Merge branch 'main' of github.com:rnemeth90/rnemeth90.github.io

Author: ryan nemeth

content/posts/2025-11-02-go-mod-breaks-tests.md

@@ -8,37 +8,34 @@ categories:
 - Golang
 - Programming
 ---
-
 ## Introduction
 If you’ve ever initialized a new Go project with `go mod init main` and later tried to write tests for it, you may have seen this cryptic error:
-
-###
+```
 cannot import "main"
-###
+```
 
 At first glance, this doesn’t make much sense — after all, your code and tests are both in `package main`. However, there’s a subtle interaction between how Go handles modules, packages, and test harnesses that causes this issue. Let’s walk through what’s actually happening.
 
 ---
-
 ## Setting Up the Problem
 
 ### The `go.mod` File
 When you initialize a new Go module for an executable project, it’s common to do something like:
 
-###
+```Golang
 go mod init main
-###
+```
 
 That creates the following `go.mod` file:
 
-###
+```Golang
 module main
 go 1.25.3
-###
+```
 
 Then you might have a simple file like this:
 
-###
+```Golang
 package main
 
 func main() {
@@ -48,73 +45,66 @@ func main() {
 func square(n int) int {
   return n * n
 }
-###
+```
 
 Now you decide to write a quick unit test in `main_test.go`:
 
-###
+```Golang
 package main
 
 import "testing"
 
 func TestSquare(t *testing.T) {
   t.Log("Test successful")
 }
-###
+```
 
 At this point, everything looks fine — until you run:
 
-###
+```bash
 go test
-###
+```
 
 and get the following output:
 
-###
+```bash
 # main.test
 $WORK/b001/_testmain.go:14:8: could not import main (cannot import "main")
 FAIL    main [build failed]
 FAIL
-###
+```
 
 ---
-
 ## Why This Happens
-When you run `go test`, the Go toolchain generates a temporary test harness (a `_testmain.go` file). This harness imports the package under test so that it can invoke your test functions.
-
-Here’s the catch: the test harness **must import the module** under its module path. When your module path is literally `"main"`, the compiler refuses — because `main` is reserved for executables and cannot be imported like a normal package.
-
-This only affects `go test` (and `go test .`), because those commands rely on the generated import. When you explicitly specify files (e.g., `go test *.go`), Go skips module resolution and compiles everything together manually, avoiding the import step entirely.
+When you run `go test`, the Go toolchain generates a temporary test harness (a `_testmain.go` file). This harness imports the package under test so that it can invoke your test functions. Here’s the catch: the test harness **must import the module** under its module path. When your module path is literally `"main"`, the compiler refuses — because `main` is reserved for executables and cannot be imported like a normal package. This only affects `go test` (and `go test .`), because those commands rely on the generated import. When you explicitly specify files (e.g., `go test *.go`), Go skips module resolution and compiles everything together manually, avoiding the import step entirely.
 
 ---
-
 ## The Fix
 
 ### Option 1: Rename the Module
 Change your `go.mod` file to use a different name, for example:
 
-###
+```Golang
 module myapp
 go 1.25.3
-###
+```
 
 Then rerun:
 
-###
+```bash
 go test
-###
+```
 
 You should now see:
 
-###
+```bash
 ok   myapp  0.35s
-###
+```
 
 ### Option 2: Move Testable Code Out of `main`
 In most cases, your `main` package should be minimal — it should only wire dependencies together. Any functions that need unit tests should live in a separate package (for example, `internal/mathutil`), which can then be imported both from your main application and from your test files.
 
 ---
-
 ## Key Takeaways
 
 - The Go test harness always imports the package under test.  
@@ -123,8 +113,5 @@ In most cases, your `main` package should be minimal — it should only wire dep
 - Reserve `main` strictly for integration and startup logic.
 
 ---
-
 ## Conclusion
-This issue isn’t a compiler bug — it’s just an edge case that surfaces when your module path collides with Go’s internal naming conventions.
-
-If you ever see `cannot import "main"` when testing, remember: rename the module or refactor your code to follow Go’s standard package layout.
+This issue isn’t a compiler bug — it’s just an edge case that surfaces when your module path collides with Go’s internal naming conventions. If you ever see `cannot import "main"` when testing, remember: rename the module or refactor your code to follow Go’s standard package layout.