added draft of go mod edit replace how-to

content/posts/go-mod-edit-replace.md

@@ -0,0 +1,57 @@
+---
+title: "Go Modules: How to replace a dependency with a local copy"
+date: 2020-03-20T20:35:25Z
+draft: false
+tags: [golang, modules]
+---
+
+Lets say you're working on a go package and you need to replace a dependency with a specific version for testing. You can use the awesome `go mod edit -replace old=new[@version]` command to do this.
+
+This command adds a `replace` directive to your `go.mod` that overrides any require statement versions for the matching module:
+
+```bash {hl_lines=[13]}
+$ go mod edit -replace github.com/gobuffalo/packr=github.com/gobuffalo/packr@v2.8.0
+$ cat go.mod
+module github.com/adamveld12/riffraff
+
+go 1.14
+
+require (
+        github.com/gobuffalo/packr v1.30.1
+        github.com/golangci/golangci-lint v1.21.0 // indirect
+        github.com/satori/go.uuid v1.2.0
+)
+
+replace github.com/gobuffalo/packr => github.com/gobuffalo/packr v2.8.0  # now points at a different version
+```
+
+Now when you build your go app, it will use `v2.8.0` of packr in place of the version specified in the `require` block.
+
+But what if you want to use it similarly to `npm link`, where you want to replace a module with a local working copy?
+
+Run the same command but omit the `@version` on the new package like so:
+
+```bash {hl_lines=[16]}
+# clone your own copy and make some edits at ~/projects/packr
+$ cd ~/projects && git clone https://github.com/gobuffalo/packr
+$ cd ~/projects/riffraff && go mod edit -replace github.com/gobuffalo/packr=../packr
+$ cat go.mod
+module github.com/adamveld12/riffraff
+
+go 1.14
+
+require (
+        github.com/gobuffalo/packr v1.30.1
+        github.com/golangci/golangci-lint v1.21.0 // indirect
+        github.com/satori/go.uuid v1.2.0
+)
+
+replace github.com/gobuffalo/packr => ../packr #now points at your local copy
+```
+
+
+When you're all finished up you can remove the `replace` directive with the following command:
+
+```bash
+$ go mod edit -dropreplace github.com/gobuffalo/packr
+```