)), "testdata")
    analysistest.Run(t, testdata, NewAnalyzer(), "complexity")
}

The analysistest.Run() function is a helper that simplifies testing linters. What it does is running our linter on package complexity in testate folder. We use NewAnalyzer() function that will return instance of our analyzer. Let's add it to pkg/analyzer/analyzer.go.

go golang linter

Go announced Go 1.15 version on 11 Aug 2020. Highlighted updates and features include Substantial improvements to the Go linker, Improved allocation for small objects at high core counts, X.509 CommonName deprecation, GOPROXY supports skipping proxies that return errors, New embedded tzdata package, Several Core Library improvements and more.

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